2002, VIII 250.162 (A)

[lbartsch]

With regards to article 250.162(A), 2 wire DC systems above 50vdc shall be grounded.

1. Can it be safely inferred that systems below 50vdc are not required to be grounded?

2. Is 50Vdc a nominal value? Many telecom sites operate at a nominal 48vdc but are actually operating at 56vdc.

3. If a system operating at less than 50vdc is grounded to meet the equipment manufacturers requirements, must part VIII be complied with?

 

[dereckbc]

Re: 2002, VIII 250.162 (A)

1. Can it be safely inferred that systems below 50vdc are not required to be grounded?

Yes, but not recommended. If you left it ungrounded you would need to provide ground fault detection, and install OCPD on both polarities. It is standard practice in telecom to always ground 24 and 48 VDC systems

2. Is 50Vdc a nominal value? Many telecom sites operate at a nominal 48vdc but are actually operating at 56vdc.

Even though it is called a 48 VDC plant, the operating voltage is 52-53 for flooded batteries, and 53-54 for VRLA batteries. You use the normal operating voltage which is above 50.

3. If a system operating at less than 50vdc is grounded to meet the equipment manufacturers requirements, must part VIII be complied with?

Not necessarily. These type of systems require much more stringent design and installation requirements than the NEC provides. Example: voltage drop, cable size, termination, securing, and grounding. Most conductors in telecom are installed in cable racks and are laced to every other stringers rather than using plastic cable ties.

 

[lbartsch]

Re: 2002, VIII 250.162 (A)

I apprceiate the response. I am surprised to see your reference to actual voltage as opposed to nominal, I thought that NEC always used preferred the use of nominal potentials.

 

[dereckbc]

Re: 2002, VIII 250.162 (A)

The only time you are going to have less than 50 VDC on a 48 VDC plant is when there is no AC power applied to the rectifiers. So in essence the voltage 99.5% of the time is 52-54. Nominal voltage on batteries ranges from 2.16 to 2.29 per cell.

I have had this discussion many times with engineers from all over the country representing multiple companies before. But here is what always wins out, cost!. If you have a ungrounded system, you need ground fault detection, and OCPD's on both polarities on every distribution/branch circuit in the system $$$$. If you ground one polarity, no GFD, and only one OCPD per polarity?

There are some other issues, but cost is enough to win the debate. When and if all the manufacture's (minority, but still a few, mostly UK) quit connecting the battery returns to the equipment frames, then you can consider leaving the battery system ungrounded, but not until that time. Even then cost will rule.