Switch Failures

[Andyhall]

In the last few months we have twice had the same type of problem concerning switches on our network. The power has been isolated by electricians working in part of the school. When the mains power has been switched back on to the particular building that had been isolated the mains power to at least one switch has failed in the network cabinet. The switch is then broken. Other switches in the same cabinet are OK and the cabinet was protected by a UPS. I would have thought the UPS would have protected the

switches from any pwer surge caused by switching the mains power back on at the curcuit breaker

 

This happened a few months ago with a 3COM 4200G switch and has now happened again in a different building with two Netgear Gigabit switches JGS516. This is causing us problems in both time and money.

 

Anyone had this type of problem before?

[Lurch]

It won't have been a surge but the switched mode power supplies in the switches that will have failed, usually just the capacitors going pop (quietly, usually). They like doing that when you power cycle them after they've happily sat there working for Months\Years. Anybody with anything made by Linksys will have had the same problem at some point.

[Andyhall]

Thanks for your comments. Is there anything we can do about this besides disconnecting all the switches from the power supply before we switch mains back on and then switching each unit on one at a time.

[Butuz]

Ideally you should run a small UPS in every cabinet for your switches - to protect them from surges and sags, and also to give you 10 mins or so of network uptime even if the power fails.

 

Butuz

[Lurch]

Not a lot you can do really, other than running them through a UPS which you're doing anyway. If they're going to go they're going to go. Depending how old they are I'd have a go at sending them back but failing that they will most likely be a fairly easy repair, cheaper than new switches anyway.

[Rammie]

My guess is that your switch failures are indeed switched power-supply faults, but the most common problem causing your symptom is annoyingly simple. It also affects TVs, videos, DVDs etc etc. Anything with a switcher, in fact.

 

99 out of 100 switched-mode power supplies use a 'soft-start' resistor to get the input oscillator running at power-up. After the transformer starts to transfer energy, a feedback winding and rectifier feeds the input chopper 'properly' and the soft-start resistor has no further function (until the power is disconnected again). Snag is that many manufacturers spec this resistor badly and it can burn out during normal operation without any external indication. You won't know until you pull the power and plug it in again later. I concede that this is not the only fault you can have, but it's a very common one.

 

The resistor is usually in the range of 120K to 470K and it will have about 300 volts across it during normal operation. Regrettably, most modern resistors are designed for low voltage circuits and although the power rating is not exceeded, they can nevertheless fail. I've fixed loads of power supplies by replacing the burnt out resistor with a high quality 2W metal oxide component. CAUTION! working on power supplies is only for experienced engineers and technicians!!! There are lethal voltages inside the box, which can be sustained long after disconnecting the mains.

 

Hope this helps someone:)

[Oops_my_bad]

I am considering a simple 1u "on-line" UPS for our core switch, and surge strips in all the satellite cabs - is this a worthwhile thing to do bearing in mind what rammie has said?