Hi-pot test of a circuit with a diode

[bramdam]

We manufacture crank case heaters and they are designed with an 800V 1A STD Recovery diode. We do not Hi-pot the heater with the diode in it we only test to make sure we have an ohm reading in one direction and an OL in the other.

However, our customer installs our heater into a system and Hi-pot the entire system. They do a 1 second test at 1.8 kV and then a ramped test maxing out at 3.25kV for one minute. I claim they are damaging if not destroying the diode by doing that test with it in the circuit which is leading to hi-amp readings at their final QC station if not total failures.

Not being an expert, I am just looking for some verification that their test could be or is damaging the diode thus causing failures. Now how and if I can design around their test is another issue.

Thanks

 

[berkeman]

We manufacture crank case heaters and they are designed with an 800V 1A STD Recovery diode. We do not Hi-pot the heater with the diode in it we only test to make sure we have an ohm reading in one direction and an OL in the other.

However, our customer installs our heater into a system and Hi-pot the entire system. They do a 1 second test at 1.8 kV and then a ramped test maxing out at 3.25kV for one minute. I claim they are damaging if not destroying the diode by doing that test with it in the circuit which is leading to hi-amp readings at their final QC station if not total failures.

Not being an expert, I am just looking for some verification that their test could be or is damaging the diode thus causing failures. Now how and if I can design around their test is another issue.

Thanks

Can you post a schematic of the system and how your heater is hooked up? Generally Hi-Pot testing is done with protection components removed from the circuit...

 

[bramdam]

Schematic for heater diode installation

The schematic is rough but the circuit is simple. We integrate the diode into one of the leads of the heater after we test it and before we ship it. I think I attached the schematic.

Thanks

 

[berkeman]

The schematic is rough but the circuit is simple. We integrate the diode into one of the leads of the heater after we test it and before we ship it. I think I attached the schematic.

Thanks

Is the Hi-Pot voltage applied differentially across the heater wires (unlikely), or in common-mode with both wires with respect to the overall ground of the system (case of the heater?)?

 

[bramdam]

It is the common-mode.

 

[berkeman]

It is the common-mode.

Then the diode should not be affected.

 

[bramdam]

Mr. Berkeman,
I am still not convinced that the hi-pot test does nothing to the diode. When we receive rejects back, the diodes are bad. The heaters make it past the hi-pot test but are rejected at final QC for pulling high amps. They pull the high amps because the diode no longer is functioning as it should. Here is their test procedure, do you see any potential here? I am still not comfortable with an 800V diode being tested at such high voltages. Thanks for your help.

Typically a 1 second test is performed.
Eg 460 volt unit
Test would be performed on main input line of unit L1,L2,L3 to Frame Ground. It would pass through a set of fuses and a side switch on the contacor
Test would start and voltage would ramp up to 3.25KV in 5 seconds then hold that value for 1 second.
There is also an Arc Sense feature that is turned on. It is not a requirement but used to make sure that the operator does not move the test wand for the 6 seconds while the test is performed
If the test indicates a failure the test would be performed after the heater fuses ( load Side) at 2.71 KV where the ramp up time is still 5 seconds however it would have to hold that voltage for 60 seconds.
If there is a failure here the heater would be disconnected from all circuits and the minute test would be performed directly to the component wiring.

 

[berkeman]

Mr. Berkeman,
I am still not convinced that the hi-pot test does nothing to the diode. When we receive rejects back, the diodes are bad. The heaters make it past the hi-pot test but are rejected at final QC for pulling high amps. They pull the high amps because the diode no longer is functioning as it should. Here is their test procedure, do you see any potential here? I am still not comfortable with an 800V diode being tested at such high voltages. Thanks for your help.

Typically a 1 second test is performed.
Eg 460 volt unit
Test would be performed on main input line of unit L1,L2,L3 to Frame Ground. It would pass through a set of fuses and a side switch on the contacor
Test would start and voltage would ramp up to 3.25KV in 5 seconds then hold that value for 1 second.
There is also an Arc Sense feature that is turned on. It is not a requirement but used to make sure that the operator does not move the test wand for the 6 seconds while the test is performed
If the test indicates a failure the test would be performed after the heater fuses ( load Side) at 2.71 KV where the ramp up time is still 5 seconds however it would have to hold that voltage for 60 seconds.
If there is a failure here the heater would be disconnected from all circuits and the minute test would be performed directly to the component wiring.

