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- New Jersey
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- Journeyman Electrician
If the yellow line is the current flow then yes but it may only be for an instant. The OCPD on the secondary should open.
SSBJ clarification
- Thread starter Pinnie
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wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
The quote you were responding to was predicated on "a bond in more than one location". That means a bond in the secondary system in more than one location, not the fact that you have both an MBJ (for the primary system) and a SBJ (for the secondary system). The diagram has only one bond on the secondary system.
So you are correct that in your diagram the current would primarily only flow in the right half of the squiggly yellow line, to the SBJ at the transformer, to XO, to complete the circuit. Very little current would be expected in the left half of the squiggly yellow line. No current would flow past the ground bar in the service where the EGC connects to the GEC, so you should remove that little bit of yellow squiggle to the meter. But some current would flow to that EGC/GEC junction, along the GEC for the service to the GES, along the GEC for the transformer, and back to the transformer X0.
However, that path on the primary side EGC, the service GEC and the transformer GEC is in parallel with the fairly short path of the SBJ itself. So the current would divide in proportion to the resistances. If all those conductors are the same size, and the roundabout path is 50', while the SBJ is 6", you'd get 1/100 of the current taking the roundabout path.
Cheers, Wayne
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wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
To illustrate the parallel paths, the purple path below is the intended path for ground fault current at the panel on the right, and the orange loop is an alternate segment that is parallel with the SBJ, and so would carry a little current.
In practice, is the diagram accurate about the GEC going directly to XO, or could it be landed on the same bar as the SSBJ is connected to? If the latter, the orange path would only be in parallel with a very short piece of terminal bar, rather than the SBJ, and so the current on the orange path would be even smaller.
Cheers, Wayne
Gotcha I was drawing this and I got it right. It’s honestly not a bad thing that there’s a parallel path through the GEC in case someone undersizes the SBJ or a similar issue. Are GECs allowed to be a ground fault path or do we think that will change. Couldn’t that potentially energize gas pipes or create an arc somewhere in the building causing a fire? That’s probably why Mike says it should be a short of a wire as possible
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wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
This example illustrates that GECs can end up as part of an additional ground fault path for installations in compliance with the NEC. And 250.121(A) exception can allow the same conductor to act as both a GEC and an EGC, so certainly such a conductor can be part of the intentional ground fault path due to its role as EGC.
So I think the question you want to ask is can cutting a GEC that is not also acting as an EGC ever eliminate the sole ground fault path (in an otherwise compliant installation) and let a ground fault occur without tripping a breaker? And I think the answer is that only if the ground fault is to the GES (a grounding electrode or one of the bonding jumpers). And even then, in an example like in the diagram where there are multiple GECs for multiple systems, it would probably require cutting all of them.
Cheers, Wayne
I guess I try to separate GECs and EGC in my conceptualization because they serve two distinctly different functions while both being called “ground”. I never really thought about until now that GEC also can carry fault current even though that’s not their intended design per se.