Seperately Derived System

[rich000]

A UPS with a 480V input has a 208V output (delta-wye transformer). I understand that this equipment needs a 5 wire feed (L1-L2-L3-G-GEC).

I have a practical and theory question.

Does the GEC need to all the way back to the building ground or does it just get bonded at the service or building steel?

Also, what is the purpose of the extra ground wire (GEC)? Is this just a backup to the grounding conductor?

 

[dereckbc]

Re: Seperately Derived System

Q1.Does the GEC need to all the way back to the building ground or does it just get bonded at the service or building steel?

A1. The requirements for GEC SDS systems are covered by 250.30(4). Basically the nearest practical location in the order of:
1. Effectively grounded structure metal member of the structure (building steel).
2. Water pipe within 5 feet of building entrance.
3. Other electrodes specified by 250.52.

The code gives the minimum requirements, but there are some performance methods used in data centers like using the signal reference grid under the raised floor, or rebar stubbed up for the purpose.

Q2. Also, what is the purpose of the extra ground wire (GEC)? Is this just a backup to the grounding conductor?
A2. Good question and you will get different answers. IMO it has to do with current transfer ratio of the transformer, and the distance of the primary input EGC. For example with a step down transformer, the output current is higher than the input, therefore the primary EGC would not be thermally capable of clearing a secondary fault to local building steel. Couple that with the distance and the added impedance, the impedance may not be low enough to operate the secondary OCPD.

Then there are some technical performance issues with respect to high frequency common mode noise involved with long EGC circuits. But I will not open that can of worms now. Hope that helps.

 

[rich000]

Re: Seperately Derived System

Thank you. Grounding (SDS) has always been a hard part for me.

I know doing it per the NEC code is the way, but why not just oversize the ground for the secondary current of the transformer?

 

[dereckbc]

Re: Seperately Derived System

Originally posted by rich000:
why not just oversize the ground for the secondary current of the transformer?

Good question, and that would probable work for most installations.

But what if it were a transformer in a high-rise building on say the 60th floor with the EGC originating in the basement and the fault occured on the secondary to building steel on the 60th floor? Doesn't it make since to bond the Xo to the nearest building steel located on the 60th floor, rather than have the fault travel all the way down to the basement via building steel, then back up on the EGC on the primary circuit? Can you imagine the voltage that would be developed at the point of the fault if done that way, and the shock hazard to every one on the 60th floor?

Sorry to ask you the questions, but It makes you think about about it and see my point. Hope that helps.

Dereck

[ March 17, 2005, 11:33 AM: Message edited by: dereckbc ]