250.148 & Seperately Derived Systems

[Jbeighey]

I want to get an opinion on a bonding issue that I have run into in a multifamily high rise building. Each condo unit has a split mechanical system with a condenser located on the roof. The rooftop condensers are fed from each individual condo unit in a number of typical electrical risers. The condensers feeders in a typical riser terminate at the roof in terminal blocks within a NEMA 3R metal enclosure. These feeders will change to L/T Flex conduit and continue on to each individual condenser. I have a question concerning 250.148 that says that circuits spliced or terminated in a metal box will have to have to associate EGC's connected to the box.

Would we be creating a problem by effective connecting all the grounds together in the described enclosure per 250.148, when many of the circuits originate from separately derived systems? If a condo unit lost its means of ground at the unit panel and then had a fault, would the fault not continue up this riser and possible dissipate onto the grounds of the other condo units? I am open to any comments concerning this issue.

Thanks,

Jerry

 

[JohnJ0906]

Since the fault it trying to return to it's source, I don't see where it will "dissipate". It will take all possible paths back to source - the X0 of the transformer supplying that circuit. If a path doesn't lead back to it's source, I don't think the current will take it.

 

[rook81108]

The term "separately derived system" has nothing to do with meters or house numbers. It is a separate transformer with a neutral not shared with existing equipment.

If this service has two separate pole or pad mounted transformers serving the same building then the disconnecting means are expected by code to be grouped together where all grounds are also expected to be connected as well. So your grounds should be common to each other already.

As long as you are installing the circuits to code and getting approved/ inspected, the possible faulting of the circuitry by wear and tear or abuse by others that would cause the issue you are concerned with should not worry you in the least.

 

[Jbeighey]

Rook

Every 3 floors is served by a transformer creating the separately derived systems. The building does share a common grounding riser, but the concern I have is the path in which a fault may return to ground through one of the separate systems.

In the scenario that I describe with a unit losing its ground, there is the possibility that you have created enough loop impedance through the circuit in question to keep the fault level down and therefore not trip the OCPD on the faulted circuit.

Yes, I do agree that a proper installation does eliminate the possiblity of a problem of this type.