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Where's the main disconnect? Are those two breakers in the photo fed by a tap or by a branch circuit coming from the breaker panel situated elsewhere?

The neutral can be bonded to earth anywhere between main disconnect and breaker panel. Grounding conductor starts at that bond and runs downstream from there.

Marc

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What's the safety problem in this case?

If it was done "correctly" at the time, there should be a grounding electrode at the post and another at the garage. If the neutrals and grounds are tied together at the garage (as they should have been according to that era's requirements), then you'll probably get current travelling through the earth between the garage GES and the post's GES. In some cases people or animals might actually become part of that path. That's one of the reasons why the rule changed.

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What's the safety problem in this case?

If it was done "correctly" at the time, there should be a grounding electrode at the post and another at the garage. If the neutrals and grounds are tied together at the garage (as they should have been according to that era's requirements), then you'll probably get current travelling through the earth between the garage GES and the post's GES. In some cases people or animals might actually become part of that path. That's one of the reasons why the rule changed.

Going off course here a little but the impression I get is that the rule was changed in the 2002 NEC (250.32) because incidental parallel conductive paths between the two buildings (steel water pipe, steel conduit, etc) would end up carrying neutral currents if the neutral were bonded to the grounding system in both buildings. It's the same problem that happens when an EGC is bonded to the neutral in two locations in a house: neutral currents will end up flowing thru the much smaller EGC and overheat it.

Electricity normally flows in the Earth for a variety of reasons. It's why some expansive industrial sites have multiple ground rods inserted at points along a long circular path on the property and are joined together along with all metallic structures and electrical systems onsite. It equalizes the local electric potentials of the Earth.

JMHO.

Marc

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They separate at the garage. So, cant clear a ground fault that way?

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Right. At least I think so. 250.32 B (exception 3) says that installation shouldn't have any GFCI devices in the garage.

Marc

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What's the safety problem in this case?

If it was done "correctly" at the time, there should be a grounding electrode at the post and another at the garage. If the neutrals and grounds are tied together at the garage (as they should have been according to that era's requirements), then you'll probably get current travelling through the earth between the garage GES and the post's GES. In some cases people or animals might actually become part of that path. That's one of the reasons why the rule changed.

Going off course here a little but the impression I get is that the rule was changed in the 2002 NEC (250.32) because incidental parallel conductive paths between the two buildings (steel water pipe, steel conduit, etc) would end up carrying neutral currents if the neutral were bonded to the grounding system in both buildings. It's the same problem that happens when an EGC is bonded to the neutral in two locations in a house: neutral currents will end up flowing thru the much smaller EGC and overheat it.

Actually, the whole parallel-conductive-path-between-buildings concept was introduced in the 1999 edition. It was abandoned in 2008. The issue isn't so much one of overheating as having current in unexpected places.

Electricity normally flows in the Earth for a variety of reasons. It's why some expansive industrial sites have multiple ground rods inserted at points along a long circular path on the property and are joined together along with all metallic structures and electrical systems onsite. It equalizes the local electric potentials of the Earth.

JMHO.

Marc

True. But they install those systems because electricity flowing through the earth is not a desirable thing.

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They separate at the garage. So, cant clear a ground fault that way?

Click to Enlarge
tn_201721814933_4.jpg

65.22 KB

Right. At least I think so. 250.32 B (exception 3) says that installation shouldn't have any GFCI devices in the garage.

Marc

Actually, that's talking about ground fault protection of equipment (GFPE), which is almost certainly not going to be present in this garage. It's fine to have GFCIs in there.

Ground faults aren't going to be cleared because there's no low impedance path back to the neutral.

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