Separately Derived Systems and Generators

[lbarnes]

Please forgive the invitation for a re-hash of old topics. I have spent some time this afternoon in the archives, and could not find anything which directly addresses my question.

First, some preliminaries for the benefit of my education:
According to bennie(member#800), the term 'separately derived system' (SDS) originally described systems totally isolated from the serving utility, and this definition still applies to these systems. His gripe is that this term has been extended to apply to other systems (transformers, in particular) which are not totally isolated. In theory, I agree with his point. By following the grounding requirements outlined in the NEC for SDS's, we introduce ground loops into our buildings.
Charlie(member #59), however, made the point that due to the very nature of our installations (metal conduit and building steel connecting everything) we already have ground loops, so we would do well to admit it and make the connections deliberate.

Is this a fair assessment of the beliefs regarding SDS's? If not, set me straight.

Now for the real question on my mind. For my purposes, the generator is permanent, not portable or vehicle-mounted. It is my understanding that 700.7(D) requires ground-fault indication under certain conditions, and that in order to provide this indication, use of a SDS is required.

Again, if I am off base, somebody call me out.

Finally, the question. Other than the above circumstance, does the NEC require a generator to be connected as an SDS? Even if not required, is there a good reason to do it?

 

[dereckbc]

Re: Separately Derived Systems and Generators

Originally posted by lbarnes:
Charlie(member #59), however, made the point that due to the very nature of our installations (metal conduit and building steel connecting everything) we already have ground loops, so we would do well to admit it and make the connections deliberate.

Is this a fair assessment of the beliefs regarding SDS's? If not, set me straight.

It is my understanding that 700.7(D) requires ground-fault indication under certain conditions, and that in order to provide this indication, use of a SDS is required.

Finally, the question. Other than the above circumstance, does the NEC require a generator to be connected as an SDS? Even if not required, is there a good reason to do it?

Well I will try to help out. First any generator using GFP should be configured SDS. If not, you are inviting nuisance tripping of the service and generator. It is not required by the NEC, just good design practice.

Charlie is correct about loops, they exist. In my business we call it either the "integrated ground plane" or "common bonding network" (CBN). It can work for you or against you. The main problem with CBN is common mode noise. External currents are allowed to flow including leakage current from unbalanced or wye distribution,lightning, and power faults. The big offender is the leakage current circulating around. This circulating current causes voltage differences in the ground system that can interfere with sensitive electronic equipment that uses ground as a signal reference. Common mode noise can be eliminated by use of an SDS like a transformer or battery plant and employing single point ground techniques of the power supply and equipment.

One technique we use in the telecom industry is to isolate all the power supplies and equipment from the CBN by use of isolators to prevent any contact from the CBN. We call this a "isolated ground plane" or "isolated bonding network" (IBN). The power source and equipment is referenced to a single ground bar, and then the ground bar is connected to the GES which could be the nearest building steel, water pipe, or ground ring. This forms a single point ground, and prevents any currents from flowing in the equipment. The benefit lightning, external faults, and noise are prevented from interfering or upsetting equipment operation.

Another good reason to configure a generator as SDS is two fold. One is fault path impedance. If the generator is not SDS, a fault has to travel all the way back to the N-G at the service then back to the generator grounded terminal adding unnecessary impedance to clear a fault. The other reason again is noise. A SDS with a bond will not develop as high of a N-to-G voltage as a non SDS. A lot of single phase equipment that has switch mode rectifiers are sensitive to this voltage. SDS minimizes this voltage drop.

Hope that helps.