I know how a single pole gfi breaker works. By measuring the difference between the current going out on the hot and coming back on the nuetral. But how does a 2 pole gfi breaker work when you hook up a hot tub and you do not use the nuetral? Is it measuring the imbalance between the two hot legs? Say if one leg was to go to ground and suddenly draw an excessive amount of current? Thank you.:p

Let me ask you how you think a 2 pole breaker works when you use the neutral. That's the more complicated case.

If one leg went to ground and drew an excessive amount of current a normal breaker would trip from overcurrent. But a GFCI breaker is not a normal breaker. Name two things a GFCI breaker does. Name something it doesn't do. And we'll go from there.

For some reason I didn't think you would bite.
GFCI's use transformers to add or subtract currents. When there is a difference of about 16 mA the GFCI opens the circuit. Thus a resistor smaller than R=120/16mA to ground on either leg will trip the GFCI. The GFCI also detects a grounded neutral using a different mechanism.

The 2 pole case:
We know that I(L1)+i(L2) = I(N); neutral carries difference of L1 and L2
We also know that L1 is 180 deg out of phase from L2.
So to get the difference of I(L1) and I(L2) the primary would be wound in the same direction.

Let's rearrange the eqn to read I(l1)+i(L2)+i(N) = 0
With a GFCI we want I(L1)+i(L2)+i(N) ~> 16 mA to trip the breaker.

So, we can wire the transformer primary with the currents from L1, L2 and N and check the magnatude. Anything greater tha +-16 mA will trip the 2 pole GFCI. This is why the Neutral of the GFCI has to be attached to the load side. Adding or subtracting just depends on the direction of current flow.

The special case of no neutral to a hot tub for instance just forces I(N) = 0.

Does this make sense?

For some reason I didn't think you would bite.
GFCI's use transformers to add or subtract currents. When there is a difference of about 16 mA the GFCI opens the circuit. Thus a resistor smaller than R=120/16mA to ground on either leg will trip the GFCI. The GFCI also detects a grounded neutral using a different mechanism.

The 2 pole case:
We know that i(L1)+i(L2) = i(N); neutral carries difference of L1 and L2
We also know that L1 is 180 deg out of phase from L2.
So to get the difference of i(L1) and i(L2) the primary would be wound in the same direction.

Let's rearrange the eqn to read i(l1)+i(L2)+i(N) = 0
With a GFCI we want i(L1)+i(L2)+i(N) ~> 16 mA to trip the breaker.

So, we can wire the transformer primary with the currents from L1, L2 and N and check the magnatude. Anything greater tha +-16 mA will trip the 2 pole GFCI. This is why the Neutral of the GFCI has to be attached to the load side. Adding or subtracting just depends on the direction of current flow.

The special case of no neutral to a hot tub for instance just forces i(N) = 0.

Does this make sense?

Yes it makes sense. The reason I didn't BITE as you called it is I have been away from my computer for several days. I have been through the IBEW's apprenticship program and have been an electrician for almost 17 years. If I wanted to be TALKED DOWN TO I would go see the electrical engineers on site and have a conversation with them. They always know how to make it work on paper but us guys in the field are the one who have to make it functional in the field. You sound just like one of them. I was looking for an intelligent answer from someone who knows, not a talking to from some a-hole who has to act like he is god's gift to elecrticity. In the future if you would like to talk down to people, go do it somewhere else!!

Burrhead:

Sorry if I offended. I have been diagnosed with psycological issues which might cause these kind of problems. I'm even glad that you understood the explanation. Sometimes I have trouble there as well. Not having any idea of the background of a person makes it harder to come up with an explanation. Do you have any other questions?

KISS

Just to speak on Kiss's behalf, I am sure he sincerely did not mean any offense. I did understand where he was coming from with his technique, just a bit rough around the edges.

I am glad your in the trade thou, it helped you understand the answer.

If you were a layperson, your eyeballs would be spinning.

He is working on his answers to keep in step with his name.

I do need to ask where the 16 amps comes from. All GFI devices for personnel protection are set for 5 Milliamps difference.

Tk:
Yep, your right. Between 3 and 7 mA. I'll admit, I didn't look it up. I should have known better anyway because of the amount of current to stop the heart is around 10 mA. eLCOSH : Electrical Safety: Safety & Health for Electrical Trades (Student Manual) - Section 2 (http://www.cdc.gov/eLCOSH/docs/d0500/d000543/section2.html)

Thanks tk.