Losing neutral in the utility system

[jim hardy]

My perception of sophies opinion, and my perception may be off, is that it is somehow acceptable to not adhere to much of a standard at all in the USA. It is quite the opposite.

Quite so, well said.

I took it as a British ethnic joke poking fun at "the colonies'. Even Mary Poppins did that ,

Mr. Dawes Jr: In 1773, an official on this bank unwisely loaned a large sum of money to finance a shipment of tea to the American colonies. Do you know what happened?
George: Yes, sir, I think I do. As the ship lay in Boston Harbor, a party of the colonists dressed as red Indians boarded the vessel, behaved very rudely, and threw all the tea overboard, making the tea unsuitable for drinking. ... Even for Americans.

Where i live is quite rural. In most of the country the homeowner would have had to get a permit and inspections.
We're an anachronism, it's still like the 1950's around here. It's assumed you'll do the right thing.
I called my electric company,. They were fine with me pulling my own meter to do some repairs on the wall behind it, and said if i need it disconnected at the pole while i work just call and they'd send out a crew to do that for me. Try THAT in a big city !

old jim

 

[sophiecentaur]

I think we should all do an 'exchange' with someone 'on the other side' and have a play with our respective systems and do a bit of installation, with a certificated sparks to supervise. That would be interesting. The two systems really are poles apart, in many senses.

Apart from the (perceived) higher voltage involved in the UK, I can't fault it. The (expensive) plugs and sockets are a bit on the clunky side for smaller appliances but they are very confidence inspiring - nice thick brass pins and nearly all plugs are moulded these days. Individual and appropriate fuses mean that the ring will often stay on whilst a faulty appliance will be isolated by its own fuse.
US members are worried about the 230V but the history of shock injury in UK is very sparse. This will be because the perceived risk is greater. People are just more careful, I guess. 'The Ringmain' is viewed with deep suspicion in the US but it has (afaiaa) very few problems and is a cheap way to provide an unlimited number of outlets around the walls of your home, with only two or three (maximum) circuits needed in most homes.
The US split phase system worries me and I don't think it would be good for the present distribution in the UK. https://www.physicsforums.com/threads/losing-neutral-in-the-utility-system.886787/members/averagesupernova.7949/ 's problem with a poor neutral connection could be shared with every third house in the street, with the two anti phase volts drifting up and down to accommodate the imbalance. I haven't managed to see any advantage with the split phase arrangement - except backwards compatibility with old installations and appliances.

I took it as a British ethnic joke poking fun at "the colonies'.

lol. As someone with what you guys still refer to as a "Tyrannical Government", I can appreciate your sensitivities to that sort of thing but the only US citizens I know personally are really quite nicely behaved and above reproach.

 

[jim hardy]

I can appreciate your sensitivities to that sort of thing

Sensitivities ?
I'm not offended. My roots are in the US's Southern Ozarks, "Li'l Abner" country , where self deprecating humor is respected.

this belongs in Youtube classics :


i was hillbilly before it was cool, and have remained so long after...

I think we should all do an 'exchange' with someone 'on the other side' and have a play with our respective systems and do a bit of installation, with a certificated sparks to supervise. That would be interesting.

That would be fascinating. You really get to know somebody by working with them, and i suppose that's part of the lure of PF. We work together vicariously through the third party of the forum..
I'm still chewing on concept of "Magnetic Vector Potential", many thanks to @dlgoff and @vanhees71 ... One develops a feeling of kinship here.

old jim

 

[krater]

The main issue with the DIY perception is the implication that a utility, should one request, will happily leave three wires hanging from a pole with taped-up ends, and a homeowner, electrician, or otherwise can happily do what they want just as long as they send a check every month. To the contrary one is required to file a legal affidavit with some utilities saying that a service which is to be connected complies with all applicable rules of said utility as well as any other locally enforced codes. Anyone desiring to do things their own way can DIY all the way to the prime mover (or alternate). Even then it is quite possible to be violation of the local law.

A google search reports estimates of roughly a hundred million homes and five million commercial buildings in the United States. There are maybe a quarter as many homes in Great Britain and probably many fewer commercial and industrial premises. The United Kingdom clocks in at something under a hundred thousand square miles, the greater USA places some distance over 3.5 million. Imagine the disparity the average inspector might have between two given jurisdictions and you can begin to understand the disparity between inside wiremen on both sides of the Atlantic.

Jim's tale of you-pull-it as applied to a meter was once more or less the rule, albeit a somewhat dangerous one, even if you didn't call and ask. Now that meters have become far smarter, pulling a live meter can be a good way to get a line truck to show up at your house within an hour as the utility sees your service go offline.

Jim, does your utility do "live metering", where the arrangement is xfmr-meter-disconnect-mainbus? If so, then I'm more than a little surprised the utility gave you blessing to remove it. As you said though, out in the styx...

...repeat after me...NO...LOADS...CONNECTED...*click*!

EDIT: "Live" metering confused with "Dead" metering. Subtle but crucial...

 

[krater]

Now then, time for a few of my own tales of the elusive neutral conductor:

One day, several years ago, I was called to the top of an emerging parking structure to check out an electrical problem. The rodbusters were building their PT deck and were being "nailed" by their cable stressing machine, which entailed maybe a half-horse hydraulic pump, attached to a tank and ram, contained within a wheelbarrow. When someone touched the metal case of the stressing machine while simultaneously touching another piece of ambient metal, one would possibly receive a hell of a jolt while the machine was running.

