Sub panel bonding question

[arlen4720]

It won't. The GFCI trips when the current through the neutral isn't exactly the same as the current through the hot wire. Regardless of where the neutral is bonded to the ground, as long as it is upstream of the GFCI device all of the neutral current goes through the neutral on the GFCI. If the neutral touches ground downstream of the GFCI it provides a parallel path for some of the neutral current and will trip the GFCI immediately.

You are right.
It wasn't till the 2003 NEC ( maybe 2000) where it was required to run 4 wires to a separate building. For many years separate buildings had their neutrals bonded to the grounding electrode and the building's branch circuit equipment grounds. So, there was always current flowing through the ground rods and earth back to the premise service point.
Not a desirable situation, but certainly a common one.

 

[buickanddeere]

I thought one of the problems with bonding the neutral and the ground at the sub panel, was that current can flow on both the neutral and ground and breakers may not trip at the sub panel. Is this correct?

Although neutral current does flow back to the pole transformer, the transformer neutral goes to ground via ground rods at the pole or back to the sub station transformer via the ground grid. Am I off base on this statement?

Pretty much have it straight but the breaker will trip.

 

[MNBobcat]

It won't. The GFCI trips when the current through the neutral isn't exactly the same as the current through the hot wire. Regardless of where the neutral is bonded to the ground, as long as it is upstream of the GFCI device all of the neutral current goes through the neutral on the GFCI. If the neutral touches ground downstream of the GFCI it provides a parallel path for some of the neutral current and will trip the GFCI immediately.

Well said. Makes sense to me.

 

[buickanddeere]

Ground Fault Breaker senses neutral and line current on it's output terminals. As long as the current is within 5 milli amps, no trip. Having the neutral and ground messed up supplying the pony panel isn't going to trip the GFI.

 

[hr3]

You are right.
It wasn't till the 2003 NEC ( maybe 2000) where it was required to run 4 wires to a separate building. For many years separate buildings had their neutrals bonded to the grounding electrode and the building's branch circuit equipment grounds. So, there was always current flowing through the ground rods and earth back to the premise service point.
Not a desirable situation, but certainly a common one.

If current follows the past of least resistance it will follow the neutral back to the panel if there is a fault,, I see what your saying but the neutral would have to be degraded to a point of failure before the ground rod will start seeing flow.. and this is why the forth wire came to be..

 

[arlen4720]

If current follows the past of least resistance it will follow the neutral back to the panel if there is a fault,, I see what your saying but the neutral would have to be degraded to a point of failure before the ground rod will start seeing flow.. and this is why the forth wire came to be..

The neutral will see the majority of the current, but the earth is still a parallel path. The earth path will be flowing current in accordance with ohms law, and the better ground you have, the more current will be there. A good ground system may be on the order of 3-5 ohms. A mediocre ground may be 25 ohms. Both cases will flow noticeable current.