Ground fault protection

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mathan1987

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Hi,

How does the ground fault protection work during the Ground fault? Is it to protect the equipment or personnel? How does the ground fault is cleared by the Circuit breakers normally?
 
mathan1987 said:
Hi,

How does the ground fault protection work during the Ground fault? Is it to protect the equipment or personnel? How does the ground fault is cleared by the Circuit breakers normally?
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GFI is for equipment. 20 and 30 mA are common enough smaller units. After that, whatever you, as an EE, set the parameters at.

GFCI is for personnel. 5 milliamperes
 
ptonsparky said:
GFI is for equipment. 20 and 30 mA are common enough smaller units. After that, whatever you, as an EE, set the parameters at.

GFCI is for personnel. 5 milliamperes
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Just a small quibble...I think he term now used for GF above the GFCI threshold is GFPE - ground fault protection equipment. As I recall UL made this official.
 
GFPE is required for 277 v services, feeders and branch cuts 1000 amps and above. A 277 volt ground fault won’t burn clear, 120 volt does not have enough energy to maintain an arc. There is a section in the NEC on GFPE, I have never installed GFPE so it’s not my area of knowledge.
GFPE is also used for heat tracing at a 30 ma trip
GFCI requirements are in 210.8
Also look at definitions for each in Art 200
 
tom baker said:
GFPE is required for 277 v services, feeders and branch cuts 1000 amps and above. A 277 volt ground fault won’t burn clear, 120 volt does not have enough energy to maintain an arc. There is a section in the NEC on GFPE, I have never installed GFPE so it’s not my area of knowledge.
GFPE is also used for heat tracing at a 30 ma trip
GFCI requirements are in 210.8
Also look at definitions for each in Art 200
Click to expand...
Yeah i am talking about the ground fault protection of the breakers above 1000A
 
As originally designed, circuit breakers were intended to trip on the excess current caused by a fault. By good bonding a ground fault was supposed to be a short circuit causing enough current flow to trip breakers 'instantaneously'.

It was learned that an arcing ground fault has enough impedance to prevent rapid over current tripping of large services.

The ground fault detection of a large service uses similar physics to a GFCI, using some form of residual or ground bond transformer to detect current flowing on unintentional paths. But the threshold current is quite different, possibly in the 100s of amps.

The purpose of this ground fault detection is to trip the main in the event of a ground fault that might not trip the main on overcurrent. Ideally this ground fault threshold is high enough that branch circuit breakers trip on overcurrent, rather than taking out the entire service when a branch circuit faults to ground.

Jon
 
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