The cause? It was easily traced to the temporary construction service to the project, whose main purpose was to provide electric baseboard heat to the GC superintendant trailer during the winter months, with a backboard-mounted 240/120v single-phase service fed from a pole-mounted transformer that was located some three or four hundred feet away from the main disconnect. The two meager eight-foot copper-clad-alloy ground rods driven into several feet of pollution abatement sand fill was a poor substitute for the very, very good Earth reference that was located some couple thousand feet of (minimally sized) conductor away, in the form of about sixteen size ten or eleven reinforcing bars which were connected to structural pylons sunk one hundred eighty feet into the bedrock of "the big river".

The solution? Disconnect and insulate the equipment ground. The "neutral" was much too heavily leaking to "ground" far after the main disconnect.

The several hundred or so permanently connected amps we had running around this complex were fed through locally grounded systems which were interconnected via thousands of feet of buried, bare, AWG 3/0 copper which probably provided a better ground plane reference than any of the "grounding electrode systems" that we installed onsite.

Fun fact of NA wiring: the term "neutral" has been systematically removed from NFPA70 : National Electrical Code as of the last couple code cycles, and ubiquitously termed "grounded conductor." Terminoligly can be very important in this discussion.

 

[krater]

...I haven't managed to see any advantage with the split phase arrangement - except backwards compatibility with old installations and appliances.

That's like saying that nobody is making 120VAC motors anymore except for backwards compatibility, when 120V systems are widely available. If everybody said tomorrow, there will be no more except 200V motors, your argument would have creedence. However, should you ask ANSI, OSHA, ect, one hundred twenty volts to ground is still a commonly acceptable maximum potential to ground than any given operator should be regularly exposed to. Your RCDs on the east side of the Atlantic seem to correspond to what electricians on the NA side call "ground fault protection for equipmnent." This is a fuzzy term that can range up to thousands of amps. However, a GFCI Class A on this side of the pond corresponds to a device interrupting at FIVE MILLIAMPS... take a look again at your crazy ring arrangement!

 

[sophiecentaur]

That's like saying that nobody is making 120VAC motors anymore except for backwards compatibility, when 120V systems are widely available.

I know my comment was the sort of comment Historians make, to explain some past event - or the present situation. It was speculative. of course but our whole lives revolve around the status quo. There is very little pressure for the US to change to the higher voltage system so why should they change? But, given the choice of starting with 230V, it would make a lot of sense. 110V uses more copper and it clearly is not suitable for high power appliances (you only use it for small appliances) but you have millions of homes with 110V wiring and appliances are sold for use at that voltage. That is what I meant by backwards compatibility.
The 'safety' aspect for the 110V system was probably a lot more relevant when the original choice was made and wiring used rubber and cotton insulation etc.. PVC and other plastics are so much better for strength and durability and, as I have already commented, shock accidents are extremely rare in the UK.
I think you would have rioting in the streets in the US if there was a move to change.

 

[sophiecentaur]

When someone touched the metal case of the stressing machine while simultaneously touching another piece of ambient metal, one would possibly receive a hell of a jolt while the machine was running.

That sort of accident can only happen when the temporary or permanent installation are not right. A system with a Live and Neutral conductor can be covered with an RCD and that will detect whenever there is an imbalanced current. Whatever is done about bonding other metal structures together and to the local Earth, an RCD will protect users. The only possible objection to active protection like an RCD is reliability. That argument would stop us using ABS in motor cars and fly by wire aircraft and it is a non-starter these days.

 

[krater]

That sort of accident can only happen when the temporary or permanent installation are not right. A system with a Live and Neutral conductor can be covered with an RCD and that will detect whenever there is an imbalanced current.

In this case an RCD, as well as the GFCI that actually did exist somewhere along the circuit was useless. The stray current was coming from the connection between the utility neutral and the equipment ground. The neutral currents at the main were not finding a good enough path back due to inadequate grounding and said current was taking all available paths to ground, as always, and one of those existed between the equipment ground and the steel of remotely located foundation columns.

GFCI/RCD watches for L-N imbalance but is indifferent to what happens on the equipment ground. Also the only device which would have had any chance to catch the imbalance would be the main which as I understand would not be an RCD even in Europe. It is important to remember as well that should you be unfortunate enough to wind up between the line and neutral of a circuit while you are relatively well insulated from ground (think dry wood floor), a GFCI/RCD will happily cook you as from its point of view you are totally indistinguishable from a load. Injury from electric shock is a function of current and duration. Higher voltage = more current = a close call at low voltage is a death sentance at a higher voltage. This is all before we address the issue of reliability; ABS braking systems and aircraft are systems are normally well monitored and maintained. When is the last time YOU tested your RCD/GFCI? If it wasn't manufactured in the last decade or so it probably lacks any self-diagnostic or failure indication capacity. I personally don't put that sort of faith in (especially electro-)mechanical devices.

The resources angle is a good one to point out since a lower voltage is obviously a tradeoff that requires larger conductors. No doubt mineral scarcity carries more weight for nations that don't have millions of acres of mine sites within their own borders. I wonder, at what point would any savings in material from only requiring half the amps to power equipment be outstripped by the replacement cost in resources to junk all the old low volt stuff and manufacture everybody a new fridge, TV, all new wall plugs, ect. My guess, that point is long, long gone.

I think any riots over a new system voltage would be marked by large crowds all carrying signs that simply read, "WHY?"