If the Hi-Pot voltage is applied to both wires, then there is no voltage across the diode. It seems unlikely that would be able to do any damage to the diode.

How often do you get failures? What percentage of the devices fail final QC for bad diodes? If it's a high percentage, then I'd recommend testing the diode by hand before and after the Hi-Pot test, to see if it really is failing there. What other tests are run on the assembly?

 

[bramdam]

We get enough failures to be on the radar of our customer. And it is always for high amp draw with the occasional hi-pot. Less than 5% but as of recently there were 3 failures in a short time frame. The diode is enclosed in SS armor and sealed so it is difficult to test on the final product without destroying the heater. We hi-pot instantly when we get to the 3.25 kV at the plant so I have no idea how any of them pass.

The heater assembly is ohms tested and hi-potted at 1.8 kV. Then the diode is installed and final ohms to verify diode installation is taken. The heater is installed into the system at the customer locale and hi-potted in accordance to the previous standard. Assuming it passes, it is then tested for amp draw in final QC which is where the majority of rejects occur.

I appreciate your help, it is hard to find anyone who knows about diodes let alone hi-pot testing.

 

[berkeman]

We get enough failures to be on the radar of our customer. And it is always for high amp draw with the occasional hi-pot. Less than 5% but as of recently there were 3 failures in a short time frame. The diode is enclosed in SS armor and sealed so it is difficult to test on the final product without destroying the heater. We hi-pot instantly when we get to the 3.25 kV at the plant so I have no idea how any of them pass.

The heater assembly is ohms tested and hi-potted at 1.8 kV. Then the diode is installed and final ohms to verify diode installation is taken. The heater is installed into the system at the customer locale and hi-potted in accordance to the previous standard. Assuming it passes, it is then tested for amp draw in final QC which is where the majority of rejects occur.

I appreciate your help, it is hard to find anyone who knows about diodes let alone hi-pot testing.

Are you sure the Hi-Pot with the diode is done by shorting the two wires together and applying a high voltage to the two wires together with respect to the grounded metal of the enclosure? Do you have a picture of this test?

 

[bramdam]

Schematics

This is what they gave me.

 

[berkeman]

This is what they gave me.

Where does the diode go? How is the Hi-Pot test performed (L1+L2+L3 to Earth Ground?)

 

[bramdam]

The heater is wired to L1 and L2 after some fuses. The diode is either in L1 or L2 since there is only one in the heater and I am sure they do not know which lead contains the diode for it is encased. The only thing they explained was that the hi-pot test is hooked into L1, L2, and L3. They do not specify if they tie those together. But they are hooked to frame ground on the other side of the test.

 

[bramdam]

Sorry, print says it is hooked up to L2 and L3. But only two legs.

 

[berkeman]

The heater is wired to L1 and L2 after some fuses. The diode is either in L1 or L2 since there is only one in the heater and I am sure they do not know which lead contains the diode for it is encased. The only thing they explained was that the hi-pot test is hooked into L1, L2, and L3. They do not specify if they tie those together. But they are hooked to frame ground on the other side of the test.

Sorry, print says it is hooked up to L2 and L3. But only two legs.

Is the unit powered by L2 and L3 during the test? Or is the connection to AC Mains open circuited during the test? Usually it is open circuited, but I'm suspicious that the test is putting a differential voltage across the diode, which could certainly damage it...

 

[bramdam]

The unit is a 480V unit and our heater is either a 240 or a 120 depending on the style called for. Are you asking if the heater is powered up? They state that there is an auxiliary contact that needs to be closed for the heater to turn on. What if it is not closed during the hi-pot test? Could that create a potential difference? I guess what question should I ask the plant in terms of their test so I can bring value added info back to this chat?

