Burnt Ethernet circuit after years of operation

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#1
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Can anyone throw in some thoughts on this?

 

Alarm panel with an Ethernet connection to a Cisco switch.  The alarm panel goes offline.  I see the board is charred near the Ethernet RJ45.  The circuitry looks like this:

 

RJ45 ----> TVS diodes ----> Isolation transformer ----> Controller  

 

The TVS diodes are blown off the board and the board is charred around them with blackened PCB.  This would tell me the switch it was plugged into had a spike?  The switch is plugged into a UPS though.  A very expensive one.   The PoE configuration on the switch is set to "never" so there should not have been 48 volts applied to the port ever.  

 

Why would they protect an isolation transformer with TVS diodes?  The panel manual says to not allow PoE or the TVS diodes could be damaged.  The switch has not been touched for years.  What would cause this to happen?  Any ideas?  Something catastrophic happened, any thoughts would be helpful.  Thanks!

 

Could a switch go haywire and cause this?  There were no storms in the area, for those who think it may have been lighting.  Mystery....

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Earth fault?

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cnewbie wrote:
The PoE configuration on the switch is set to "never" so there should not have been 48 volts applied to the port ever.  

Never underestimate the capacity for users (or even installers) to do Really Stupid Things!

 

I have seen alarm boards where the telephone interface has been charred much as your describe - and it turned out to be that mains power had been connected to the phone line terminals!

 

surprise

 

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Kartman wrote:

Earth fault?

 

That was my first thought.

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Use a multimeter to help look for unknown source due to component failure, incorrect component polarity, dendrite short, mislabeled switch....

It all starts with a mental vision.

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earth fault,

some wise guy did put 48V on the other side of the cable not knowing that the switch does not like it.

Note that the other side might have gone in to overload protection only after the TVS diodes had already burned and they then just started using another port as there seemed to be a wiring issue with this specific port.

We had a new infrastructure when we came here and some of the RJ45 port did not behave as expected, turned out that connections were missing or reversed, so in that respect you can expect everything, even from a well known installer.

 

still might be the result of a Lightning strike the fields created might have damaged the diodes a long time ago and they might have been slowly deteriorating, until breaking point. At that point they just collapsed and caused the short and the unit to fail.