-48v Cell Site Battery Backup system....Problems..

[Jrudrow]

Well to start I have not worked with much electricity let alone a -48v system. I have done quite a bit of reading with what I can find across the internet, and other books/manuals I have access to. I am just a little loss on this one.

We just recently installed a -24v, -48v, battery backup system at out cell sites for wireless internet. Problem we are having is the racks have become positively grounded. This has cause a problem on the some of the sites equipment...(Older switches with a RPS system installed on them)..I went through all the install guides, and everything on the full system and cant seem to find any fault.

We are using ICT branded items. These have a Ground(Earth) lug on the back of them all. Our contractor didn't hook any of these up, and I believe this might be part of my problem. Before I hook them up however I thought I would post on some forums and see what others thought....

Sorry to have bothered you guys and gals here, however I just wanted to get some other opinions from around the world! Thanks in advance for the help.

 

[Volta]

Welcome.

When you say "the racks have become positively grounded", do you mean from day one, or do you find this has happened as time goes on?

If the ground lugs on the back are floating, but are internally connected to the negative terminals, how have the positive terminals contacted the building ground(s)

 

[ceb58]

Not sure on your set up. Our microwaves are 48 vdc but are negative ground. Our racks are bonded to the grounding bar for the power supply to the microwave and it is also bonded to the grounding halo.

 

[GoldDigger]

Well to start I have not worked with much electricity let alone a -48v system. I have done quite a bit of reading with what I can find across the internet, and other books/manuals I have access to. I am just a little loss on this one.

We just recently installed a -24v, -48v, battery backup system at out cell sites for wireless internet. Problem we are having is the racks have become positively grounded. This has cause a problem on the some of the sites equipment...(Older switches with a RPS system installed on them)..I went through all the install guides, and everything on the full system and cant seem to find any fault.

We are using ICT branded items. These have a Ground(Earth) lug on the back of them all. Our contractor didn't hook any of these up, and I believe this might be part of my problem. Before I hook them up however I thought I would post on some forums and see what others thought....

Sorry to have bothered you guys and gals here, however I just wanted to get some other opinions from around the world! Thanks in advance for the help.

Definitely do not just connect the ground lugs on the back of some items without understanding the problem.

1. Some DC equipment is designed so that it can operate from a 24 or 48 volt differential between its + and - terminals, regardless of which one is grounded. I will say that these have an isolated DC input.

2. Some devices internally connect either the + or the - terminal to the ground terminal, the rack mounting hardware, and in general any exposed metal surfaces. And also potentially to the logic or communications ground of any interconnecting cables. Such devices are designed and built for only one DC power polarity with respect to ground. I will call these positive ground and negative ground devices.

3. Some DC power supplies provide an isolated 24 and 48 volt output which can be left floating or get its ground reference from the connected equipment or have a switch or jumper selected ground polarity. Chances are that your original AC0-driven supply was of the floating type or was configured for negative ground.

Now, it appears that the addition of the battery backup (which is NOT floating for safety reasons) has forced the supply terminals into a positive-grounded configuration. But some of your load devices are connecting their negative power terminal to their "ground" reference. The result of this will be that the whole panel, if not solidly grounded, will be at -48 volts. Not desirable.
The bottom line is that if you have a combination of power supply and loads, some of which require that the negative be grounded and some of which require that the positive be grounded, you CANNOT easily use them in the same system.
The only solution may be to provide both -24, -48 and +24, +48 volt supplies, using each to power the corresponding equipment.
It is not hard (just expensive) to get a DC supply which will take -48 volt input and produce +48 volt (relative to ground) output.

 

[don_resqcapt19]

I thought that the tradional 48 volt DC for outside plant phone systems was positive ground, and that they kept this tradition for the cell system equipment.

 

[GoldDigger]

I thought that the tradional 48 volt DC for outside plant phone systems was positive ground, and that they kept this tradition for the cell system equipment.

It was and they have, using positive ground so that the hot lead is at -48 to reduce the effects of corrosion on the wires.
But a lot of network and other electronics equipment, as well as PV battery and inverter systems has been designed for negative ground (+48), which is not necessarily compatible, while some gear may designed in a way that does not require a particular side of the 48 volt supply to be grounded.

 

[Jrudrow]

I have a teenagers birthday party going on tonight. I will try to get something written up in the morning to explain my situation a tad more. I have made a few drawings just to help some of the other people involved in this project understand. I might just upload those pictures here in the next 48 hours or so.


I do want to thank everyone who has posted so far! Very helpful people here. I think I might of found a normal "forum" hang out for myself. I should be able to add some of my 2 cents in here and there somewhere on here.

Thank you again guys and gals!

 

[eHunter]

I have a teenagers birthday party going on tonight. I will try to get something written up in the morning to explain my situation a tad more. I have made a few drawings just to help some of the other people involved in this project understand. I might just upload those pictures here in the next 48 hours or so.


I do want to thank everyone who has posted so far! Very helpful people here. I think I might of found a normal "forum" hang out for myself. I should be able to add some of my 2 cents in here and there somewhere on here.

Thank you again guys and gals!

Are there any site specific engineering documents/drawings or schematics for this installation that you can refer to?
All of the wireless jobs that I have been involved with have reams of written specification documents, layout drawings, detailed schematics, detailed equipment connection diagrams and written detailed scope of work all for that specific site.
Then there are additional micro management instructions on the elaborate grounding system and the mandatory use of T&B Kopr-Shield.