+48V DC to -48VDC

[bcm]

Dear Electrical Gurus,

My company owner asked me to answer the below email from one of our clients. No pressure there....

What they are trying to do in hooking this converter up with the inverters seems far too simple. I have just about no DC experience, so I'm struggling to explain what's wrong with his logic. Can any of you pros help me clue in what's wrong with this and what he might actually need to do?

Thanks so much!!

BCM

----------------------------------from client---------------------------------------
The T-1/fiber media converters we have only come in and ac version. I have found some +48Vdc inverters that we could run for our battery plant I think but everything else in the cabinet is -48Vdc.

Seems like I would be able to just go + to + and ? to ? and still get +48Vdc for the inverter device. My concern is the + is the return for the -48 Vdc and I fear I might create some kind of short using a +48V device. Any thoughts?

I tried to hook it up through a fused port at one of our sites yesterday and it seemed to work until I plugged something into the ac outlet then it tripped the battery disconnect breaker?

 

[mivey]

Not sure what his issue is. +48 Vdc is the same as --48Vdc.

 

[hbiss]

Let me take a stab at it. They have devices (media converters) that operate off of regular 120v power. They are wanting to power them off the existing 48VDC battery plant for the UPS's because it's uninterruptable. So they are trying to use a 48VDC to 120VAC inverter.

What I think is happening is that the battery plant is positive ground. The negative input (standard negative ground) on the inverter is bonded to the case and the ground pins on the 120V receptacles. When he plugged a cord into the receptacle a DC short was created back through the grounds on the media converters and whatever they are wired to.

To test this out lift the grounds on the power cords with one of those grounding adapters. The fix is to use a 120V to 120V isolation transformer.

-Hal

 

[bcm]

+48V DC to -48VDC

Thanks so much! I'll see if this can point them in the right direction.

BCM

 

[wireguru]

how about getting non-janky media converters that are available with -48vdc power supply? MRV is one such manufacturer.

 

[wireguru]

as far as the inverter, perhaps in their +48v 'solar power' type inverter they got the battery negative is tied to the EGC so it creates a short when connected to a -48v supply where battery + is grounded.

there are manufacturers out there who make telco type inverters for -48vdc operation. Heres one http://www.majorpower.com/products/

 

[bcm]

+48V DC to -48VDC

Buy what he really should use? A novel thought, and I sure wish I could just say that!

I highly suspect that this client is trying to make this work with what he has available on the cheap which is why these questions have come up.

Thanks for everybody's input!

BCM

 

[wireguru]

do you have a make / model of the media converter?

 

[bcm]

+48V DC to -48VDC

No model #s or even equipment mfrs. I provided the email question verbatim as it was given to me.

 

[wireguru]

No model #s or even equipment mfrs. I provided the email question verbatim as it was given to me.

ok. i was wondering if it was a single media converter with a wall wart type power supply, a rackmount media converter chassis, or something else. If its the former it would be simple to replace the wall wart with a 48vdc input switching power supply.

 

[bcm]

+48V DC to -48VDC

I'm not sure what type of converter they're trying to use. They may come back to me with more questions, and I'll have to delve deeper into the problem. I'll be sure to ask about the converter type if it does come back up.

Thanks!

 

[dereckbc]

The T-1/fiber media converters we have only come in and ac version. I have found some +48Vdc inverters that we could run for our battery plant I think but everything else in the cabinet is -48Vdc.

I will take a stab at this as I have ran into this a few times over the years using some customer premises equipment (CPE) on Foreign Exchanges. The problem is the power supply itself having the output bonded , or referenced to the chassis. Solution was use a 48 volt power supply with 3 options and not fixed:

• Positive reference
• Negative reference
• Float

Sounds like you got one with only 1 fixed option, the wrong one.

 

[zbang]

----------------------------------from client---------------------------------------
The T-1/fiber media converters we have only come in and ac version. I have found some +48Vdc inverters that we could run for our battery plant I think but everything else in the cabinet is -48Vdc.?

Just asking- why don't they just get 48v converters? They're almost as common as dirt.

 

[Jerry the Phone Guy]

Floating the case of the +48V supply is a code violation, because it is considered a separately-derived source. Same goes for the neutral of the 120VAC iso xfmr.

I only see two legal solutions:
1) Bring 120VAC to the rack
2) Buy a new -48VDC supply