Re: Question About Wiring Regs.

[Nigel Orr <nigel@xxxxxxx>]

At 03:17 31/10/00 -0800, you wrote:
>I seem to recall folks saying that you should not run Low Voltage
>cables beside mains, to comply with wiring regs.

Not quite like that.  You shouldn't run mains cables beside any other
cabling if there isn't adequate insulation between them, but that's not
usually a practical issue (mains twin and earth is insulated
adequately).  The real problem is that the mains current will induce noise
in the low voltage cables.  The extent of the noise induced is dependent on
the mains cable type, the amount and frequency of current flowing in it
(50Hz and induced harmonics), the distance to the low voltage cable, the
type of low voltage cable, and the devices connected at each end of the low
voltage cable. (Phew!)

Because that's so complex to calculate, a 'rule of thumb' of separating
mains from control/signal cables by about 60cm/2ft is usually adopted.  I
have some articles from various journals with some more detailed specs at
home, which suggest smaller spacings for different classes of cable but
basically, get them as far apart as possible, but anything over 1m won't
help in most situations.

>itself as the light switch through something like a crydom SSR. This
>would seem to need LV and Mains in the same pattress box.

That's a different issue, where the risk is direct connection of exposed
mains conductors to exposed low voltage ones.  Best/only solution is a
physical barrier between the sections (eg plastic, with an opto-isolated
electrical interface), the exact way to do it safely is probably best left
to someone who knows their stuff, as there are some potential hazards which
might not be obvious (things like "what happens if this wire comes
loose
from a terminal block and pings out against this one".  )

>What I don't want to do is to make my house unsellable because it
>doesn't meet wiring code. What's the opinion of the assembled throng?

First off, sadly, it's unlikely that anyone will ever find out.  Even a
wiring check is unlikely to even notice that there are odd cables into some
pattress boxes... so it's more an issue of making it "actually
safe" than
just making it conform to regs.  IEE wiring regs mostly ignore connected
appliances and just work towards the safety of the mains wiring
installation.

If it's a commercial product, the isolation should be adequate for the
purpose, so just install it carefully (cable grommets where it goes through
metal cases, adequate fusing and earthing etc)

If it's homebrew, and you think about the design carefully, it's unlikely
to ever result in a short between mains and low voltage, but the
consequences could be very serious.  It's not unusual for an enclosure to
have mains and isolated low voltage inside (eg mains adaptors!), you might
find some useful ideas from taking other equipment apart and seeing how
it's been done.

Personally, I'd recommend you acquaint yourself with a professional
electronics control engineer who knows what they are doing, and pass the
risk to them!  And no, I don't mean me :-)

Nigel


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