SSBJ fault return path

[lm227]

Looking at the diagram below, why is the SSBJ needed (where red X is shown) if the fault return path green arrows are shown through the neutral bus to X0?

 

[Sea Nile]

Good morning, looks like you drew the red x on a grounding conductor.

Are you asking why all metal enclosures are bonded together with grounding conductors? Or are you asking something different?

 

[Dsg319]

Looking at the diagram below, why is the SSBJ needed (where red X is shown) if the fault return path green arrows are shown through the neutral bus to X0?

Because the neutral conductor is not bonded to the transformer.So if you have a ground fault on the secondary side of the transformer it is connected to the neutral via the SSBJ and SBJ inside the disconnect.

 

[lm227]

Because the neutral conductor is not bonded to the transformer.So if you have a ground fault on the secondary side of the transformer it is connected to the neutral via the SSBJ and SBJ inside the disconnect.

In the diagram the N to G bond is in the secondary disconnect enclosure so a ground fault at the load for example would return to the secondary transformer windings following the green arrow pathway indicated. Wouldn’t this return path still be valid even if that green wire marked X were removed?

If I have a ground fault at the transformer enclosure, the transformer enclosure is bonded to the primary OCPD device upstream.

 

[lm227]

Good morning, looks like you drew the red x on a grounding conductor.

Are you asking why all metal enclosures are bonded together with grounding conductors? Or are you asking something different?

Per the NEC that green conductor is the SSBJ. My question is does it serve any purpose as a fault return path to since the first OCPD is downstream. Assume the transformer enclosure is already bonded on the primary side.

 

[wwhitney]

The green line with the red X is required to bond the transformer enclosure. It would be part of the fault path if, say, X1 faulted to the transformer case.

Cheers, Wayne

 

[LarryFine]

In the diagram the N to G bond is in the secondary disconnect enclosure so a ground fault at the load for example would return to the secondary transformer windings following the green arrow pathway indicated. Wouldn’t this return path still be valid even if that green wire marked X were removed?

If a secondary conductor contacted the enclosure, there would be a L-N-voltage difference between the transformer and the disconnect enclosures.

If I have a ground fault at the transformer enclosure, the transformer enclosure is bonded to the primary OCPD device upstream.

If it's a primary conductor fault, that will work. But, if it's a secondary conductor fault, only the electrode conductors will be a pathway to carry fault current.


Added: Think about why meter bases are bonded to the neutral.

 

[lm227]

The green line with the red X is required to bond the transformer enclosure. It would be part of the fault path if, say, X1 faulted to the transformer case.

Cheers, Wayne

If X1 faulted to the transformer case why would I want to send fault current to a downstream breaker? Opening the breaker at the secondary disconnect would not clear a ground fault at the transformer enclosure.

 

[retirede]

If X1 faulted to the transformer case why would I want to send fault current to a downstream breaker? Opening the breaker at the secondary disconnect would not clear a ground fault at the transformer enclosure.

Correct, but without the jumper, the transformer enclosure is at a elevated voltage relative to everything else that is bonded, creating a hazardous situation.

 

[Sea Nile]

If X1 faulted to the transformer case why would I want to send fault current to a downstream breaker? Opening the breaker at the secondary disconnect would not clear a ground fault at the transformer enclosure.

I'm just guessing, but you are right that the breaker in the panel won't clear the fault. My guess is an OCPD on the line side of the transformer (not shown in pic) would open due to a direct short between x1 and x0 of transformer.

 

[wwhitney]

If X1 faulted to the transformer case why would I want to send fault current to a downstream breaker? Opening the breaker at the secondary disconnect would not clear a ground fault at the transformer enclosure.

