WS-12-DC recommended fuse size at full load

[jsweere]

I am in process of deploying a handful of the WS-12-DC switches across a few sites. I am powering them with a -48VDC rectifier system with a string of batteries attached for those fun no-power days. I am curious what the recommended fuse size is for this switch running at full capacity. I stupidly put a 2A in to begin with not thinking, and immediately blew it. I'm possibly running up to 3 SAF links and 3-5 mikrotik 911/912's per site. I'm guessing a 15A may be sufficient, but wanted to double check.

[Dave]

a 10 amp fuse should be fine.

[sirhc]

Well I doubt you will run it to capacity but lets make a safe assumption on things

The switch as (4) 48VH ports 1-4 each capable of 72 watts so assume 72 x 4 = 288 watts

Then assume you have ports 5-12 loaded to max of .75A each which will never happen in the real world but lets pretend so 8 ports x 36 watts = 288 watts

Assume switch needs 10 watts max so 288 + 288 + 10 = 586 watts / 48V = 12.2 Amps

Do you really think you will actually drive 12.2A????

Probably a 10A fuse would be more then enough which Dave beat me to it.

[jsweere]

Ok. that sounds about what I was thinking.

That being said...when I had the 2A fuse in there, laptop connected to port 12 for configuration, everything was cool. As soon as I connected a RB912 to port 1 (poe supposedly off on the port) the fuse blew. I double checked my ethernet connection to make sure I didnt have any miswires, and double checked my power connections as well. Connected to port 2 the second time, same blown fuse. Technically, the 912 I connected shouldn't have even been noticed by the switch since POE was set to off and no power should have been applied.
???
I'm confused.

[Dave]

jsweere

Be careful, something sounds wrong...remember, out output POE power is not isolated from input power (ie...shared ground, the black (-) terminal on our input connector is connected to the negative power on POE port lines). Almost sounds like you have a power ground issue.

Dave

[wtm]

I would be REAL concerned with powering by -48v. supply (Positive ground). You need to check to make sure ALL equipment connected to the supply (not just the switch) does NOT have the "-" (negative) connected to the chassis ground. If ANY of the equipment does, you will FRY the entire system when it is connected!

Some equipment has isolation capacitors on their boards that lift the negative in the unit, and isolate it from the chassis ground. These units will work, but care MUST then be taken to make sure that anything plugged in down the line further does not the negative grounded.

The standard for the radio and computer industry is "-" (negative) grounding. Telco industry and microwave has a standard for using "+" (positive) grounding.

Crossing the 2 requires lots of pre-planning ! If you further decide to do this, make sure you put up a BIG sign in the facility, to alert and check for negative grounds BEFORE plugging equipment in !

They do make some DC to DC power supplies that will shift the grounding for use on your system.

[yoder]

I know from experience that certain equipment doesn't like the positive grounds. Rocket M5s don't like it but Nanobeams and Nanostations don't seem to care. I think I have a Routerboard 1100 running off a -48 rectifier, but that might be using an inverter as well.

Keith

[jsweere]

I know the subject of this thread kind of changed, but we spent the entire day today labbing the situation in the shop. So I'm pretty positive, my positive ground is the real problem I am having here. All my other equipment is -48v, but the Netonix switch is not. So as soon as the radios mounted on the tower (usually just a mast on top of a building grounded to an electrical conduit) is plugged into the netonix, it blows the port. So maybe I can find a DC-DC converter that will switch the ground for me to be able to power the netonix with this -48v system? Any hints or tips that may help me out?