AHJ and engineer differing opinions- SDS/transformers

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housemoney

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Currently using the EGC as the GEC permitted by 250.121 exception without a separate GEC. Inspector is requesting a "local GEC" (circled in red below) to "ensure transformer voltage stability to ground and maintain a redundant effective ground fault current path in the event the EGC/GEC is disconnected upstream". He cited Soares: "the dual purpose conductor would have to be continuous from the equipment to the grounding electrode (red flagging terminations on equipment grounding bars in the upstream panels that feed the transformers). Ironically, the client's engineer says we don't need it as long as I'm using grounding bushings on the RMC all the way back to the service.

What's the consensus here? Is a separate GEC (circled in red below) necessary or going to do anything for these SDS's (75kVA step down transformers 480:120/208V)? All the potential grounding electrodes in the vicinity of the transformers (building steel or cable tray which is bonded to building steel) are tied together in the grounding electrode system back at the service equipment, some 300-400 feet way from the transformers.


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In my opinion, the building steel connection would be preferable to the EGC connection.

If nothing else, it reduces the chance of potential differences between the two surfaces.
 
There's nothing in 250.30(A)(5) that says the GEC needs to be continuous. I see why it should be and it seems like that section should reference to install per 250.64.
 
The 2014 code would have required the local steel to be used as the grounding electrode for the SDS. In the 2017 code, the language requires that the SDS be connected to the building or structure grounding electrode system.
That being said, a connection to the local steel is still required by 250.104(D)(2).
 
don_resqcapt19 said:
The 2014 code would have required the local steel to be used as the grounding electrode for the SDS. In the 2017 code, the language requires that the SDS be connected to the building or structure grounding electrode system.
That being said, a connection to the local steel is still required by 250.104(D)(2).
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Which the "building steel" likely is a part of the GES. One could certainly make a bonding jumper from said steel to a water pipe electrode or to a CEE.
 
don_resqcapt19 said:
That being said, a connection to the local steel is still required by 250.104(D)(2).
Click to expand...
Doesn't the OP's scenario meet Exception 1? Assuming the EGC/GEC meets the GEC requirements all the way back to a point on the grounding electrode system where the building metal frame is already bonded.

Cheers, Wayne
 
kwired said:
Which the "building steel" likely is a part of the GES. One could certainly make a bonding jumper from said steel to a water pipe electrode or to a CEE.
Click to expand...
The AHJ is pushing us to install a half dozen "local GEC's" on all the transformers out in process areas, all a few hundred feet away from the E-rooms. The shared EGC/GEC wire runs through RMC, a couple J-boxes and a distribution panelboard ground bus on it's way back to the service GEC connection. I do have grounding bushings on these runs but can see where a "local GEC" might make sense? Never seen it be required however
 
wwhitney said:
Doesn't the OP's scenario meet Exception 1? Assuming the EGC/GEC meets the GEC requirements all the way back to a point on the grounding electrode system where the building metal frame is already bonded.

Cheers, Wayne
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It probably does meet the exception, but it appears that the intent is for a connection to the building steel in the area of the SDS. This was better in the 2014 code where you had to use local steel or a local water pipe as the SDS grounding electrode.
 
The metal structural frame of a building shall be permitted
to be used as a conductor to interconnect electrodes
that are part of the grounding electrode system, or as a
grounding electrode conductor. Hold-down bolts securing
the structural steel column that are connected to a
concrete-encased electrode that complies with
250.52(A)(3) and is located in the support footing or
foundation shall be permitted to connect the metal structural
frame of a building or structure to the concrete
encased grounding electrode. The hold-down bolts shall
be connected to the concrete-encased electrode by welding,
exothermic welding, the usual steel tie wires, or other
approved means.
Click to expand...
If the building steel does not meet these requirements, you can't claim a connection to building steel is a connection to a GE.
 
So what is the advantage of having a short local connection, rather than a longer connection back through the service? Is the resistance of the longer path really going to be high enough to cause problems? If so, what/how?

Cheers, Wayne
 
wwhitney said:
So what is the advantage of having a short local connection, rather than a longer connection back through the service? Is the resistance of the longer path really going to be high enough to cause problems? If so, what/how?

Cheers, Wayne
Click to expand...
That's exactly what I was hoping others with bigger brains that I could shine some light on!
 
housemoney said:
That's exactly what I was hoping others with bigger brains that I could shine some light on!
Click to expand...
Part of what is trying to be accomplished here is to have no voltage difference between the grounded conductors (which is also where the EGC originates) of the two separate systems.

If you have building steel that meets what was quoted in post 10 then local connection to that is fine, and possibly even less resistance than conductor sized from 250.66 or 250.104.

Simply driving a rod (or two) at a remote SDS leaves you with earth to rod resistance between the two systems.
 
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