EDIT: Another point from the safety side. Power limited operator circuits have been moved to ever decreasing voltages as the cost of installing relatively tiny, isolated control circuits in appliances has been matched with the increase in safety afforded by only placing 24, 12, or even fewer volts in close contact with a potential operator. In other words, even though 120v is accepted as safe enough, it is widely recognized that, put plainly, lower is safer. Do you really think anyone would incurr the added cost of a low volt power limited control circuit that doesn't expose an operator to mains voltage if it had no safety benefit whatsoever?

 

[sophiecentaur]

signs that simply read, "WHY?"

One very good reason is in that described incident, which would not occur in the UK system. If the wiring involves no connection between Neutral and earth, downstream, then an inadequate (HR) Neutral (or an inadequate Live - same status) would give an output with Volts that would dip whenever a load was added and 'someone' would be aware of the problem. A user who ignores lights dipping has only himself to blame, if there is no good reason for it - and only the supply company can advise him that "it's ok in this premises". An RCD cannot be operated if the neutral current, downstream, is shared with an Earth because there will be a built-in current imbalance.
As far as I can see, the reason for the move to 12V and lower for appliances is due to the fact that an increasing number of appliances can be operated from batteries or car systems. The gauge of cable needed for 12V heating and other power devices would make it very unattractive. Who would want a 3kW electric kettle, working at 12V? The cord/flex would be incredibly fat / heavy and connector contacts would need to be maintained regularly to make sure they were adequate.
I can't understand the apparent obsession for wanting the Neutral to have a local connection to earth. I have already made the point that the Neutral conductor has the same status as the live conductor so why not treat it in the same way? The 220V circuits in US houses have two conductors that are treated with the same 'respect' so why should the Neutral be different. If the circuits that carry power are completely isolated from the local Ground (define it as zero potential) and all non-electrical metal is bonded to that, then what is done with the Neutral, somewhere up the line is not relevant to the safety situation on site. Systems that require power to be carried through the Earth may have their place but I cannot see where they should come in for house wiring. What is the advantage?

. take a look again at your crazy ring arrangement!

That is just being Xenophobic. It is a very safe (and cheap) way to provide many outlets around every room, without the need for multi-way adaptors, long leads to appliances and many star radials, run in parallel to every outlet around the wall. You don't appear to understand much about the rationale behind the design and have just taken against it because it is unfamiliar. One of the reasons that it is successful in the UK is that the regulations are tight and level of inspection is pretty stringent. The clunky connectors are something we all get used to and they are not an issue. Read around about it and you could even grow to love it!
Your comment on the available range of RCDs is hardly more relevant to the situation than saying that fuses exist from 10mA to 1kA so they are of no use; they have to be selected for the purpose.

 

[krater]

One very good reason is in that described incident, which would not occur in the UK system. If the wiring involves no connection between Neutral and earth...

You're most likely incorrect. If I understand what was pointed out elsewhere in the thread, the "protective earth" or equipment ground as it is properly called are simply connected on the utility's side, and not the customer's. The connection still exists. Unless one presumes some sort of magical infallibility to the design of the utility's system there is absolutely no reason the exact circumstances cannot occur. Think, in the noted example would the same phenomenon have existed if the utility's transformer was so well grounded that virtually no current found a return path along any other means? You note yourself that you can observe a slight potential difference between PE and neutral. Imagine if the nearest spot they were connected together was thousands of feet away electrically. Not good. The nonexistance of mains level RCD means nothing is done to mitigate the hazard.

Your comment on the available range of RCDs is hardly more relevant to the situation than saying that fuses exist from 10mA to 1kA so they are of no use; they have to be selected for the purpose.

5mA is a personnel protection standard widely recognized in NA and elsewhere, from what I'm seeing your equal is 30mA and we are talking at twice the voltage, meaning it can get through twice the path resistance. Ventricular fibulation occurs between 50-100mA and this is why in NA the 30mA standard is considered for equipment protection only and is insufficient to protect people from electric shock. You call me xenophobic, I call you reckless and where does it get us? But go ahead and argue how a smaller tolerance is somehow less safe.

 

[sophiecentaur]

You note yourself that you can observe a slight potential difference between PE and neutral.

Yes. In the same way that your two split phase lines show a large voltage to local ground. It isn't particularly relevant.
Suppose, hypothetically, the company were to supply you with three wires. one with 120V, one with 10V, in phase with the first and one with 100V 180° in phase and they also give you an Earth conductor at 0V relative to the spike in your garden. What would be the problem? You can connect 220V and 110V equipment as normal. The earthed water and gas pipes etc would be bonded to the incoming Earth conductor and also to the spike. Your protection equipment in the house would be quite happy with it and, should there be a path to Earth, through an equipment case (with, of course, an Earth wire), there is no danger of shock. A fuse might blow if the path resistance is low enough but an RCD will also have operated. If the supply company do not happen to have a 'good' earth, your RCD will still operate whilst a fuse may not blow (no problem). The only possible problem would be if the company neutral was disconnected AND there happened to be a connection between your local neutral and your Earth conductor due to a fault within the house. The latter fault would have already tripped an RCD but, if there were no such connection in the house, the supply would have been interrupted. A nuisance situation but not a hazard.
By allowing the possibility of Earth return as an allowable condition, it would be possible for the neutral interruption to go unnoticed and for a dangerous potential to exist somewhere amongst the Earth network.
It strikes me that the Neutral path to the transformer is treated less 'reverently' than the other two paths and that seems to have meant that neutral is not designed to the same standard. No one would be surprised if one of the live cables came into contact with the ground and you had problems so what's the inherent difference? IS it just to do with cost?

