-48 VDC grounding issue 3

[pressedfortime]

I am currently doing bids on remote stations that have no AC systems they Are all 48 24 12 VDC Central office with batteries, solar and 48 VDC gensets.
My question is what type of earth ground is required I cannot find it in the NEC codes,engineering book or the power engineer who is 1800 miles away LOL
I design RF Wan LAN systems not power LOL but I need to be a power person (sounds like a breakfast commercial)i just got stuck with this bid big whine LOL where is that 10 amp dc breaker any way LOL

 

[advidana]

check with the manufacture of the equipment it is usually different for each application or manufacture. I am a electrical engineer and that is whatI have to do. Then I check with the NEC code to see if it is ok.

 

[busbar]

You might find something interesting in ch. 11 at: [~3MB]

http://www.qwest.com/wholesale/downloads/2003/030225/PUB77350IssMApr03.pdf

Fuse overcurrent protection seems popular.

http://www.bussmann.com/shared/library/catalogs/Telpower_Cat.pdf

 

[GOTWW]

Per the 1996 NEC handbook, photovoltaic 2-wire circuits under 50V do not require either conductor to be bonded to ground. However the equipment grounding shall remain per NEC 250. Alot of sites bond the positive +Vdc, and fuse the negative. I am not sure why but it may be related to electrolysis effects (i.e. corrosion of the grounding rod, etc.)

 

[gordonl]

On DC systems I prefer to run ungrounded with ground fault detection and alarming to a central office. That way when a ground fault occurs a technician can be dispatched to repair/resolve without the whole system going down. This may be exactly what you need since it sounds like these are likely remote sites. (As previously stated your system has to be built to code, or have an exemption)

 

[GOTWW]

Gordonl, thats interesting, how does one pull off ground fault detection on an "isolated" system. What you must actually mean is that one line is referanced to ground by way of a resistance, simular to medium voltage ground fault detection.

 

[gordonl]

No, you measure voltage from +ve to graound and -ve to graound, and balanced is healthy, when one becomes grounded time to find the problem.

A resistive divider is used to reference the DC to half way between, with the centre of the divider grounded. The size of the resistor determines the sensitivity.

 

[GOTWW]

Good idea, wish I would have thought of that. An alternate method might be by using capacitance measurements from ground to the + and - leads after everything is installed. This way there would be no DC coupling to ground, other than leakage.