Quick GFI Lesson Required

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  #1  
Old 07-02-07, 11:59 AM
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Quick GFI Lesson Required

I need to understand how a GFI works when there is no Nuetral coming from spa to GFI/Shutoff. I have a Cutler-Hammer 50A GFI/Cutoff and it gives explicit instructions that 3-wire spa connections (240v 2 hot/1 GND) obviously will not make use of the Nuetral lug on the GFI. The pigtail is connected to the neutral bus as well the neutral from the service panel also connects to this bus. The breaker neutral lug is left open. Explain how the GFI breaker detects faults in this case.
 
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  #2  
Old 07-02-07, 12:04 PM
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The current on one hot leg is compared to the current on the other hot leg. A difference in current (above the threshold) is an imbalance and trips the GFCI.
 
  #3  
Old 07-02-07, 01:02 PM
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The general form for a 2-pole GFCI is this:

N = A - B
Current in the neutral should equal current in hot leg A minus current in hot leg B when no fault is present.

In your case, current in neutral (N) always equals zero, but the relationship still applies.

N = 0

0 = A - B

0 (+B) = A - B (+B)

B = A

So it simplifies to current in hot leg A must equal current in hot leg B when no fault is present. If A is not equal to B when N is zero, then a fault must have occurred and the breaker trips.
 
  #4  
Old 07-02-07, 02:20 PM
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Very interesting. Good to know. I would have guessed N=A+B. 2 hots feeding the load with the single nuetral as the return. Which brings up an interesting thought...

Example 1 for simplicity: 2 hots & neutral from service panel (for a 220v feed). "-->" indicates example of current flow. So your analysis would yeild A= N+B

so is the current flow still....(size of arrow indicates relative current magnitude)
Hot 1: current flow A ---->
Hot 2: current flow B -->
Neut: current flow N <--
(seems B and N of opposite polarity would yeild zero)

Or is it...
Hot 1: current flow A ---->
Hot 2: current flow B <--
Neut: current flow N <--

or... since the current is actually a 60Hz cycle we're talking about absolute values for comparison sake resulting in (ignore arrows for direction)...

Hot 1: current flow A ----
Hot 2: current flow B --
Neut: current flow N --
But you would think the flow from A and B would be the same thus my thinking of N=A+B.
 
  #5  
Old 07-02-07, 03:11 PM
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When considering alternating current, there is no direction of current flow. More specifically, the current reverses direction 120 times a second. To add to the complication, the reversals of one hot are opposite the reversals of the other hot.

If you really want to understand this, you need to study alternating current. Make sure you are looking at sine waves when you do.

Is this just a theoretical academic exercise, or do you have a real problem to solve?
 
  #6  
Old 07-02-07, 03:11 PM
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You do not understand how 240 volt power works in North America (European power is different).

For a straight 240 volt load, the current on the A leg equals the current on the B leg. The current for the A leg returns on the B leg, and the current for the B leg returns on the A leg. In this case there is no excess current on either leg, so there is no current on the neutral.

For a combination 240/120 volt load Any excess current on either leg cannot return on the opposite leg because it's current is less. This excess current returns on the neutral.
 
  #7  
Old 07-02-07, 03:13 PM
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you are all correct

"I would have guessed N=A+B."
Actually, you are correct. Neutral current IS the sum of both legs. In fact, that's what happens when a multiwire circuit is miswired (both legs are the same phase).
But, realize in using legs that are 180 degrees out of phase, that when you do the A plus B addition, the B is negative phase (exactly), so the magnitude addition turns into a subtraction.
Vector math is usually needed when you are dealing with time variant waveforms. Its only possible here since we are dealing with a very simple example of Alternating Current. The difficulty here is that you can't tell if the writer of these equations is working with only the magnitude of the current or also working with phase angle.
 
  #8  
Old 07-02-07, 03:19 PM
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Thanks All. I have a BSEE (Electronics) but that was 20 years and have since gone into management...although still inquistive having never really had a conversation on the below topics.

I do not have a problem as a result of the below questions, and therefore will no longer reply and keep everyone guessing if there is an unqualified gremlin playing with the service panel. This started as a basica question which now my curiosity on learning will be redirected to books as such.

However the last 2 replies, do make perfect sense.

Thanks for your help. Vince
 
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