 

[sophiecentaur]

5mA is a personnel protection standard widely recognized in NA and elsewhere, from what I'm seeing your equal is 30mA

Is this relevant to the neutral problem?
I agree that there is quite a difference and I have no idea why two different specs are applied, bearing in mind that both will have been arrived at after looking at a lot of statistics from accident data. (Pity we can't subject humans to a more rigorous study of the effects of shock. If we could, then a more reliable figure could be used. haha)
I remember a similar set of information was found to indicate the decompression times for diving at depth. But the naval divers who supplied the information were more like guinea pigs because they were actually ordered to explore the envelope. Likewise, I seem to remember being told that American (I think) soldiers were used to establish the relative risks of exposure to nuclear weapons by getting them to stand at various distances from test explosions and then studying their health over their subsequent time in the army. Drug testing has been done, using convicts, in various parts of the world. So much for Ethics.
The difference between the figures could represent something about the statistics about the difference between overall risk that each system poses. Or it could be just figures, snatched from the air - like the recommended alcohol and salt intakes. You can bet the guy who actually took the decision knew very little about health or engineering. He should have been a member of PF.

The connection still exists.

(Earth / neutral)
Of course there is a connection because, if there were not some defined potential to earth, there would be the possibility of the three phases and neutral from a transformer drifting out to any, unspecified potential - kVs even. The neutral of the transformer only needs to be loosely 'moored' somewhere in the region of 0V, rather than tied up hard to a cleat in the electrical sea.
I asked, higher up, for opinions about the pros and cons of local N/E connection, rather than just one connection back at the substation and no one offered one. I always assumed that a floating neutral ensured a better balance between the supply volts at each (single phase) consumer along the line and less I²R losses.

 

[sophiecentaur]

You call me xenophobic, I call you reckless and where does it get us?

OK let's not fall out about this. The ring system confuses pretty well everyone who hasn't used it. The diagrams in many UK school books are actually wrong. What is true is that it is very successful, with very few 'incidents'. You cannot have a star system at the same cost, to provide the useful number of outlets that you find in most UK homes and offices. Hence, it continues to get my vote.
AS for RCD's, you pays yer money and you takes yer pick. I guess I might feel a bit safer with a lower cut out current but I would probably get very cross about the increased number of spurious trips. Hardly "reckless" though. Perhaps, as I get older, I may be more susceptible to cardiac arrest with a fault current of 30mA - we shall see.

 

[krater]

Well the one point I would like to make would be your theorized 10V main lead. What happens when you get between it and the 0V protective earth? Of course current will flow. If you are part of this current path it can be, well, uncomfortable. Even if, say, a resistance of just the right value might fall between the two lines, it could involve enough current to create enough heat to start a fire.

I think where a lot of the difference in perception comes from is the way OCP is handled in the two situations and really has little to do with how many grounded or ungrounded lines are coming in, and at what potential. First off, let me point out that especially until recently there was not commonly an equal to the RCD found in NA branch circuit panelboards. Beginning in the 1990s standards came about to require ground fault protection for personnel in selected areas, most often where someone is likely to encounter electricity and moisture or moist, well-grounded surfaces at the same time. This was normally handled at point of use, or the receptacle (plug-in, outlet maybe...the thing the prongs go in. Terminology!).

There were a few reasons why normally the preferred route. Sometimes due to what's called multiwire circuiting it was not possible to install a panelboard device that could watch current going out and coming back on the pair comprising a circuit, and rewiring would have been necessary. Also, at the time the already costly GFCI devices were especially costly in circuit breaker form, maybe three to five times the price. Either way it was usually most economical to install the few receptacles needed in the bathrooms and kitchens than to install even one GFCI breaker.

Here's where I'm going with this as it relates to the local grounding electrode. Aside from places where one stood at a decently high risk for a shock, the only sort of current monitoring on a general use branch circuit was the relatively dumb thermal-magnetic trip unit in the 15 or 20 amp breaker, connected in series with the ungrounded conductor or "hot". Nothing watched, let alone interrupted the neutral no matter how much current flowed on it, or where for that matter. This did pretty well in keeping wires from melting. It didn't do much for shock hazard though, but since the system was designed with one conductor referenced at 0V to ground anyway, it made sense to create a local connection to ground potential via a grounding electrode system and connect that 0V conductor to it.

Another reason this makes sense is that it gives transients on the line as well as potential fault currents between ungrounded conductors and either grounded or grounding conductors another path to facilitate the clearing of some device meant to interrupt the fault. If the only paths to and from the transformer are the wires comprising the transformer's secondary it has some implications for eletromagnetic choke effects that could hinder an OCP sufficiently that the circuit conductors could fail before the circuit was broken. You still talk of Earth return as if it is used as a normal current flow path which has been depreciated for quite some time. However under a fault condition, with respect to paths for current return, more is better.

You seem to get the impression that the neutral has a depreciated place in the circuit hierarchy; I would say we just use it to its fullest potential, pardon the pun. You won't convince me that letting a neutral float to arbitrary height from a local ground is OK so long as some electronic device is upstream from me that will *try* to break the circuit if it senses something is amiss. What happens if that device fails? Better hope you're only floating a couple volts above ground. Better also hope that your zero connection at the transformer is quite solid as your role as a parallel path is defined by all the other paths available. And you better hope that no situation ever comes along where amps, wire length and electromagnetism conspire in such a way that some large number of amps find a way out around your main and get "three stooges syndrome" on the way back to the transformer. I don't like designing things with the hopes that nothing breaks and everything will just keep working fine...

