Why SSBJs?

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
At a service, metallic equipment upstream of the service disconnect gets bonded to the grounded service conductor. But if I understand correctly, for an SDS where the System Bonding Jumper is at the secondary disconnect/OCPD rather that at the transformer, a separate Supply Side Bonding Jumper is run back to the transformer case.

Why does the NCE requires this separate SSBJ, rather than permitting bonding the transformer case to the secondary grounded conductor? What's different between the service and the SDS?

Thanks,
Wayne
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Why does the NCE requires this separate SSBJ, rather than permitting bonding the transformer case to the secondary grounded conductor? What's different between the service and the SDS?
The transformer is permitted to be bonded to the neutral at the transformer but then you would not bond the neutral at the secondary disconnect where the OCPD is. If you bond in both locations then the SSBJ and any metal raceways would be carrying some neutral current.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
The transformer is permitted to be bonded to the neutral at the transformer but then you would not bond the neutral at the secondary disconnect where the OCPD is. If you bond in both locations then the SSBJ and any metal raceways would be carrying some neutral current.
I'm talking about bonding the transformer case to the secondary neutral (like you would bond a service neutral to a meter base), and then having no bonding jumper between the transformer at the secondary disconnect (relying on the secondary neutral, just like we rely on the service neutral). Yes, if the secondary conductors are in metallic conduit, the conduit would end up in parallel with the secondary neutral, just like at a service.

If you like, we can flip the question around--why don't we bond services like we bond SDSs? That would mean keeping the service neutral insulated from all service equipment, and then running an SSBJ back from the service disconnect (where the MBJ is) towards the service point to bond any metal enclosures/conduit containing the service conductors.

Cheers, Wayne
 

don_resqcapt19

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Staff member
Location
Illinois
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retired electrician
Simply costs to the utility...until recently the utility provided most of the service conductors between their transformers and the service point. A supply side bonding jumper would increase costs to the utility. A change to require the supply side bonding jumper for the service would likely require changes in the state public utility commission rules and in the NESC, unless the service point for all services would be at the utility transformer terminals. That would be difficult for the typical overhead residential service where there are 5 to 10 houses fed from a single transformer using taps to overhead conductors.

Not sure there really is a difference, other than there are likely fewer "touchable" metallic parallel paths for grounded conductor current between the service equipment and the utility transformer then there would be with a customer owned SDS.

And of course the electrons change their behavior based on who owns them.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Simply costs to the utility...until recently the utility provided most of the service conductors between their transformers and the service point.
Not quite following this argument--even if the utility keeps bonding their equipment to their neutrals on the utility side of the service point, the NEC could require an SSBJ be run with the service conductors on the premises side of the service point. That SSBJ would bond all the equipment on the premises side of the service equipment and ultimately be bonded to the MBJ at the service disconnect. While the neutral would be kept insulated on the premises side of the service point.

Doing this would remove neutral current from the cases of service equipment and metal conduit enclosing service conductors.

Cheers, Wayne
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Doing this would remove neutral current from the cases of service equipment and metal conduit enclosing service conductors.
That assumes that any metal conduit on the utility side of the service point is not continuous with metal conduit on the premises side of the service point. Obviously not an issue when the service point is overhead with conductors in free air, but for underground services it might be an issue.

Cheers, Wayne
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
Not quite following this argument--even if the utility keeps bonding their equipment to their neutrals on the utility side of the service point, the NEC could require an SSBJ be run with the service conductors on the premises side of the service point. That SSBJ would bond all the equipment on the premises side of the service equipment and ultimately be bonded to the MBJ at the service disconnect. While the neutral would be kept insulated on the premises side of the service point.

Doing this would remove neutral current from the cases of service equipment and metal conduit enclosing service conductors.

Cheers, Wayne
I guess I just don't see the current on the metal parts of the service equipment to be a real issue. (I think this falls in the "if it is not broke, don't fix it")
The fault clearing time for ground faults would be slightly longer unless the SSBJ is required to be the same size as the grounded conductor.
Under open neutral conditions, we still have the same hazardous voltage on the metal parts of the equipment.
 
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wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Yes, but as I said, the NEC would have to require that the utility transformer terminations be the service point for all services, to make the SSBJ effective.
Sorry, why? Why can't we just have two different bonding schemes on the two sides of the service point?

Say for an aerial residential service, where the service point is the connections at the top of the weatherhead. The fault path for a hot to weather head fault would be along the SSBJ, to the service disconnect, to the MBJ, to the service grounded conductor, back to the transformer.

Cheers, Wayne
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
Sorry, why? Why can't we just have two different bonding schemes on the two sides of the service point?

Say for an aerial residential service, where the service point is the connections at the top of the weatherhead. The fault path for a hot to weather head fault would be along the SSBJ, to the service disconnect, to the MBJ, to the service grounded conductor, back to the transformer.

Cheers, Wayne
I see what you are saying, I just don't see an issue requiring any type of change.

What real issue would be solved by this?
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
I see what you are saying, I just don't see an issue requiring any type of change.

What real issue would be solved by this?
No, I'm not advocating one system or another. I'm just contrasting the bonding strategy used at the service to the bonding strategy used at an SDS, and asking if there's a technical reason for the difference, or it's just a historical accident.

Thanks, Wayne
 

Tulsa Electrician

Senior Member
Location
Tulsa
Occupation
Electrician
Let's say we do like you suggest for for discussion.
X0 is bonded to case. Are we using case to ground X0 or are we using X0 to ground the case. The open netural would be grounded at this point.
Keeping that in mind are you going to have a GEC at the first means of disconnect or would you require one.
Sounds like old school of doing and still done today unfortunately. I see it all the time.
Using the primary EGG for GEC.
I also see it done both at trans and panle.
Here was there arguments. The trans is bolted to steel and the steel is in concrete. Then there's, the case is bolted to the concrete.
Then there is, a jumper is required in the panle because it has a main just like a new service.
In the pics you can see the three phase trans has a gec and the panel it feeds has a system bonding jumper. Done four months ago.
Then the single phase 480 to 120/240 use an EGC kinda like your reference. I say your grounding the netural and not using the grounded conductor for bonding.

Not look at pic with the panel. No SSBJ.
See how what your proposing can easily become an issue and be confusing.
Edit add pic of four wire leaving trans with no SSBJ.
 

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wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
See how what your proposing can easily become an issue and be confusing.
Confusing for those used to doing it the current way. Not inherently any more confusing for someone learning from scratch.

So that brings me back to my original question: is there a technical reason for the difference in bonding strategies for the service upstream of the MBJ (use the service neutral) vs an SDS upstream of the SBJ (use a separate SSBJ run from the enclosure with the SBJ)? Or is just historical?

The only difference I see in the two applications is that the POCO's service neutral is already earthed at the POCO transformer. While the SDS neutral is only (directly) earthed at the location to which we bring the GEC. Still, if you bring the GEC to the transformer, and the SBJ is at the first disconnect, that seems entirely analogous to the service arrangement.

Cheers, Wayne
 
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