[mythtv-users] Protecting against power surges

[Simon Hobson]

Rob <bertaboy at gmail.com> wrote:

> Does anyone here have good recommendations for how to properly (if there is such a way) protect their gear from electrical surges?  Depending on the environment, surges can enter the network via the AC lines or networking lines.  I've traditionally relied on metal oxide varistors (MOV), but began reading up on them lately and am feeling less comfortable with the idea of a device degrading and silently failing.  Is there any reliable protection which doesn't involve air-gapping the networking lines and running the protected electronics behind series-mode surge protectors?

As you've hinted at, it's not a simple problem. And one that some people make a good living from dealing with !

I've seen first hand the problems of multiple connections into a device - my boss from a few years ago lived out in the sticks and had all the problems caused by overhead power and phone lines coming via different routes. Every time there was a good thunderstorm, they'd lose fax machines etc. Plain phones are fine as there's no path for the surge to go from phone line to anywhere else, but a modem or fax is connected to a power supply and so there is a path to earth - with the surge bypassing the limited isolation on the line interface of the device.

Worst I've seen was a customer's laptop. He worked in Madagascar where they have "proper" thunderstorms. A strike had gone in via the phone line, jumped the isolation transformer (leaving heat distortion on the moulded case), blown the modem chip apart, and then arced over to the main logic board.

The most basic requirement is that ALL the connections should go via one point where you apply common protection. The reason for that is that lightning strikes involve very fast rise times, the sort of speeds where you have to consider the inductance of cables - we're into radio frequency thinking territory. In effect, even thick cables represent a high "resistance" to lightning induced surges. So if you have separate power and network protection, separated and connected to earth(s) via cables, then the inductance of the connecting cables means that you can still get serious voltage differences between them.
If you put all the lines together, then the local change in earth reference doesn't matter - everything on that device sees the same common mode voltage on all lines.

The problem with that is you run the risk of sending the surge back out through the protected lines and damaging any equipment that is outside your protected zone.

So it really comes down to working out what the risks are, and how much you want to spend !
If you want to go overboard, you'd use a double-conversion UPS (you won't find those in the budget section) and fibre-optic connections for the network(s). Nothing within the protection zone would connect via anything but the UPS or the fibre network connection.
For a budget setup, I'd be looking towards grouping as much of my equipment (including network switch) in one place so it can be inside a single protection zone. Protect it with good quality surge protection (spark gap + MOV ?) and filters on both power and network. In a domestic (and most commercial ones as well) you'll struggle to find a "good" earth short of digging up the garden and burying large quantities of earth grid. So you'll just have to rely on the mains earth and accepting that surges will cause local changes - hence the need to a single protection zone.

For stuff outside "the zone", make another smaller protection zone in the same way. Use another network switch even if one isn't needed - it can act as a sacrificial device for larger surges that might get through and is cheaper than most devices you'd be protecting.