 

[berkeman]

The unit is a 480V unit and our heater is either a 240 or a 120 depending on the style called for. Are you asking if the heater is powered up? They state that there is an auxiliary contact that needs to be closed for the heater to turn on. What if it is not closed during the hi-pot test? Could that create a potential difference? I guess what question should I ask the plant in terms of their test so I can bring value added info back to this chat?

So your heater is powered by the differential AC Mains voltage difference between L2 and L3, when a contactor is closed, right? The question to ask is how the Hi-Pot voltage is applied to L2 and L3 during the test, and whether the contactor is closed during the test.

In my experience with Hi-Pot testing, both of the input wires would be tied together (in this case L2 and L3, so there could not be any AC Mains voltage connection for L2 and L3 during the test), and that pair of wires has the Hi-Pot voltage impressed on it with respect to the chassis/Earth ground of the device. You could ask whether this is how they are doing the Hi-Pot test of your unit.

If on the other hand, they are keeping the AC Mains connection to L1, L2, L3 during the test, and only applying the Hi-Pot voltage to one line at a time with respect to chassis/Earth ground, that could potentially cause some issues, I would think.

 

[bramdam]

I have shot them some questions and will keep you posted. Thanks again for the help.

 

[berkeman]

Sounds good. BTW, what is the reason for the diode? Are you just wanting half of the heating effect from the AC heating element?

 

[bramdam]

To the best of my knowledge that is the reason. This is more of a warming element than heating.

 

[jim hardy]

Golly one hates to think about changing a product
but one of these in parallel will protect your diode... small enough to fit ?

http://www.littelfuse.com/~/media/e...tors/littelfuse_varistor_ma_datasheet.pdf.pdf

I take it this is the heater in bottom of the compressor to keep refrigerant from condensing there?

old jim

 

[bramdam]

Thanks Jim. This is UL product and changing anything on it would come with agony. However, it is not impossible. If we were to recommend or incorporate a surge protector how would we hook it up. The diode is integrated in series into a lead and inaccessible. Putting this surge device in parallel with the diode would not be possible given the design.

You are correct on the purpose of this heater, very low wattage.

 

[jim hardy]

I appreciate the pain of "requalifying", been there.
I thought perhaps the varistor could be placed inside the same heatshrinked segment of wire as the diode and soldered right to it(if indeed that's how you mount it) .
Your best bet would be to send one of your technicians to witness how your customer tests the product.
Your working guy and their working guy are closest to the problem. Might be as simple as making him aware he needs to jumper both ends of the heater together when hi-potting.
And sending a rep might salve the wounds from failing parts. I've been on that end, too.

old jim

 

[bramdam]

Testing Procedure

I think we are onto something. Below is the testing procedures and I see the very likelihood that the diode is experiencing the differential not once, but multiple times during this testing. Looking forward to your feedback.

Testing Procedure: Attached is the schematic for the system.

Separate tests are performed for each lead.
So in the example below each red dot represents an individual test
15 tests will be performed for this product
3 on the main Voltage line L1,L2,L3
And individual tests on each contactor on the load side total 12
All contactors are NOT energized while performing the tests

 

[jim hardy]

I see it applied to the motor windings, but it is not clear to me where the heaters are on that schematic.

Is this like my home unit, heater energized when motor isn't ?
Then, during that test with the contactors open the L1-L2-L3 are not connected by the motor winding.
That means you may well have the highpot DC (~2500 Volts?) pushing current from one L into another L through your diode . There should be only enough current to charge the distributed capacitance and whatever leakage is present. I'd guess that is enough to hurt an 800 volt diode.

At this point that's just speculation but not wild speculation.

Somebody who's real practical-minded needs to see it done.
Do you have a schematic of their unit that shows your heater ? Or am i just too dumb to find it on what you've already posted?
I take directions well, so don't be bashful about pointing out the obvious for i need and appreciate it.

old jim

 

[jim hardy]

Aha ! Maybe that's them in attachment to post [STRIKE]24[/STRIKE] post # 11, (doggone aspergers ! ) ...
below the text block, CCH1 and CCH2 through B contacts C1AUX and C2AUX ?

