During a recent class in Las Vegas, we were talking about separately derived systems (SDS) and how they relate to portable generators and isolation transformers. I had just been reading Soare's Book on Grounding and Bonding the night before so I was feeling pretty confident about what I was saying. And then I had one of those learning opportunities.

 

This particular class had a lot of very experienced production electricians, several of whom worked for Cirque du Soleil. Cirque is known for their very skilled personnel and they are almost fanatics about training, so when one of the master electricians questioned the validity of a statement I made about bonding an SDS, I had to think hard about what I was saying. The situation was this:

If you have an isolation transformer, as many audio techs like to have, then it is considered a separately derived system because it meets the criteria as defined by the NEC, which is:

"A premise wiring system whose power is derived from a source of electric energy or equipment other than a service without any direct electrical connection, including a solidly-connected grounded circuit conductor to supply conductors originating in another system" - (quoted from the NEC)

 

An SDS should have one and only one bonding jumper. The neutral conductor  should be bonded to the grounding conductor by the system bonding jumper at the isolation transformer, at the first disconnecting means or overcurrent protection device, or any point in between. There should be no connection at all between the neutral conductor and the grounding system downstream of the system bonding jumper. 

 

When I made that statement, the electrician said it can't be true because the neutral bus bar in all of the panels is in contact with the chassis, which, in turn, is bonded to the grounding system. I didn't challenge that statement at the time because I wasn't 100% sure. Now I am. And here's why.

 

According to Article 250.6 of the NEC, "The grounding of electrical systems, circuit conductors, surge arresters, surge-protective devices, and conductive normally non-current-carrying metal parts of equipment shall be installed and arranged in a manner that will prevent objectionable current."

If the neutral bus is bonded to the grounding system downstream of the system bonding jumper, then the neutral current will divide between the neutral conductor and the grounding conductor, which is the "objectionable current" referred to in 250.6. Therefore, they must be isolated from each other except at the source. 

 

For more information, see Article 250.30 - Grounding Separately Derived Alternating Current Systems.