Bonding a Separately Derived Service

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Scotty51

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For 38 years I've worked as an electrician and ALWAYS drove a ground rod next to a transformer and bonded XO, and then the "first point of service" so that any ground fault would return to the first point of service.
Recently, while working on a new service here at the college, one of my crew members contacted this forum to get clarification of where to bond the neutral for this new service. He was instructed not to bond the "first point of service" if the transformer neutral was bonded.
This leads me to believe that the fault now has to go back to the OCP serving the transformer and I don't understand that.

Case in point is the power company. They build a transformer bank for your service and bond their new neutral to ground. You then connect your service to this feeder and bond your "first point of service". There is an obvious contradiction here that I would appreciate anyone clearing up for me.
Thanks for your time.
 
There are various things we call bonding and grounding that some times get confused. You may also be confusing an actual service with a SDS which is a separately derived SYSTEM

If the transformer is a SDS you are required to use the closest existing GE. You can't just pound in a rod and use it.

In any case, neither the GEC or the GE has anything do to with clearing a fault, even if you believed that for 38 years.
 
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Scotty51 said:
For 38 years I've worked as an electrician and ALWAYS drove a ground rod next to a transformer and bonded XO, and then the "first point of service" so that any ground fault would return to the first point of service.
Recently, while working on a new service here at the college, one of my crew members contacted this forum to get clarification of where to bond the neutral for this new service. He was instructed not to bond the "first point of service" if the transformer neutral was bonded.
This leads me to believe that the fault now has to go back to the OCP serving the transformer and I don't understand that.

Case in point is the power company. They build a transformer bank for your service and bond their new neutral to ground. You then connect your service to this feeder and bond your "first point of service". There is an obvious contradiction here that I would appreciate anyone clearing up for me.
Thanks for your time.
Click to expand...

The first thing is we need to do is call things the same as the NEC if we want clear NEC answer.

There is no such thing as a 'separately derived service' in the NEC. There are services and there are separately derived systems. The rules for each are different.

A service can only come from a utility and of course services have specific rules tat only apply to them.

On the other hand an SDS (separately derived system) has different rules.

If an SDS is bonded at the source you do not bond it again at the first disconnect except under some very specific and odd conditions.

So how exactly is this new SDS being wired, where is the bond, where is the grounding electrode conductor running to?
 
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Bonding a Separately Derived System (my bad)

Bonding a Separately Derived System (my bad)

We are establishing a sub-panel from a 480V board and have bonded the transformer to a ground rod, and bonded XO to the case and the rod. The real question is, why can't you ground the xfmr and bond the first panel in this situation?
 
This is confusing.

Are you saying you have an existing 480v Panelboard that is currently being fed from a Transformer,
and your going to come out of the existing Panelboard and set a subpanel?

and

That the Transformer feeding the existing 480v Panelboard, currently has a bonding jumper from XO to the
case of the transformer and then attaches to a ground rod by the transformer,and that's it?
 
Lets look a a few sections from 250.30 reagrding your question:
Fist, the grounding electrode(ground rod) itself:

(4) Grounding Electrode. The grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system.
The grounding electrode shall be the nearest one of the following:
(1) Metal water pipe grounding electrode as specified in 250.52(A)(1)
(2) Structural metal grounding electrode as specified in 250.52(A)(2)
Exception No. 1: Any of the other electrodes identified in 250.52(A) shall be used if the electrodes specified by 250.30(A)(4) are not available


As you can see, a ground rod is the exception to the normal grounding electrode.

We will then need a system bonding jumper:

(1) System Bonding Jumper. An unspliced system bonding jumper shall be installed. It shall be sized in compliance with 250.28(A) through (D). This connection shall be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices,


As far as connecting the grounding electrode, that connection shall be made at the same point you establish your bonding jumper as shown:
(5) Grounding Electrode Conductor, Single SeparatelyDerived System.
A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived phase conductors. It shall be used to connect the grounded conductor of the derived system to the grounding electrode as specified in 250.30(A)(4). This connection shall be made at the same point on the separately derived system where the system bonding jumper is connected.
 
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