[mythtv-users] OT:Protection of htpc from power surge through incoming cable

Simon Hobson linux at thehobsons.co.uk
Wed Nov 6 12:00:47 UTC 2013


Rajil Saraswat wrote:
>I have a cable connected to my settop box which feeds analog video via
>composite cable to my htpc (using PVR 500). The htpc is connected to
>the LCD TV via hdmi.
>
>Recently, there was a power surge and the TV blew off. We suspect that
>there was a power surge through the incoming cable which killed the
>TV. The TV technician said that the TV PCB is all black.
>
>Is there any way to stop power surge through the incoming cable in the future?

It's very tricky getting this sort of thing right - and many people do it wrong !

Example of getting it wrong. User has a problem with surges in the phone line damaging the modem. So they get a surge protector extension lead with phone sockets - and routes the phone line through it as well as plugging in the modem. The next surge is diverted into the earth wire of the socket strip, so the earth in the socket strip now has a spike on it. This feeds up the mains lead fo the modem and puts a spike into "the electronics" which now appears down the serial line to the PC - so now instead of damaging the modem, the spike damages the PC as well as the modem ! If the PC is also plugged into the protected socket strip, then something else (eg printer) gets damaged as one eld of the parallel cable gets a spike while the other doesn't.
Yes, it's an old example, but you can translate to current technology fairly easily.

What you really need to do is create a protected zone. Nothing (power, data, antenna cable, ...) should connect through the boundary of this zone without going past a single point - and at this single point, everything should go through suitable surge protection connected to a common grounding point.

The idea is that any external surge coming in through a cable gets shunted (mostly) to ground via the surge protector. This may well cause a momentary change in potential of the grounding earth - but by having everything within the protected zone sharing one set of protection, the whole lot changes potential at the same time (ie any residual surge is common mode across everything).

Where people go wrong is they'll put a surge protector on one part of the system, or they use multiple surge protectors that aren't co-located - and when a surge comes in, part of it gets converted into a different one that then goes and affects other parts of the system. Remember that if we are talking about lightning induced surges, these have very rapid rise times and you start having to think about cables as inductors and normal DC/low frequency AC rules just don't apply. Eg, just a few yards of cable (even fairly heavy cable) can appear as a high impedance to these very fast rise time pulses).

So for full protection you need to route everything coming into the house via one point - that's mains power, phones, cable services (unless fibre in a metal/condutor free cable), aerials, etc. Take everything through surge protectors, which must all be bolted to one heavy piece of copper - and then earth the copper. Don't forget metallic water or gas pipes - in the UK these are required to be bonded to the mains earth (typically using 10mm^2 cable) but a length of this cable is a high impedance to lightning induced spikes.


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