Good point, I overlooked that. Of course the X1 conductor between the secondary terminals and the secondary OCPD has no direct OCPD, so the only way that fault would clear would be by inducing sufficient load on the primary to trip its OCPD. But:

Suppose you remove the green conductor with the red X. The transformer case is still bonded to the secondary disconnect case, the long way around: the primary EGC is bonded to the transformer case, and to the primary neutral at its MBJ, which is bonded to the GES, which is bonded to the secondary disconnect case, which is bonded to X0. So rather than sending fault current from a X1 - case fault through the primary system EGC, the X'ed green conductor confines most of it to the local system.

Also, in the perhaps unlikely case that an ungrounded secondary conductor after the OCPD faults to the transformer case, then the X'ed green conductor will allow the OCPD to work as expected, without imposing (much) fault current on the primary EGC.

Cheers, Wayne

 

[lm227]

Good point, I overlooked that. Of course the X1 conductor between the secondary terminals and the secondary OCPD has no direct OCPD, so the only way that fault would clear would be by inducing sufficient load on the primary to trip its OCPD. But:

Suppose you remove the green conductor with the red X. The transformer case is still bonded to the secondary disconnect case, the long way around: the primary EGC is bonded to the transformer case, and to the primary neutral at its MBJ, which is bonded to the GES, which is bonded to the secondary disconnect case, which is bonded to X0. So rather than sending fault current from a X1 - case fault through the primary system EGC, the X'ed green conductor confines most of it to the local system.

Also, in the perhaps unlikely case that an ungrounded secondary conductor after the OCPD faults to the transformer case, then the X'ed green conductor will allow the OCPD to work as expected, without imposing (much) fault current on the primary EGC.

Cheers, Wayne

That's an interesting point. Say there was a ground fault at the transformer enclosure, wouldn't it be better if the fault current returned to the primary source, let's say at the supplying service disconnect MBJ as the sole pathway to open an upstream device to clear the fault? With the SSBJ bonding the enclosure to the secondary disconnect and the primary side EGC bonding to the primary disconnect I see parallel paths for fault current to take which we are usually trying to avoid.

 

[wwhitney]

That's an interesting point. Say there was a ground fault at the transformer enclosure, wouldn't it be better if the fault current returned to the primary source, let's say at the supplying service disconnect MBJ as the sole pathway to open an upstream device to clear the fault?

Fault current originating at an unfused secondary conductor will only return to the secondary transformer coils (its source), not to the primary side (barring an internal isolation failure in the windings). So there will be no OCPD in the conductive current path. The only way a primary OCPD opens is if that secondary fault causes a primary overload.

Cheers, Wayne

 

[lm227]

Fault current originating at an unfused secondary conductor will only return to the secondary transformer coils (its source), not to the primary side (barring an internal isolation failure in the windings). So there will be no OCPD in the conductive current path. The only way a primary OCPD opens is if that secondary fault causes a primary overload.

Cheers, Wayne

If the transformer case is energized and faults to ground (whether it be through contact with a primary conductor, secondary conductor, lightning strike etc.) why wouldn’t there be fault current on the primary EGC that is bonded to the case as well? Wouldn’t this fault current take any path proportional to the impedance that is available?

 

[wwhitney]

If the transformer case is energized and faults to ground (whether it be through contact with a primary conductor, secondary conductor, lightning strike etc.) why wouldn’t there be fault current on the primary EGC that is bonded to the case as well? Wouldn’t this fault current take any path proportional to the impedance that is available?

The current path back to the source depends on what faults to the case. If it's lightning, which is due to an earth-air potential difference, the current would literally flow to/from earth, i.e. the GES. If it's an ungrounded primary conductor, then it would flow back to whatever other transformer (possibly utility owned) supplying the primary side. And if it's an ungrounded secondary conductor, it would flow back to the transformer's secondary coil.

So for the last case, if the red X'ed conductor is absent, it can still do that via the "long way around" path I described. With the X'ed conductor in place, you are correct that current will take all paths in proportion to their impedances. But the path through the X'ed conductor is likely much shorter, so its impedance should be less, and most of the current will take that path.

Cheers Wayne