I have seen many SS diodes clamp and appear to never de-clamp. So they
get tossed in a bad pile. Then we test them some time later on the bench
and they work fine... until they're in the field again and we have weird
problems and can't figure out why.
On 2/19/2016 2:15 PM, Adam Moffett wrote:
I would have assumed they weren't supposed to short the + and - wires
when clamped. That would be the opposite of protection lol. I also
don't get why the short to stay there with the power shut off. I
spent a good 15-20 minutes checking things with the power shut off,
and the short didn't go away until I disconnected and reconnected
wires on the SS. I would have guessed the clamping would stop when
the over current or over voltage condition went away.
I did eventually find literature saying that the 200SS can take up to
6.5amps. This BS uses 90 watts on bootup, and maybe 60 watts in
normal operation. The spec sheet says 125 Watt max. So somewhere
from 1.5 to 2.5 amps.
Maybe that particular SS is just broken in some way? If so, it's a
dangerous mode of failure.
On 2/19/2016 2:30 PM, George Skorup wrote:
Definitely sounds like the SS was clamping. Maybe the SS thought a
surge occurred at initial startup of the power supply? And FYI, I
have used Traco TSP 48v supplies with -48 radios just fine, output is
floating.
On 2/19/2016 10:40 AM, Adam Moffett wrote:
...my first thought of course was maybe it was hooked up wrong the
first time. The four 16ga wires hold their shape pretty well and I
didn't have to re-bend them to fit, so I'm pretty sure they were on
there correctly.
People talk about SS's "clamping" or "clamping to ground". Could
that cause a short? I never tested the power wires against ground.
If you're thinking +48 and -48 mixed on the tower, there are only
two devices on the tower right now. One base station and one
backhaul, and they're both -48. There's some 24v stuff in the
enclosure at the bottom (switch, site monitor), but none of those
have a ground referenced to power and they're isolated from the
other stuff by a 48v to 24v converter.
On 2/19/2016 11:27 AM, Adam Moffett wrote:
So I went out to a new location yesterday afternoon just to turn
the breakers on. The power co had just plugged in the meter a few
days prior.
The Traco 48v power supply there immediately went into an alarm
state. After some troubleshooting I found a dead short on the -48v
power bus (terminal blocks with jumpers actually). The short ended
up being on the power cable to a Wimax base station, I read 0 ohms
between + and - on that cable.
There's a Motorola 200SS surge suppressor within 5' of the power
bus, and then about 80' of wire to the base station. I checked
resistance between + and - on each side of the SS and read 0 ohms
both ways. There are actually 4x 16ga wires going to the top, so
two were screwed down to + on the load side of the SS and two on
the - side.
That whole part was backstory. This is where the mystery starts.
I pulled all 4 wires off the load side of the SS to see if I could
narrow down which pair was shorted, and I couldn't find a short. I
checked the load side and the supply side of the SS and it was open
on both sides. So then I reconnected the wires to the SS and
checked again - no short. I plugged the BTS back into the
bus...powered up just fine.
So where did the short circuit come from and where did it go? In
hindsight it seems like it had to be in the SS or before it. If it
was shorted at the top there would have been > 0 ohms of resistance
with the 80' of wire.