That argument...is a non-starter these days.

As far as the 5mA protective standard it actually has more to do with children whos smaller bodies are more succeptable to shock than an average adult. So we figure that if the adult safety value is 50mA, then one order of magnitude less should be more than safe for everybody. I would imagine there has been quite a lot of human research of some form relating to electric shock due to its very, very wide range of applications. Do you suppose research done to develop pacemakers for example would be somewhat relevant to designing a device made to actually protect the heart from shock? I certainly would.

I don't agree with your dismissive attitude towards the development of GFCI personnel standards. This stuff wasn't handed down by the king or written up by "the guy" you speak of, it was developed by multiple groups of people, entites, and industries, who form a consensus standard. And I can't understand why you would expect some increase in spurious trips. First of all if no fault exists that let's current out of the circuit why should anything trip, ever? And secondly, do you not realize that scores and scores of 5mA GFCI devices are in use every day, even on things like vending machines? What, do you think someone has to go around and reset GFCIs every week or two so that they can ensure their sodas are still selling?

 

[sophiecentaur]

You are being a bit over-protective about the system you are familiar with. 'Different' doesn't necessarily mean 'better' or 'worse'. I am not 'attacking' your neutral = Earth system. Rather I am casting doubt on it and I want to hear a good argument in favourite of it. I really haven't heard one yet - except that it is established practice so there's no point in changing.

What happens when you get between it and the 0V protective earth?

What happens if you get between any non-zero potential conductor and earth? The system should not be exposing you to the possibility of touching neutral or live sides.

This was normally handled at point of use, or the receptacle

As in the UK (you have introduced another thing here, which is just confusing the main issue). Electric Showers and outdoor (garden) outlets are protected by RCDs. ('power breakers' etc)

the only sort of current monitoring on a general use branch circuit was the relatively dumb thermal-magnetic trip unit in the 15 or 20 amp breaker,

That was used in UK where the Earth resistance was too high for many faults to blow a fuse. If you can't measure the imbalanced current (i.e. years ago) that's all you can do. It protects against fire, only.

You seem to get the impression that the neutral has a depreciated place in the circuit hierarchy;

From the fact that you seem to regard the Earth conductor as a possible backup for the neutral circuit, I would say that is precisely the situation with your neutral circuit. How about using the Earth as a possible return path for the live? Obviously a nonsense idea because, by definition, the neutral potential is 'safer'. It is not unheard of for systems to use two floating power conductors. Which one would you choose for the Earth return then?

As far as the 5mA protective standard it actually has more to do with children whos smaller bodies are more succeptable to shock

You keep bringing in extra issues. The tripping current is decided upon by a committee, on the basis of statistics and total cost - same as many other 'recommended levels' throughout our lives. There will be many other factors taken into consideration and, despite what you think, the decision is made 'on balance'. As it happens, the US decided on a different value to the chosen UK value. I doubt that you have read all the committee papers and you really can't know the arguments used to justified the chosen levels. Yes; the US standard is tighter than the UK standard but what statistical effect does it have? Data is needed here and not a gut reaction. I would also 'feel' safer with a 5mA level, of course. Perceived danger is very important to us.

First of all if no fault exists that let's current out of the circuit why should anything trip, ever?

In an ideal world, you are right but elements in ageing water heaters are notorious for small but finite Earth currents. There is always the possibility of an unbalanced current at switch on. Google "Spurious RCD trips". It's a rich vein of information.

 

[sophiecentaur]

Even if, say, a resistance of just the right value might fall between the two lines, it could involve enough current to create enough heat to start a fire.

I just re-read this. Isn't the fuse supposed to deal with that?

 

[jim hardy]

When someone touched the metal case of the stressing machine while simultaneously touching another piece of ambient metal, one would possibly receive a hell of a jolt while the machine was running.

The cause? It was easily traced to the temporary construction service to the project, whose main purpose was to provide electric baseboard heat to the GC superintendant trailer during the winter months, with a backboard-mounted 240/120v single-phase service fed from a pole-mounted transformer that was located some three or four hundred feet away from the main disconnect. The two meager eight-foot copper-clad-alloy ground rods driven into several feet of pollution abatement sand fill was a poor substitute for the very, very good Earth reference that was located some couple thousand feet of (minimally sized) conductor away, in the form of about sixteen size ten or eleven reinforcing bars which were connected to structural pylons sunk one hundred eighty feet into the bedrock of "the big river".

The solution? Disconnect and insulate the equipment ground. The "neutral" was much too heavily leaking to "ground" far after the main disconnect.

A picture would help me understand that one.
Had there been an all metal path for fault current to flow between that machine and the"separately derived system" 's transformer winding at construction service , which code requires, (even if that path were just the neutral conductor, ) the guys could not have experienced more than the voltage drop along that all metal path..

Seems to me something was elevating local Earth potential atop that garage so by disconnecting those ground rods you isolated your rodbusters from whatever that was.

 

[sophiecentaur]

A picture would help me understand that one.

Definitely. It strikes me that, if the neutral was high resistance, there would / should have been visible effects like dipping lights when a motor / heater was turned on etc..
I know you guys could take offence again but the described scenario (affairs) seems so random that I can't imagine that setup occurring in the UK. (Could be my rosy tinted spectacles, of course). Construction site setups are the highest risk*, of course and that's why they have transformers all over the place, special connectors for everything and 110V appliances.
*Next to rock concert stages.

