System Bonding Jumper

[DARUSA]

But the EBJ on the load size of the disconnect will be sized base on the table 250.122 or I'm wrong.?

 

[raider1]

But the EBJ on the load size of the disconnect will be sized base on the table 250.122 or I'm wrong.?

Correct, after the first overcurrent protective device on the secondary of the transformer you would size the EGC based on table 250.122.

Chris

 

[raider1]

I do not see a conduit as meeting the requirements of 250.102(A). I do agree that 250.30(A)(2) implies the use of something other than a wire conductor as the equipment bonding jumper. It doesn't tell us what else can be used, so we jump to 250.102(A) to find that information.

Don, I can see what you are saying, but 250.102(A) says a bonding jumper shall be a wire, bus, screw, or similar suitable conductor, could a metallic raceway not be a suitable conductor?

For what it is worth I have always used a wire for an equipment bonding jumper on the secondary of a transformer.

Chris

 

[Smart $]

Don, I can see what you are saying, but 250.102(A) says a bonding jumper shall be a wire, bus, screw, or similar suitable conductor, could a metallic raceway not be a suitable conductor?

For what it is worth I have always used a wire for an equipment bonding jumper on the secondary of a transformer.

Chris

What good does a grounding conductor bonding the transformer enclosure to the system disconnecting means serve anyway? Couple that with the fact the transformer enclosure is required to be grounded to the primary supply.

 

[don_resqcapt19]

What good does a grounding conductor bonding the transformer enclosure to the system disconnecting means serve anyway? Couple that with the fact the transformer enclosure is required to be grounded to the primary supply.

What good does the primary EGC do for a secondary side fault? The equipment bonding jumper on the secondary side bonds the transformer enclosure to the secondary grounded conductor via the line side bonding jumper.
With a transformer installed per the NEC, I would expect that a secondary ground fault on the line side of the secondary OCPD would cause the primary OCPD to open.
With just a primary grounding/bonding connection to the transformer, the impedance of that fault path back to XO may be high enough to prevent the primary OCPD from quickly clearing the secondary ground fault.

 

[erickench]

In the case of a two conductor secondary the transformer conductors are considered protected by the primary overcurrent device therefore in this case there is no disconnect in the secondary. The SBJ would be at the transformer.

 

[mark32]

What good does a grounding conductor bonding the transformer enclosure to the system disconnecting means serve anyway? Couple that with the fact the transformer enclosure is required to be grounded to the primary supply.

What good does the primary EGC do for a secondary side fault? The equipment bonding jumper on the secondary side bonds the transformer enclosure to the secondary grounded conductor via the line side bonding jumper.
With a transformer installed per the NEC, I would expect that a secondary ground fault on the line side of the secondary OCPD would cause the primary OCPD to open.
With just a primary grounding/bonding connection to the transformer, the impedance of that fault path back to XO may be high enough to prevent the primary OCPD from quickly clearing the secondary ground fault.

This is an interesting topic/debate. Smart and Don are legendary members here and they seem to disagree on something that I have recently pondered being sparked by this thread
http://forums.mikeholt.com/showthread.php?t=124616
there I asked about the same graphic that was posted on this thread by Augie, my question here is the same, which follows as, "In the second example in the first graphic (Post #2), does the need exist to use an equipment ground from the trans to disconnect? I'm looking at pg. 303 and 304 in my "Illustrated Guide to the NEC" by Charles Miller and his graphics are similar to yours except there is no EGC being used when the system bonding jumper is installed at the disconnect."

In addition, if the system is wired as such, having the SBJ being installed at the disconnect and not having a jumper from XO to the transformer's frame, how exactly would a secondary fault to ground (Metallic raceway for example), line side of the sec ocp, behave and what path would the fault take? Would it flow back to the primary's transformer? If so, that confuses me somewhat as the secondary system would be from a separately derived source. Electricity tries to get back to it's source but in the example above, how can the sec fault accomplish this w/out the XO being bonded to the transformer? Don, you described what I'm trying to grasp, perhaps you could dumb it down for me a little.

 

[don_resqcapt19]

...
In addition, if the system is wired as such, having the SBJ being installed at the disconnect and not having a jumper from XO to the transformer's frame, how exactly would a secondary fault to ground (Metallic raceway for example), line side of the sec ocp, behave and what path would the fault take? Would it flow back to the primary's transformer? If so, that confuses me somewhat as the secondary system would be from a separately derived source. Electricity tries to get back to it's source but in the example above, how can the sec fault accomplish this w/out the XO being bonded to the transformer? Don, you described what I'm trying to grasp, perhaps you could dumb it down for me a little.

Mark,
You are correct that the current from a secondary fault has to return to the secondary of the transformer. If there is no XO bond, the secondary is an ungrounded system and a single ground fault only creates a "grounded system". It does not cause current flow.

If the system is a grounded system you need a low impedance path to the system bonding jumper so that in the event of a ground fault on the line side of the secondary OCPD, there will be enough current flow that the primary OCPD quickly clears the fault. In a transformer installed under the rules of the NEC that should happen. This would be in contrast to most utility transformers where the primary OCPD is not normally sized to open with a secondary fault.

In the case of a metallic secondary conduit, that would be the fault clearing path, even though I am not convinced that the code permits it to serve as the required "line side bonding jumper", it would probably do as good a job as a wire bonding jumper would.

If there is not a solid metallic path between the location of the system bonding jumper and the transformer enclosure there will be a delay in the opening of the primary OCPD in the event of a secondary conductor short to the transformer case. There would not be low impedance path from the fault to the location of the system bonding jumper. As long as the fault exists there will be a shock hazard, and if there is enough current there will be a fire hazard.

 

[Smart $]

This is an interesting topic/debate. Smart and Don are legendary members here and they seem to disagree on something...

For the record, I agree with Don. I assumed the role of devil's advocate to pose the question... and he nailed it

 

[mark32]

Thank you guys,

A thought came to me this morning, in reference to your last paragraph Don, you mention "There would not be low impedance path from the fault to the location of the system bonding jumper" agreed, but my confusion arose when I tried to figure out the fault's path, in this scenario. Would the fault flow back to the primary's GEC and in turn seek to get to the secondary's GEC which may or may not be a low impedance path?

 

[don_resqcapt19]

T... Would the fault flow back to the primary's GEC and in turn seek to get to the secondary's GEC which may or may not be a low impedance path?

If the only path is via the earth between the two grounding electrodes, it would be very rare to have a low impedance path.
If there is some type of common grounding electrode such as the structural steel of a building or a metal underground water piping system the impedance of the path will be much lower. It will not be as low as when there is a supply side bonding conductor installed between the transformer and the system bonding jumper.
Remote paths (current into the fault and the fault return current follow different routes) will always have a higher impedance (in AC systems) as the result of the inductive reatance which increases with separation.

 

[erickench]

And if the primary does'nt protect the secondary and a fault occurs on the line side of the OCPD then you would have a burnout as you would in an actual utility service, assuming the conduit is metal and provide's the low resistant path to the SBJ.
Now we see why we have transformer vaults.

 

[mark32]

OK, so we can agree that installing the SBJ at the trans and running the GEC from there is a better design than installing the SBJ (And GEC) at the disconnect or panel?

 

[Smart $]

OK, so we can agree that installing the SBJ at the trans and running the GEC from there is a better design than installing the SBJ (And GEC) at the disconnect or panel?

IMO, with emphasis, yes.