SSBJ (Sorry about the length of this post)

[Jamesco]

I am confused about the nature of, and connections of, the SSBJ. The NEC has no specific definition for this phrase. Let’s start with this:

• Utility owned transformer. Local ground rods connected to center point of WYE via GEC sized per 250.66. No N-G bond at transformer location.
• Run A, B, and C conductors from transformer X1, X2, and X3 to main breaker on service panel.
• Run N conductor from transformer X0 to neutral bar on service panel.
• Connect GEC(s) to neutral bar, and run it (them) to grounding electrode(s).
• Run "main bonding jumper" from neutral bar to ground bar inside main service panel.
• Run jumper (I don’t know what to call it - "equipment bonding jumper," perhaps?) from ground bar to enclosure of main service panel.

OK so far?

Now here is what I think:

• You need one more conductor run from transformer to main service panel.
• It is called the “Supply Side Bonding Jumper” (SSBJ).
• The SSBJ is to be sized per 250.102(C).

NOW: This is the part I am not clear about:

• I think that, at the transformer, the SSBJ is attached (essentially) to the transformer enclosure. I don’t know if there is a lug or a bar for this purpose.
• I also think that, at the main service panel, the SSBJ is attached (essentially) to the panel enclosure. I don’t know if it attaches directly to the enclosure or to the ground bar.

• Are these the correct connection points?

• One final point: I think the only reason for the SSBJ is to provide a path for clearing a fault that takes place at the transformer, from a hot conductor to the enclosure. The fault path is as follows:
  • A fault connection occurs between X1 (or a point downstream, but still internal to the transformer) and the enclosure.
  • If there is no SSBJ, the enclosure would be energized but no current would flow.
  • With the SSBJ routed as described above, current would flow from the fault point to the SSBJ connection, along the SSBJ to the main service panel enclosure, to the ground bar, to the neutral bar, along the neutral conductor back to the center point of the WYE, thus completing the circuit.
  • There will be sufficient current flowing along this path to activate the primary side overcurrent device.

So, how far off base am I?

WOW! Is that from the spec book or your own words?

I agree!

If there is no SSBJ, the enclosure would be energized but no current would flow.

Until someone walked up and touched the transformer on a wet dewy morning. Current might/would flow then. Step potential....

 

[augie47]

IMO, if its a utility transformer you don't need a SSBJ to the transformer. 250.24 allows a bonding jumper from the transformer XO to a grounding electrode and transformer enclosure.

{The same thing is true for a SDS (customer owned) if the transformer is located outside and meets the requirements of 250.30(A)(1) Exception 2}

Other than as shown in the { } above, a customer SDS would require a SSBJ from the transformer to the 1st disconnect. (There are some other limited exceptions)

 

[electrofelon]

In '11 we were introduced to a new term, a supply side bonding jumper (SSBJ). That conductor was required between the SDS and the 1st disconnecting means and there was no exception.

That is interesting, I was not aware of that. Seems like a pretty big blunder to not allow omission of the ssbj when the "Bond at both ends" exception is utilized. Actually it basically makes it so the exception could never be applied because you must have a parallel path with the ssbj.

 

[Jamesco]

IMO, if its a utility transformer you don't need a SSBJ to the transformer. 250.24 allows a bonding jumper from the transformer XO to a grounding electrode and transformer enclosure.

But that's not what was spec'd by the OP in his original post.

No N-G bond at transformer location.

The transformer XO neutral conductor is grounded/bonded at the service equipment inside the building. From there connected to the Grounding Electrode. The SSBJ bonds the transformer enclosure to the grounded neutral conductor at the service equipment. The SSBJ is required in this case.

{The same thing is true for a SDS (customer owned) if the transformer is located outside and meets the requirements of 250.30(A)(1) Exception 2}

Other than as shown in the { } above, a customer SDS would require a SSBJ from the transformer to the 1st disconnect. (There are some other limited exceptions)

 

[electrofelon]

But that's not what was spec'd by the OP in his original post.