 

[jim hardy]

I know you guys could take offence again but the described scenario (affairs) seems so random that I can't imagine that setup occurring in the UK. (Could be my rosy tinted spectacles, of course). Construction site setups are the highest risk*, of course and that's why they have transformers all over the place, special connectors for everything and 110V appliances.

Well, going back to that link you posted in #62(if numbers still hold)
you and we should have an all metal path back to the transformer winding from which the current originates.

The two meager eight-foot copper-clad-alloy ground rods driven into several feet of pollution abatement sand fill was a poor substitute for the very, very good Earth reference that was located some couple thousand feet of (minimally sized) conductor away,

Hmmm.
IF that couple thousand feet of 'minimally sized conductor ' served as both neutral AND earthing conductor, as is allowed at a service entrance,
THEN the drop along it could explain the shock especially if things were wet. I guarantee a 12 boat battery will zap you good when your skin is soaked with seawater.
Nowadays that'd be disallowed under code for neutral and protective conductor can join only back at that man disconnect and that local panel would be called a sub-panel , so there'd be a separate conductor for fault and/or leakage current ...

I don't know how long ago Krater fixed that one. Might well have been under a previous revision of the US code.

Every little lesson like this improves our ability to work systems in our head. This thread, along with my guest house wiring, have clarified several points in my alleged brain... Codes evolve as we (collectively) learn from our mistakes.

old jim

 

[sophiecentaur]

. . . . and I am right that at least someone should have spotted the dimming lights or groaning motors - which must have been powered, to account for the vast voltage drop.
Someone should have had a real roasting about that.

 

[jim hardy]

BTW for what little this is worth
Going back to the events of post #19, incoming neutral and hot wires swapped at panel, placing 240 volts across half the circuits in my neighbor Harry's house...
Yesterday i replaced his refrigerator's freezer fan . It had burnt up in the minute or two we had 240 on it. Surprisingly it still made ice cubes but the lower compartment wasn't getting cold enough.
There was a mild buildup of ice on the evaporator so I suspect we'll have to replace the defrost timer too ... Compressor and evaporator fan are thermally protected thank goodness.

 

[jim hardy]

. . . . and I am right that at least someone should have spotted the dimming lights or groaning motors - which must have been powered, to account for the vast voltage drop.

You'd sure think so.
Universal motors with brushes are more tolerant of out-of-spec voltage than induction motors.
My retired carpenter friends reminisce about running their 110 volt worm drive Skilsaws on 220 . "They really go!" .
Ahh youth.

Induction motors will burn up from either over or under voltage.

 

[krater]

Diagram of this not exactly open neutral situation

The red lines are circuit conductors L, N... the very faint green line after the service point is the equipment grounding conductor or protective earth. It existed everywhere along that path, I don't believe they were ever connected together again anywhere down the line, yes it was some time ago but that hasn't been permissible for some time either.

As for noticing any other effects of the situation, well, apart from flickering fluorescents at the main trailer there simply weren't many lights connected nor were there that many non-intermittent motors as this was a temporary construction service for a 98% concrete structure.

 

[sophiecentaur]

A nightmare!

 

[krater]

From the fact that you seem to regard the Earth conductor as a possible backup for the neutral circuit

You misunderstand. The closest thing I've said to that is probably the fact that the neutral could concievably be a backup (or replacement) to ground. That was once standard (cost related) and still exists regularly in dwellings. Scary. Here's the thing, and yes it's been alluded to already, so long as circumstances, for example wood construction and dryness, ensure that one remains isolated the system works just fine to prevent ground faults from starting a fire with little or no potential for shock.

I can see by your many many citings of RCDs that you are very comfortable in total reliance on them. Unless they are priced at hundreds of dollars apiece I judge your trust as poorly placed. Once again, do you TEST your RCD? Ever? How old are they? Do you have any idea how much GFCIs and the standards behind them have changed in NA just in the last decade? How do you know it will detect any leak in the neutral, and trip in such a condition, instead of happily frying you if you somehow end up solidly connected between neutral and local ground potential?

Consider a situation where your utility loses your PE connection in the circuit that serves your electrical works*. Maybe with a mostly underground system that's difficult to imagine but we will consider it since a great many power lines still exist on poles. You don't own or control the PE connection so as far as maintaining it you are at the mercy of the utility. Even if you were in an underground situation in NA this is not a situation you ever have to worry about. Why is that? Because at this point of the system the "PE" is the NEUTRAL conductor! Yes, it's the zero potential wire but since it's zero it is used as a fault current path for non-circuit metal parts as well. So when the neutral connection is lost on a UK system, the lights go out. When the neutral connection is opened on a NA system, well, something is likely to go, somewhere. A loss of PE before the service is tantamount to a loss of neutral in the NA system. But when the utility side PE goes out in a UK system, nothing happens!

This can have serious ramifications aside from the fact that it occurs without indication. Since on the system a circuit conductor exists which is zero somewhere is in close proximity to a conductor which is zero locally there is the chance of a fault developing between them. The fault will likely happen at a low potential since zeros might be relatively close. But low potential might also create the balance of a current sufficient to start a fire but too low to clear a main. The RCD is intended to prevent this but unless it is self-diagnosing with a fail-safe design it is a single point failure in the protection chain under these circumstances.

