Help me recognise what type of DC grounding is employed in this system 2

[DrDrreeeaaa]

Hi All

I need to make a deduction as to the type of earthing being employed in this DC system.

I have read as many posts on this site as I have been able to find but I cannot make a reliable deduction myself so I have had to ask the question directly. I don't have much info to go on so I hope I'm not wasting everyone's tine...

I have attached photos of the cabinet in question. It is a communications cabinet. The DC bus powers a radio that supports the site; a high level of availability is required of the radio.

The site utilises 400V MEN system with TN-C-S earthing.

The bottom of the cabinet is sitting on a wooden block and I cant find evidence of a protective earthing connection anywhere. The DC positive is connected to the body of the cabinet and the 0V is isolated from the cabinet.

The DC bus is fed from a UPS.

Wondering if anyone has enough experience to suggest the DC system is either a) ungrounded or b) positively earthed.

The problem I have is that it is proposed to run an RS485 cable into the cabinet, on which the signal ground conductor may be connected to the 400V protective earth (in the field). I am trying to predict any problems this may cause.

To all those who have lasted to the bottom of this message thanks, I know there isn't much to go on. Any help would be much appreciated.

 

[VEBill]

"The DC positive is connected to the body of the cabinet and the 0V is isolated from the cabinet."

If what you say is true, then the answer is "Positive Ground". But you should triple-check to make sure that the power supply is wired as you think (because it's relatively uncommon to have a positive ground).

Then track down where the power supply returns are located. This can help to confirm the positive ground.

Then trace out the *external* wiring to see if there are any earth grounds (hidden or otherwise).


Nobody left a circuit diagram?

 

[IRstuff]

A relatively simple answer may be to opto-isolate the interface. This would protect both sides of the interface in case of a wiring error or other problem.

As a general rule, unless the RS-485 signal originates within the same DC power supply system, you will always run the risk of power-on transients that clobber the receivers, unless they share a common ground. In many cases, sharing a ground poses different risks, so an opto-isolation scheme can often absolve you of many circuit and grounding sins.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[VEBill]

That's a very good suggestion. There are ready-made devices on the market to provide the optoisolation.

 

[mcgyvr]

positive ground is very common in telco/communications systems. (typically -48VDC supplies/rectifiers here in the US)

 

[DrDrreeeaaa]

Thanks all for your replies

I would love to isolate the interface but unfortunately there is a conflicting requirement

So, looking at the photo and the way that the DC positive is connected to the body of the cabinet... would it be done this way in an ungrounded system?, or is this a specific practice of a positively grounded system

Unfortunately I've seen all the photos I have and I cannot see any protective ground connection (to the 400V system earth)

 

[IRstuff]

This would seem to be an outright ERROR. There should never be any voltages on any exposed chassis surfaces, regardless of whether the supply voltage is positive or negative relative to true ground.

You need to verify what would happen if you added a hard ground to the chassis. Would something blow up?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[DrDrreeeaaa]

"There should never be any voltages on any exposed chassis surfaces, regardless of whether the supply voltage is positive or negative relative to true ground"

Yes, the statement you have made is what is making me think the supply is ungrounded

I am having a hard time quantifying the answer to your second question mate... haha

 

[DrDrreeeaaa]

... @ VE1BLL: Nah mate, no one has been kind enough to leave a circuit diagram...

I have decided to terminate the serial cable into the serial-ethernet converter an intermediate enclosure, then run a cat 5 LAN cable from the converter to the comm's cabinet. The transformer isolation on the ethernet should prevent any problems.

Thanks all for your responses.