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Topic Title: Machine lightning/transient protection.
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Created On: 13 January 2015 09:32 PM
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 13 January 2015 09:32 PM
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A client has a very expensive ground-source heatpump installed.
The place is in an exposed rural position - phone line is overhead, 11kV to local Tx overhead too. The machine is connected to the 'net through a separate interface box.

Last year all the circuit boards were damaged during a stormy period. It also took out the internet interface ~ though strangely enough, the broadband box and everything else connected to it all survived...

Specifying transient protection is a bit out of my field..
Can anyone suggest suitable kit for the Cat5 coming into the machine, and its 50A 1ph feed? Is this the sort of stuff that the likes of RS carry, or is it more specialist than that?

 13 January 2015 10:11 PM
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Can anyone suggest suitable kit for the Cat5 coming into the machine

My advise is don't just put SPDs local to the appliance. The SPDs short the pulse to earth so you get a large current between the source of the pulse (lightning) and the SPD - any electronics in between (router etc) gets fried. (I learned that the hard way years ago after I installed a small phone system at my parents place so Mum could phone Dad at the other end of the house rather than shout as his hearing was getting worse - the home PC got a surge arrestor extension lead (power and phone line - pre broadband days) and the next thunderstorm saw the PC's modem survive, but the expensive PBX didn't. Relocating the phone line SPD to the incoming line before the PBX solved the problem. SPDs at the earliest opportunity when services (mains, phone) enter the installation. Extra ones further in if you wish, but don't skimp on the first line ones.
- Andy.
 13 January 2015 10:44 PM
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Also, note that the phone lines are not earthed at your end, and in a sense import something close to the ground potential at the exchange. On average, yes, that's close to your local ground voltage, but may be many hundreds of volts off local ground during a nearby strike.
Therefore having something that flashes over if the incoming lines go off earth by more than a few hundred hundred volts, as opposed to the normal suppression that is just between the phone lines themselves.
As Andy says, given the time it takes for signals to travel up wires, (say a foot per nanosecond) there is no point at all having a breaker that takes perhaps 10 nanoseconds to break over, right close to the device, or discharging to the end of 10 feet of earth cable. It needs to be at least 10nanoseconds back up the wire from the potential victim,and needs to be the shorter fatter path through the surge arrestor, than through the device that will other wise take the hit.

Quite often it helps to re-route the lines as they come in so they all 'call-in' first at a common earth terminal with all the first line defence, that is near true ground as possible and then from there go onwards some distance to the devices to be further protected.

Mil handbook 419 A is probaly more than you need, but has some nice sketches of protecting cables as they enter a facility.
try about pages 1-70 to 1-80

regards Mike
 13 January 2015 11:42 PM
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run the Ethernet through a couple of fibre optic transceivers and a short length of fibre optic cable.


"If you try and take a cat apart to see how it works, the first thing you have on your hands is a non-working cat."
 14 January 2015 06:36 AM
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It is unlikely that the appliance meets all the requirements of its CE marking. I suggest you contact the manufacturer for assistance in ensuring that it does. Ask for copies of its EMC tests results, particularly the sections on transient tests. Fiddling with the installation will noy make any difference if it is inherently faulty.


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