I will state this again, if you can't guarantee that a zero will not become more than zero the safest way to handle it is to connect it to as local a zero as possible. Your system gives up on that a step before ours. Probably on cost with no real consideration beyond that. Maybe it's not a big deal there; the world is a bit smaller. It can be a big deal here. As far as knowing where the standards come from, well it's part of continued training that I attend for example informationals put on by the people who are, yes, developing and writing the standards, they are not some inaccessable oracle in a far off high-tower but are in fact regular people like you and me who don't hide their reasoning behind some smokescreen.

The statement that with no faults there will be no trips is idealized, yes. I'm familiar with the devices and their uses, yes they do nuisance trip in all kinds of situations. But you don't appreciate that 5mA point of use devices may be much more abundand than probably even RCDs, and nobody deems them useless due to spurious trips. 30mA range devices are used here, too. They are only permitted to be applied in situations where human safety is not the reason for the ground fault detection, it is an equipment protection standard to keep monster sparks from shooting out of things. Equipment protection is common in another range, the 1000A+ devices that need big overgrown RCD/GFCI circuits to keep a high impedance fault from burning down the gear without ever drawing enough amps to trip a thousands of amp device set to accommodate a large building's inrush.

And I don't consider it to be bringing in extra issues when I'm illustrating my side of an argument with the full implementation of the system I'm favoring. Just as RCDs are integral to the safe function of your side, GFCIs are necessary and required at times on my side. And you still seem to be ignoring my presentation of another reason why the more local the grounding, the better: fault/surge energy dissipation, in other words, abnormal circuit conditions that you seem intent to lump in with Earth return circuiting.

*"Service" or "Service point" = The spot where your main OCP is in UK, the spot where your main OCP and last neutral earthing is in NA. Usually at this point "you own it"

edit: zero will NOT become more than zero

 

[jim hardy]

Diagram of this not exactly open neutral situation

The red lines are circuit conductors L, N... the very faint green line after the service point is the equipment grounding conductor or protective earth. It existed everywhere along that path, I don't believe they were ever connected together again anywhere down the line, yes it was some time ago but that hasn't been permissible for some time either.

Thanks !

Good one . Ground troubles give really confusing symptoms.

An open near originating end in that thousand foot grounding conductor would let it float to somewhere between L and N by leakage, as you suggested.

I've been known to test grounding conductor's ability to clear a fault by applying one, but only downstream of a 15 or 20 amp breaker. Wouldn't do that on this one though for fear of shocking somebody back in that trailer. I will though check for voltage between L and N unloaded, then risk sacrificing a cheap analog multimeter by reading resistance L to N. That gives me confidence the green conductor is continuous. Where they use conduit for grounding conductor , loose fittings can bite you.

Thanks for sharing your mystery Krater. I love anecdotes about troubleshooting .

old jim

 

[krater]

The equipment ground was isolated at the metal-encased hydraulic stressing machine, leaving the metal case and any metal connected to "float". I checked for N-G short in the faulty machine, and found nothing (using a DVOM anyway...yes i know). The serving circuit seemed to be working properly otherwise, although for additional safety reasons a majority of construction equipment is now put together in a double-insulated package to help further protect against just these sort of conditions.

By the time I got up there to check the problem the crew had gotten another identical stressing machine that was reportedly working fine, upon examination I found that the new machine had already gotten its equipment grounding connection removed from the metal case. Funny coincidence, or maybe not so coincidental at all considering the usual environment.

This wasn't the only exceptional incident I observed at this particular site. One humid day in early summer I witnessed anomalous behavior of small bits of 18ga steel wire, a few inches long, which spontaneously exhibited magnetic behavior in contact with some of the installed structural rebar; it would become attracted to the tip of a non-magnetized metal tool and could be led around like a dog on a leash. Nobody had a good explanation for that one and I've never seen the phenomenon repeated.

 

[sophiecentaur]

Just as RCDs are integral to the safe function of your side

No they're not the only safety measure; they are only an extra backup against some faults and will also draw attention to them. If the whole of the exposed metal structure of the house / site are at the same potential then any accidental path from Live to Earth (not even enough to blow a fuse) the potential on any exposed metal will always be the same as you when you touch it. Whether the neutral and Earth are connected, this will be true. RCDs are relatively new devices and hordes of people didn't die previously when their electric kettle element corroded through from L to the Case.
I think you are basing many of your ideas on non domestic situations. What goes on 'on site' can be far worse because people take short cuts (illegally) and bend the rules but I am surprised at the number of serious problems that have been quoted due to what appears to have been plain bad practice.
I take your point about the Neutral being intended as a 'fall back' for the Earth but the risk of shock is pretty low if there is good quality connection of all exposed metal to the local 'Earth'. network What happens down the road would not be relevant except for supply maintenance staff who would presumably follow proper safety procedures when arriving at a site.

 

[darth boozer]

Just for info, in the early 1980s I worked in an RF screened room with hefty filtering on the 240V supply. While trying to trace the source of some RF interference, I tested the mains supply in the room and was able to draw about 7 amps at 14 volts between neutral and the earthed walls of the room.

 

[sophiecentaur]

Just for info, in the early 1980s I worked in an RF screened room with hefty filtering on the 240V supply. While trying to trace the source of some RF interference, I tested the mains supply in the room and was able to draw about 7 amps at 14 volts between neutral and the earthed walls of the room.

I wonder who paid for the energy.

 

[sophiecentaur]

Just for info, in the early 1980s I worked in an RF screened room with hefty filtering on the 240V supply. While trying to trace the source of some RF interference, I tested the mains supply in the room and was able to draw about 7 amps at 14 volts between neutral and the earthed walls of the room.

