In article ,
Stephen writes:
Hello,

We've had two thunderstorms inn the last three days. I realise I am
not sure what are sensible precautions to take and what are old wives'
tales: unplug the tv and don't use the phone seem plausible but keep
away from radiators, windows, and don't have a bath? Are they good
ideas or just mad?

There are two different events to protect against.
A direct hit, which is very rare.
High voltage pulses delivered via your services, due to a
nearby hit, which is more common.
(Local situation can adjust the relative importance of these.)

There are two different things to protect.
Injury and death.
Your property (both home and electronic devices).

These two orthognal sets create 4 cases, and handling each of
these should be done as a separate risk assessment.

However, in most cases, the output of a risk assessment in a
domestic situation is likely to be "change nothing".
I might only add on top of that, avoid using a landline phone
or handling the TV and aerial whilst there's a lightning storm
overhead.

See if your insurance covers damage due to lightning.
In domestic situations, that's about the only protection of
property which is generally of any use. Nothing much

Our tv equipment is plugged into one of those Belkin surge protector
extension leads, as is the computer but they look untidy on the floor.
I was thinking about getting more sockets put on the ring main to tidy
up these cables. Are there any cheap surge protector sockets, or are
the extention leads the cheapest way? I see that CPC sell some sockets
but IIRC they are £14 each for the surge protection and even more for
ones that filter RFI. By the way, is the RFI filtering worth having?
Do you know of any cheaper ones? I would have thought there would be a
demand for these protecting equipment in offices and industry?

If you do the maths, it's cheaper for the manufacturer to actually
put nothing in domestic surge protectors and then just pay out for
equipment which gets damaged.

If you want effective surge protection, that's something which has
to be designed into an installation when the wiring is laid out and
when the services are initially connected up to the house. It's not
something you can do effectively by buying a few surge protectors
and adding plugging them in to an existing domestic system.

If you're talking commercial installation and paying commercial
prices for protection, then full protection against even direct hits
is standardly provided. It would cost you many times the replacement
cost of all your domestic equipment and wiring though, which is why
it's not viable in a domestic setting.

What about telephone and tv protection? Other than the extension
leads, are protected sockets available?

Yes. It's done by having the phone cable enter the house as near as
possible to the main earthing terminal, and routed (in steel conduit
if it's any distance or unavoidably near any other wiring) to that
terminal where a surge protector is fitted (usually with gas
flashovers) and a short fat connection to the main earth terminal.

I was thinking about the aerial cable. If lightning strikes, the
current will have to travel down the cable to the suppressor in the
extension lead. Won't it have melted the coax by the time it gets
downstairs? Shouldn't the suppressor be as soon as the cable enters
the loft?

In a commercial setting where protection was required, the aerial
pole would be earthed as part of a the lightning conductor system.

To complicate things even further, we live in a dip, so we have to
have the aerial on a 16' pole and even then we need a masthead
pre-amp. That amp requires a power supply, so would I be right to
think the suppressor has to be after the power supply? Would it block
the power otherwise?

The aerial wire in an incoming service, and handled like the phone
connection above, if you really want to do this properly.

Is there anything I can do to prevent the 16' pole attracting
lightning?

No, but you can make sure a strike on it is harmless, if you want
to throw lots of money at the problem.

Whilst I realise the odds of a lightning strike are 1 in
3,500,000, there was report on the news about an unexposed house in
Derby being destroyed after lighting hit their aerial.

I guess these precautions only protect against small charges from
distant lightning? I guess a direct hit would be so powerful it would
melt whatever earthing or suppressors you had connected?

This goes back to what I said at the beginning. You have to decide
which type of event you are trying to protect against. You can do
both, but unless you have several million quid of electronics, such
as a local area phone exchange in your home, it's unlikely to be
worthwhile.

BTW, BT equipment takes about 600 lightning strikes a day, across
the UK.

One last question: is there some sort of surge protector you can fit
at the CU to protect the whole house?

Depends on how all your services are routed.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]