Appliance plug connected wrongly: live neutral earth

[shirozack]

Homework Statement

A metal casing appliance's plug is created wrongly. the earth wire is connected together with the live wire terminal. when the plug is connected to the socket, what will happen to the appliance?

Relevant Equations

na

there are 4 options.

a) the fuse blows
b) the appliance becomes live
c) the appliance catches fire
d) the appliance doesn't work.

the answer given is b) , because the earth is connected with the live. but i would like to know why issn't option d) correct instead?

since the live and earth wire are connected to the same terminal, will current even flow? does current just flow from the live and exit through the earth? meaning the appliance doesn't even work?

 

[haruspex]

Homework Statement: A metal casing appliance's plug is created wrongly. the earth wire is connected together with the live wire terminal. when the plug is connected to the socket, what will happen to the appliance?
Relevant Equations: na

there are 4 options.

a) the fuse blows
b) the appliance becomes live
c) the appliance catches fire
d) the appliance doesn't work.

the answer given is b) , because the earth is connected with the live. but i would like to know why issn't option d) correct instead?

since the live and earth wire are connected to the same terminal, will current even flow? does current just flow from the live and exit through the earth? meaning the appliance doesn't even work?

Have you quoted the question verbatim? (Is it a translation?)
Can it be read as "the earth wire of the appliance (i.e. the wire connected to the casing) is connected to the live terminal of the plug (as well as the live wire of the appliance) instead of being connected to the earth terminal of the plug"?

 

[shirozack]

it doesn't say. it just talks about the error in the plug. so the rest of the connections in the appliance is assumed normal i suppose, i.e the one end of the earth wire connects to the casing, the other end connects wrongly in the plug, earth to the live wire in the plug.

 

[haruspex]

it doesn't say. it just talks about the error in the plug. so the rest of the connections in the appliance is assumed normal i suppose, i.e the one end of the earth wire connects to the casing, the other end connects wrongly in the plug, earth to the live wire in the plug.

Ok, so why would that mean the appliance does not work?

 

[berkeman]

Homework Statement: A metal casing appliance's plug is created wrongly. the earth wire is connected together with the live wire terminal. when the plug is connected to the socket, what will happen to the appliance?

Is this a 2-prong (polarized) or 3-prong plug? Is there a picture or diagram that goes with the problem? What country is the textbook written for (AC Mains standards vary with country).

 

[votingmachine]

There needs to be a wiring diagram for me. My first reading was that the hot AND the ground for a 3-prong plug are TOGETHER connected to the same terminal. That's a short that blows the fuse.

If it is a 2-prong plug, then it is the same as if the plug was simply lugged in backwards (old plugs were the same size prongs).

If it is a 3-prong plug, and the ground is connected to the hot terminal, it matters where the hot is connected. Without that in the appliance circuit ... nothing will work.

I bet it is an old electric code question, or a question for another country. And basically the same question of a 2-prong without prong size differences. You can plug it in either orientation and complete a working circuit.

The use of the "fuse" might be a sign. Wouldn't a modern test question use "circuit breaker"?

 

[haruspex]

My first reading was that the hot AND the ground for a 3-prong plug are TOGETHER connected to the same terminal. That's a short that blows the fuse.

No, it is the earth of the appliance (which connects to the casing) that is connected to the live pin of the plug (as well as the correct wire of the motor being connected there). That does not complete a circuit so long as no other conductor contacts the casing.

 

[votingmachine]

No, it is the earth of the appliance (which connects to the casing) that is connected to the live pin of the plug (as well as the correct wire of the motor being connected there). That does not complete a circuit so long as no other conductor contacts the casing.

But the earth of the appliance connected to live pin of the plug is a miss-wiring of the appliance, not the plug.

I see the circuit you are making, and why it would not trip the breaker. But the miss-wiring is specified as in the plug/cord:

"A metal casing appliance's plug is created wrongly"

The plug connects the ground to the hot terminal. The words "connected together with" has to be read carefully. I think it indicates the ground wire is connected to the appliance live terminal, by itself, not along with the hot wire (which would trip the breaker / blow the fuse).

I have an old house and the Breaker box has a common bus for ground and neutral. (The house is properly grounded with copper rods into the earth.) That made me wonder if the question is based on that old electric code wiring, where the neutral and ground are the same?

An old metal cased toaster would still heat and toast with the 2-prong-plug in either orientation. Appliances with 2-prong plugs are rarer, but still around.

I'm inclined to wonder (like Berkeman above) whether the question is for the US. Or if it is from a really old text.

 

[haruspex]

A metal casing appliance's plug is created wrongly

yes, the plug, not the socket.

 

[berkeman]

I think we will need to wait until the OP returns to clarify the situation. (that will probably happen overnight tonight from a US perspective)

 

[shirozack]

thanks for the discussion, i figured out why the answer is B already.

because the live and neutral are both connected to the live terminal in the plug, when the current comes in from the live, it will split into the earth and the live. so the appliance will still work as normal as the current from the live will go into the live and then into the neutral to complete the circuit.

however, since the earth is also wrongly connected to the live, the current will also go into the earth. but since the earth wire's other end is connected to the metal casing, this casing will become 'live' and once a user touches it, he will form a complete loop to the ground, essentially acting like an earth wire. so the current from the live will go into the faulty connected earth wire to the casing to the human to the ground.

 

[Tom.G]

so the current from the live will go into the faulty connected earth wire to the casing to the human to the ground.

Only if the human is in contact with a ground, as in standing outdoors barefoot in a mud puddle. Of course the same result occurs if while holding the appliance the human grabs a water faucet, or anything else that is grounded (like a gas or electric kitchen stove)!

Cheers,
Tom

 

[votingmachine]

I'm puzzled by that statement the current "will split into the earth and the live"

Current will divide based on the resistance across the appliance and the resistance to the ground (which is zero). That seems like the appliance gets no current.

(My being puzzled is not the same as there being a problem in the explanation).

 

[haruspex]

when the current comes in from the live, it will split into the earth and the live.

As @votingmachine notes, that is wrong.
What "comes in" from the plug, initially, is voltage. What currents result depends on resistance of the circuits. The wrong wiring means that both the casing and the wire to the motor get mains voltage. When the appliance is switched on, that voltage has a circuit through to the neutral side of the plug and the appliance operates. When an earthed person touches the casing the voltage on that has a circuit through the person and ground and back to the power station.