GFI Mains

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wire monk

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Could you give me a little info on gfi main breakers.. What reason would a 277v ballard light trip the gri main breaker it is 600v, that had a bad ballast??
 
If the ballast had a ground fault that exceeded the setting of the GFI main, it would trip the main breaker.

Chris
 
wire monk said:
Could you give me a little info on gfi main breakers.. What reason would a 277v ballard light trip the gri main breaker it is 600v, that had a bad ballast??
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The reason is called "poor coordination". It is very rare that the minimum settings of a service entrance GFI will coordinate with typical lighting branch circuits.
The minimum current setting for most of these GFI's is 200A, the minimum time is either 0.0s or 0.1s. The typical setting for a 20A circuit breaker is +200A.

Do not simply adjust any protective device settings unless you are willing to accept the risk associated with changing the customer's level of protection during a fault condition. "Coordination by clicking" can lead to unexpected problems.
 
Putting in in simpler terms, whenever there is a fault, of any kind (ground fault, line-to-line, three-phase, etc.) there will be a large amount of current flowing through the fault point. That current comes mostly from the source (for the moment, let's ignore the additional contribution from any large motors that happened to be running at the time). The current has to flow through every breaker that is in line from the service point to the fault point. Every one of those breakers is going to be given the command to open. One will win the race. Why should you expect it not to be the main breaker?

It takes some clever design choices, including breaker selection and breaker trip settings, to ensure that the only breaker that can win the race is the one that is closest to the fault point. It is only in a very limited set of circumstances that the NEC requires us to include coordination as a design consideration. So in a high percentage of facilities, a fault in a light fixture may very well take out the main breaker, if the fault current is high enough.
 
Just a question how does or what are the calculations used to include motors into fault calculations?
 
mikeames said:
Just a question how does or what are the calculations used to include motors into fault calculations?
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This sounds like a thread hijack.

But, it is not uncommon to simply assume that a motor will contribute 6x its FLA as fault current. It does really depend on the motor, however you usually do not get the needed information unless the motor is very large or >600V.
 
jim dungar said:
This sounds like a thread hijack.

But, it is not uncommon to simply assume that a motor will contribute 6x its FLA as fault current. It does really depend on the motor, however you usually do not get the needed information unless the motor is very large or >600V.
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Ok sorry, forget I asked... Ill open a new thread if its important enough.
 
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