I imagine that the 14V Neutral voltage was due to the internal resistance of the 'hefty filtering' - i.e. the current supplied to the equipment inside the room was enough to cause a 14V drop through the N leg of the filter. Was there 14V on the N wire, upstream of the Room?

 

[Guineafowl]

I realize this argument is months old now, but I must point out a few (hopefully interesting) things...

5mA is a personnel protection standard widely recognized in NA and elsewhere, from what I'm seeing your equal is 30mA and we are talking at twice the voltage, meaning it can get through twice the path resistance. Ventricular fibulation occurs between 50-100mA and this is why in NA the 30mA standard is considered for equipment protection only and is insufficient to protect people from electric shock. You call me xenophobic, I call you reckless and where does it get us? But go ahead and argue how a smaller tolerance is somehow less safe.

Now, of course a lower fault current limit is safer. But there’s a reason for the 5/30 difference, and I think I see it...

And secondly, do you not realize that scores and scores of 5mA GFCI devices are in use every day, even on things like vending machines? What, do you think someone has to go around and reset GFCIs every week or two so that they can ensure their sodas are still selling?

I assume this is about a dedicated GFCI for each machine? If so, there’s your difference. UK circuits are usually protected in two groups fed by an RCD each. We’re probably going to move to RCBOs for each circuit one day, but that’s the way it is now. In an ideal world, those RCDs should be 5 mA, but this is reality, and earth leakages, even from new equipment, add up, and at 240V will be twice that seen in the US. Indeed, with a peak voltage of 330V or so, there may be dielectric breakdown twice per cycle that wouldn’t happen at 120V at all. You say 30 mA is a dangerous threshold, and it may be, but the real-world shock current will be the difference between the ‘normal’ leakage and the threshold.

A 5 mA RCD protecting a ring main, an immersion heater, lights, a shower... I bet you any money this will ‘nuisance trip’. This is extremely annoying. And if you squeeze safety standards too hard, what happens? At least one person out of the 70 million here will bypass it.

A final point: We’ve had the dual 30 mA RCD system here for some time now, so we’ve ‘sucking and seeing’ for a while. If a single death occurred despite the RCD working as it should, there’d be an inquest and a call for revised standards. I challenge you to find such an incident, because I can’t. The 30 mA threshold is there not because we think a shock below this is perfectly healthy, but to achieve a balance between safety and usability. As it stands, nuisance tripping is rare, and fatal shocks are rare.

 

[krater]

Now, of course a lower fault current limit is safer. But there’s a reason for the 5/30 difference, and I think I see it...earth leakages, even from new equipment, add up, and at 240V will be twice that seen in the US. Indeed, with a peak voltage of 330V or so, there may be dielectric breakdown twice per cycle that wouldn’t happen at 120V at all...

Yes I suppose this was somewhat addressed in above discussion when I believe I mentioned that 120V to ground is a maximum exposure safety standard for operators per ANSI/OSHA standards for many industries. Many, many control and operation functions that used to be carried out in potentially hazardous environments through 120V circuits are now using 24VDC or less as a control voltage, with a power limited source to further increase safety.

I have seen three or four 120V GFCI devices used for temporary power during building construction, many of which were split initally at what we call a duplex receptacle, many plugins of which had either a long extension cord or a three-way pigtail splitter splitter, or both or even other combinations thereof plugged into them, which ran multiple battery chargers/lights/electric hand tools fed from cumulatively hundreds of feet of cord. And with proper attention paid to the installation and the conditions of the cords or tools connected to them, there are surprisingly few problems with spurious tripping of the protective devices. Nearly every time it happens it's either related to a damaged cord that when removed solves the problem, or some more complicated situation involving something like a large motor that can often be resolved by re-arranging a few things.

 

[Guineafowl]

Yes I suppose this was somewhat addressed in above discussion when I believe I mentioned that 120V to ground is a maximum exposure safety standard for operators per ANSI/OSHA standards for many industries. Many, many control and operation functions that used to be carried out in potentially hazardous environments through 120V circuits are now using 24VDC or less as a control voltage, with a power limited source to further increase safety.

I have seen three or four 120V GFCI devices used for temporary power during building construction, many of which were split initally at what we call a duplex receptacle, many plugins of which had either a long extension cord or a three-way pigtail splitter splitter, or both or even other combinations thereof plugged into them, which ran multiple battery chargers/lights/electric hand tools fed from cumulatively hundreds of feet of cord. And with proper attention paid to the installation and the conditions of the cords or tools connected to them, there are surprisingly few problems with spurious tripping of the protective devices. Nearly every time it happens it's either related to a damaged cord that when removed solves the problem, or some more complicated situation involving something like a large motor that can often be resolved by re-arranging a few things.

Ah. On building sites, a site transformer is often used, stepping the 240 V down to 110V with a centre tapped PEN. This means an operator is only exposed to the potential between one tap and the PEN, i.e. 55V.

My mate’s oven was recently nuisance tripping the RCD. As it cooled, a relay would click and sometimes, not always, the RCD would go. I measured the peak fault current using an adaptor to allow a current clamp around the live and neutral combined. It never went above 30 mA, more like 9 or 10. I guess here, the whole house Earth leakage (an old supply, with one RCD protecting everything - crap) was topping up the oven’s contribution. Either that or the old RCD’s threshold had drifted. Anyway, the oven was brand new so it went back under warranty and there have been no problems since.

If your Earth loop impedance is low enough, you may have some circuits not under RCD protection - this is common for ovens. Otherwise, it’s RCDs everywhere here.