-48V DC Power Systems.

[Jake Benedick]

Why is the -48v "return" bus not grounded to an earth ground or "ground halo" in the equipment room. Is the -48v return bus "floating"?

In a +24V system the return bus is grounded/bonded to the earth halo or grounding halo...whatever you want to call it.

I understand the use of -48V for corroson issues in sensitive phone equipment. But I'm curious as to WHY the grounding difference between +24V and -48V DC power distribution systems with battery back up? Why are we not to bond/ground to return bus of the -48V system?

 

[brian john]

I have worked on -24, +24, -48 and +48 and the all all the systems I have been involved with were grounded.

 

[ActionDave]

The only time I ever work with -48 is on cell sites and, same as Brian John, it's grounded.

 

[ninjutsu]

Why is the -48v "return" bus not grounded to an earth ground or "ground halo" in the equipment room. Is the -48v return bus "floating"?

In a +24V system the return bus is grounded/bonded to the earth halo or grounding halo...whatever you want to call it.

I understand the use of -48V for corroson issues in sensitive phone equipment. But I'm curious as to WHY the grounding difference between +24V and -48V DC power distribution systems with battery back up? Why are we not to bond/ground to return bus of the -48V system?

+48 is grounded insteead of -48 because of the corrosion issue.

To prevent wire corrosion. Most phone wire is copper, and it is stripped and exposed to air at many points along its path from the telephone company central office to your phone. Also, breaks in insulation could expose small parts of it to water or soil. Any of these will allow the wire to corrode, and the usual methods of corrosion are that copper in the wire is converted to copper oxide or copper carbonate. Both of those happen as a chemical reaction where copper atoms act as positively charged ions to combine with negatively charged oxygen or carbonate ions in water or dirt. A wire with a positive charge will actually draw the negatively charged ions to the wire and will greatly accelerate corrosion. A negative charge will repel the corrosive ions and will slow the corrosion.

A negative charge will cause some copper to be lost due to a process called electro-deplating (reverse of electroplating), but the rate of copper loss is much less than would be caused by copper corrosion with a positive charge.

The phone company power supply acts like a giant battery. They ground the positive site of the supply so that any part of the system has a voltage that is zero or negative, so the whole system resists corrosion better and lasts longer.

 

[jeremysterling]

Over the last several months, I've installed twenty rack-mounted -48V DC plants (rectifiers and battery). The DC UPS installation manual requires one of two "grounding" methods. The DC supply may be referenced to a telecom ground electrode or referenced to the AC equipment ground.

If you remove the bond link between the chassis and the DC return bus and then fail to bond the return bus to the telecom electrode, the DC system will float. Is this what your system is?

Maybe your system is actually referenced to ground through the rectifier's AC supply equipment grounding conductor.

The installations I performed involved migrating the telecom equipment to the new plant and removing the old plant. Along the way, I fell into someone's "trap". I found a twenty circuit DC GMT fuse panel that had been hooked up backwards, that is, the return for the telecom equipment was fused. I found out the hard way. I pulled the fuse for the A power of a MUX and tested across the terminals and found no voltage. I should have tested the -48V lead to "ground." When the removed lead touched the rack, the upstream fuse at the rectifiers blew, dumping the local fuse panel's A bus. The lesson is, never assume anything.

With all that said, is your system really floating, or is there a bond that you cannot see?