The transformer XO neutral conductor is grounded/bonded at the service equipment inside the building. From there connected to the Grounding Electrode. The SSBJ bonds the transformer enclosure to the grounded neutral conductor at the service equipment. The SSBJ is required in this case.

For one thing, I think this was all hypothetical and the OP either misspoke or was unaware the utility would bond. Further, I don't think we are required to bond/run a ssbj to a utility transformer.

 

[Jamesco]

For one thing, I think this was all hypothetical and the OP either misspoke or was unaware the utility would bond. Further, I don't think we are required to bond/run a ssbj to a utility transformer.

All of what you say may be true. I didn't run across this thread until after the OP had posted posts #15 and #17.

Post #15
charlie b said:

Thanks for the replies.

WARNING: Curve ball is heading your way.


The building is on a US military base near Seattle. The actual serving electrical utility brings power to a couple substations on base. From there the government owns all distribution lines and transformers. But they have chosen to treat the system, from the substations to each building's transformer, as though they were the utility. I don't really know what that means. I only heard someone tell me that yesterday. So for the moment, let's consider the transformer to be owned by the utility.

I need to do some reading of the articles some of you have mentioned. I will get back to this thread later today.

Post #17
charlie b said:

The AHJ is the government. They have a group on base that performs inspections. The NEC certainly applies inside the building, as it would off the base. But that is because the government has invoked the NEC in their own set of design requirements. I don't recall seeing any mention of the NESC in that same set of design requirements. So this is a matter of some confusion to me.

My two previous posts to this thread are based on charlie b posts #15 and #17 and then going back to his original posted message.

Why is the grounding/bonding of the transformer spec'd to be wired as the OP stated in his original posted message? I think we first would need to find out who wrote it up.
Maybe the reasoning is to prevent a path for electrical stray currents, if the neutral was grounded/connect to earth using a ground rod at the transformer, through the earth when the neutral from the transformer to the service equipment inside the building is carrying load current.

Path for neutral stray current through the earth?
Grounded neutral XO to ground rod at transformer >>> through the soil of the earth >>> To any grounding electrode that is connected to the electrical service equipment grounded neutral conductor inside the building.

Is that the reasoning behind not grounding the neutral at the transformer? Beats me. We would need to find the guy that wrote the specs.

 

[jtinge]

All of what you say may be true. I didn't run across this thread until after the OP had posted posts #15 and #17.

Post #15
charlie b said:


Post #17
charlie b said:


My two previous posts to this thread are based on charlie b posts #15 and #17 and then going back to his original posted message.

Why is the grounding/bonding of the transformer spec'd to be wired as the OP stated in his original posted message? I think we first would need to find out who wrote it up.
Maybe the reasoning is to prevent a path for electrical stray currents, if the neutral was grounded/connect to earth using a ground rod at the transformer, through the earth when the neutral from the transformer to the service equipment inside the building is carrying load current.

Path for neutral stray current through the earth?
Grounded neutral XO to ground rod at transformer >>> through the soil of the earth >>> To any grounding electrode that is connected to the electrical service equipment grounded neutral conductor inside the building.

Is that the reasoning behind not grounding the neutral at the transformer? Beats me. We would need to find the guy that wrote the specs.

I also work at a government location that for years treated the medium voltage distribution as a utility and routinely called the supply into each building a service. We now recognize that the only service point is at the main substation and everything leaving is a feeder. What many legacy engineers and electricians routinely don't realize is that the NEC has many of the same requirements for outside feeders and supplies, and grounding and bonding, as for services. I developed a document we use at our site to compare the requirements for services and grounding and bonding to those for supplies, including SDS''s.

Because the definitions associated with services, supplies, and grounding and bonding have evolved over the last several decades, many older electrical types still use the definitions that they learned when first getting into the business. To show how these definitions have evolved over the years, I developed a worksheet of pertinent definitions back to the 1987 code cycle showing the evolution of some of these changes.

These documents have helped greatly in making sure every here is singing from the same song book regarding services and supplies and application of the proper sections of the code.

If you PM me, I am happy to share both documents via email since they may be too big to share as an attachment.