If so you've probably found the trouble, and without my bungling "help" ..

old jim

 

[berkeman]

Aha ! Maybe that's them in attachment to post [STRIKE]24[/STRIKE] post # 11, (doggone aspergers ! ) ...
below the text block, CCH1 and CCH2 through B contacts C1AUX and C2AUX ?

If so you've probably found the trouble, and without my bungling "help" ..

old jim

Good eyes! I now wonder even more why the diode is in the circuit. If there are capacitors in series with the resistive heating elements, why not just decrease the series capacitors instead of going to a half-wave rectifying diode?

Of course, fixing the Hi-Pot test program is probably a simpler option...

 

[jim hardy]

Capacitors ? Or normally closed (form B) relay or switch contacts (and fuses) ? Awkward naming convention makes them look like caps, sure enough.

 

[berkeman]

So C1AUX and C2AUX aren't capacitors? This kind of diagram is definitely not the kind of circuit diagram I'm used to looking at, so I could be misinterpreting it...

 

[dlgoff]

So C1AUX and C2AUX aren't capacitors? This kind of diagram is definitely not the kind of circuit diagram I'm used to looking at, so I could be misinterpreting it...

Nope. Contacts. Usually the coils are indicated by circles.

This type of drawing makes it easier to trouble shoot IMO.

 

[jim hardy]

Industrial electrical symbols are a little different from pure electronics or telephone industry.

Working in the power plant where the equipment is so diverse, one runs across a lot of different drawing 'dialects'. In the sixties and seventies it seemed every manufacturer had his own drafting standards . One quickly became "multilingual".
Some instruction manuals were almost works of art. Now that it's mostly CAD there's a lot less individuality. I never did adjust to the European drawings, though.

Letters A and B referring to a contact mean Normally Open and Normally Closed, respectively. Mnemonic is "A = Alike" for NO, B = Backward" for NC. Letter C means a SPDT contact, one each NO and NC with common flapper.
So when i said "B contacts" earlier i was referring to the diagonal slash, as explained in Don's sketch above.
The contact's name should include the same identifier as the contactor to which it is mechanically slaved, but those are hidden by the note on OP's drawing . Probably they're C1 and C2.

There are standard device numbers too - http://en.wikipedia.org/wiki/ANSI_device_numbersold jim

 

[bramdam]

Sorry I am late to this. You have found the heater and it is after the auxiliary contactor, either C1AUX or C2AUX, not sure which one is our heater. My guess is the heater is after the NO contact for what I have been told there is a selector switch that allows the heater to be turned on or off.

It sounds like the way they test could definitely be the source of the sporadic failures of the diode. Those diodes are definitely not the most robust elements and when we buy them in the tens of thousands some are bad coming out of the box.

At this time, I see a couple of different options:
1) Try to get them to change their test methods by either removing our heater from this hi-pot test (since we hi-pot the heater prior to shipping and before the diode is put in, or by tying the two leads of the heater together prior to shocking their system.
2) Re-design the heater by either including some type of surge protector in parallel with the diode or removing the diode completely. Both will be hard. The UL is one thing but to get the heater to mimic the current limitation caused by the diode may not be possible.

Thanks for taking this issue on gentlemen.

 

[jim hardy]

This 1kv zener diode would survive the hi-pot, but the leakage it allows might cause the unit to fail its insulation test.

http://www.vishay.com/docs/88554/byv26dgp.pdf

12 cents at Digikey if you want t try a couple.

 

[bramdam]

Jim,
I laid out four options to resolve this issue and they chose to re-evaluate their hi-pot testing procedure. In the meantime they will tie the leads of our heater together, after the contactor, to avoid presenting a voltage differential across it. I could not get one to fail testing it this way at our shop. They too are weary of changing the part at this time so did not bite on putting a surge device in parallel.

 

[berkeman]

In the meantime they will tie the leads of our heater together, after the contactor, to avoid presenting a voltage differential across it. I could not get one to fail testing it this way at our shop.

Hey, that's great news!