Separately Derived 3phase 3 wire System

[dereckbc]

Re: Separately Derived 3phase 3 wire System

Bennie, I cannot speak for Don, but this is exactly what I do. So where is the problem?

The only difference I see in the drawing and real life is the GEC for the transformer may go to building steel rather than directly to the service ground, but it is the same point electrically.

 

[don_resqcapt19]

Re: Separately Derived 3phase 3 wire System

Ed,
Another excellent drawing and it is in full compliance with the NEC. This is exactly what the NEC requires for the connection of a transformer that is the source of power for a SDS.
Thanks for providing these drawings. They are very helpful.
Don

 

[don_resqcapt19]

Re: Separately Derived 3phase 3 wire System

Bennie,

Connect a wire from the neutral/ground buss of a 480/277 service, run it 100 feet and stick it in the dirt. Current will flow in proportion to neutral current and impedance.

Assuming that the service grounded conductor is connected as shown in Ed's drawing (that is at the utility transformer and at the service) you are correct. There will be current flow through the ground. This occurs on all services that have a grounded conductor. The same thing happens with any equipment grounding conductor that is connected to equipment that is on or connected to the earth. These paths are in parallel with the grounded conductor from the utility transformer to the service disconnect. The only way to eliminate this current would be to install an equipment grounding conductor from the utility transformer, along with the grounded and phase conductors. Until this happens, and it is not likely to ever happen, there earth will be a parallel path for grounded conductor current. The only reason that this current flows is because we have connected the grounded conductor to earth at two different physical locations.

Now go to the X-O of the power transformer, run a wire 100 feet and stick it in the dirt. Current will flow in proportion to neutral load current and impedance.

On this one we do not agree. Assuming that the service and transformer is installed per Ed's drawing, and that we are talking about system 3 in the drawing, the grounded conductor is connected to earth at only one physical point. That point being the bonding jumper in the transformer. Extending additional grounding electrode conductors to additional grounding electrodes will not result in any of the grounded conductor current from that transformer flowing through the earth. This is different from the service installation, in that we have two physical connections to earth from the grounded conductor on the service, but only a single point of connection for the transformer.
Don

[ March 19, 2003, 11:26 AM: Message edited by: don_resqcapt19 ]