DC Power Grounding

[jsinclair]

Another DC power problem and its relationship to NEC. DC power plant. Fed from 100A, 240v AC sub panel contained within the unit. (I think this migh be a problem too.) On the DC side we have a 400A, 27vdc of potential DC power. Running (1) #2 from (-) bus of DC to MGB for "logic ground". On AC side of things, we have (1) #6 grounding the AC panel and (1) #2 grounding the frame of unit. (Project spec on the #2)

Question: Is the DC "logic ground" required to be "bonded" to the frame ground? :roll:

 

[dereckbc]

Re: DC Power Grounding

John, I think I can help since I design DC power plants and protective grounding. First thing we need to make clear is that AC equipment ground and DC equipment grounds are two separate systems with a common bond.

Question: Is the DC "logic ground" required to be "bonded" to the frame ground?

Answer: Yes, but probable not in the way you are thinking. A DC plant usually has a Master Ground Bus (MGB) associated with it, and serves as the Isolated Ground Plane (IGP) Single Point Ground (SPG). The source ground for the MGB comes from the Office Principle Ground Point Bus (OPGPB). This "source ground" is the same as a GEC in an AC system, and is typically a 750 KCM.

The MGB has a Main Bonding Jumper (MBJ) connected to the return bus of the DC power plant sized to a formula based on the number of rectifiers and batteries (typically 750 KCM). The MBJ serves the same purpose as the MBJ in an AC system connecting the grounded conductor to the Ground Electrode System (GES), in order to allow OCPD?s to operate.

As I said before the MGB serves as the SPG for the DC system. The Logic Ground Bus (LGB) originates from the MGB but is isolated from the Frame Ground Bus (FGB). The LGB purpose is to serve as a current free signal reference for data bit detectors.

The FGB originates from the MGB. The FGB purpose is the same as an AC EGC circuit. It is a current free fault-clearing path for the OCPD?s.

There are no ground loops if designed and installed correctly. That is why it is called an ?Isolated Ground Plane?.

A 24-volt system is not required to be referenced to earth, but there is still the requirement to provide over current protection for the system. If you did not reference one of the battery polarities to something (say a piece of metal magically floating in air) or earth, you would have to provide a Ground Fault Detection (GFD) system, and OCPD on both battery polarities in the primary and secondary protection systems. It is much less expensive and complicated to reference to earth, provide a common fault-clearing path, and only provide OCPD?s in the ungrounded conductor.

Question: Also, any good reference books or references for topics such as this? DC power seems to be pretty well neglected when you get into low-voltage high current areas.

Answer: Yes for a price. It is proprietary information. Some sources are Tellcordia, BICSI, ANSI/IEEE, and telephone operating companies. The best is from Tellcordia (TR-NWT-000295 Titled ISOLATED GROUND PLANES) in which all others are based upon. I also have written a engineering/installation practice for an operating company.

[ March 11, 2004, 12:39 AM: Message edited by: dereckbc ]

 

[jsinclair]

Re: DC Power Grounding

Thanks for the response. It was just what I was looking for. Let me summarize what you said so that I am sure I understand it.

1.) From the (-) bus of the DC system there should be a reference or logic ground that terminates on the MGB. Sized according to the number of rectifiers and batteries. (Typically 750 mcm) What is the formula?


2.) A frame ground from the cabinent or frame of the system that runs to the MGB. Sized according to Article 250.

_____________________ (-)
|
|
MGB
|
|_____________________ Equipment Frame


I have found myself a copy of Telecordia Standards and will start immersing myself in it.

Thanks again.

John

 

[dereckbc]

Re: DC Power Grounding

John, you are not quite on track yet.

Question: 1.) From the (-) bus of the DC system there should be a reference or logic ground that terminates on the MGB. Sized according to the number of rectifiers and batteries. (Typically 750 mcm) What is the formula?

Answer: The conductor you speak of is not the logic ground. It is a bonding jumper, or sometimes called a battery reference conductor, depends on which company or standard you are reading. It serves the same purpose as the MBJ in an AC service entrance or a separately derived system (SDS). It is part of the fault clearing. A battery plant is a SDS. The MGB as it is called, is equivalent to a ground bus in a transformer. The Logic Ground and Frame Ground are two separate circuits that originate from the MGB.

Try looking at it this way. Consider the battery plant as a SDS step-down/isolation transformer that you are going to configure with an IG bus (LRB) and an EGC bus (FGB). You would be required to bring a GEC to the Xo lead. (The bonding or reference conductor jumper connected between the MGB and either the negative or positive bus in the DC plant). Now to complete the IG bus (LRB) you install a bus bar and bond it to the transformer ground bus (MGB). Your EGC (FGB) originates from the transformer ground buss (MGB), and the IG (LRB) bus originates from the transformer ground bus (MGB).

To make matters even more confusing, in a small DC plant you can use the return bus in the plant as a MGB, FGB, and LRB. You just simply run a GEC from the return bus in the plant to the OPGPB, and arrange the ground circuit conductors in a matter that divides the ?integrated ground plane? from the ?isolated ground plane?. Confusing huh? It is suppose to be, keeps the Communist confused.

As for the size of the various bonding jumpers it depends on the size of the plant which I will not go into. But as a rule of thumb if the plant is less than 600-amps they are 2/0 AWG, all others 750 KCM.

Question: 2.) A frame ground from the cabinet or frame of the system that runs to the MGB. Sized according to Article 250.

Answer: Not exactly. There are several frame ground conductors.

1. There is a dedicated frame ground conductor installed from the MGB and power plant frame work..
2. Another dedicated frame ground conductor installed from the MGB to the battery racks.
3. Another dedicated frame ground conductor installed from the MGB to the cable racks or raceways.
4. Another dedicated frame ground conductor connected from the MGB and FGB. The FGB is used to bond all the terminal equipment being supplied by the battery power plant. In a small system a FGB may not be needed, and just a single conductor from the MGB or battery return bus.

The size of the conductors is based on the resistance of .005 ohms or less (connection and compression resistance included) to the most distant end. The minimum conductor size allowed is a 6 AWG limited to 10 feet or less to achieve .005 ohms or less. Once the conductor size is calculated you check the minimum requirements of 250, but I have never seen a conductor calculated smaller than NEC 250, it is always much larger.

This is getting to involved, send me your email address via PM and I will send you a engineering/installtion standard I wrote for ANSI and a telephone company.