DC WISP switch

[flameproof]

Greetings,

We have a network deployment involving Ubiquiti gear at a cell tower operator. They have Delta power supplies (CellD 40) that provide -48V DC, but we're not sure if they ground all their tower metal, racks etc. to the positive side.

If we were to place a WS-12-250-DC at this location, and simply flip the wires around, would the switch short the negative to the chassis? I guess the question is - does the switch provide isolation, or is chassis directly connected to the NEGATIVE input of the switch?

Failing this, even if we install an isolated DC-DC converter, eg. TEP 150-4815WI, which provides input of 18-75 V and output of 24 V, would we get screwed by the positive grounding? In this scenario, we'd feed the positive and negative from the TEP to the WS, so its inputs would be isolated, so the whole installation upstream would effectively be floating.

Not sure how to proceed...

[Dave]

flameproof

Sorry for the slow response, one of the days...our dc return is isolated from our chassis ground. So if you just flip the leads around, you should be ok, as I believe that is what a bunch of other people are doing.

However, that being said, assuming nothing else in your system has earth ground bonded to any of the power rails. You could always use a meter & check resistance from your power leads to chassis, but more than likely they are isolated.

I.E. Our power rails are isolated form chassis ground.

Dave

[lligetfa]

Dave, I want it to be clear whether you are talking about your smart DC power supplies or the dumb DC switch? I think on some PoE powered kit, that the kit may tie the PoE negative to frame ground.

[Dave]

Les, all of our dc-dc models have the inputs floating in relationship to chassis ground.

[lligetfa]

Dave, perhaps I didn't phrase my question well. I understand that Netonix floats the inputs relative to the switch chassis. What I wondered is whether the negative input into the smart DC power supply is bonded to the negative output that feeds the switch.

flameproof said:

They have Delta power supplies (CellD 40) that provide -48V DC, but we're not sure if they ground all their tower metal, racks etc. to the positive side.

If they do bond to the tower and a PoE device also bonds to the tower, using the dumb DC switch that could be a problem.

If the input and output of the smart DC are bonded it could be a problem. If they are not, it could solve a problem.

[Dave]

Les

Yes, the negative input & output of our DC switches are bonded together.

Dave

[flameproof]

Thanks for the replies guys - I think we'll try to get clarification on the grounding used at the tower, but as long as they don't tie their supply ground to tower/chassis ground, we'll be OK - in effect they would give us a floating voltage (eg. like in phone lines) with a separate ground that's connected to the tower & all the other metal around.

I'll report back on findings!

[Travis_WVN]

We're really wondering about this too, have a similar situation. Collocation on a cell tower with Sprint -48VDC gear at the top.

We have our poly-plastic equipment box halfway up, and a 10-gauge armoured DC cable running up to it to power a WS-12-250-DC switch.
At the bottom in the telco shed are 4 AGM batteries in series to get 48V, which we send up the cable.

Still working on a way to deal with surge suppression on the DC line, without draining my battery bank into the tower metal (tried using a Transtector DCOD up at the top, and almost killed the battery bank before I realized what was going on) We do have a bus bar up at the top that we have our radio lugs and switch lug connected to, and #2 down to the bottom bus bar, so that's in place.

Another concern is with the Ubiquiti gear we're using - is our DC return weakly bonded to the tower via the antenna mounts?

Anyone else dealing with this sort of situation and has input, it'd be most welcome - winter is almost here, and I'll lose easy access to the site soon.

[sirhc]

Keep in mind that UBNT and most other low cost radios also and DC negative to Earth ground which is why we started doing this bonding as of 9/7/2016.

If you take a Rocket and use an Ohm meter and test between PIN 7 and 8 to the outside of the SMA connector you will see they are bonded so the radio will gain access to the Tower Earth Ground via the SMA jumper and sector or dish antenna.

Too many people were not properly bonding their tower ground rods to their service ground rods and thus the Ethernet cable became that bond and the cables carried a lot of ground current.

Also a lot of DC chargers have DC negative and Earth ground bonded as well. Once again a simple Ohm meter test will verify this.