"Lurch" wrote in message
...
On Mon, 9 Aug 2004 14:51:52 +0000 (UTC), "Rick Hughes"
strung together this:
The Best Practise oft talked about is not to have lighting on a 30mA trip
duwe to nusisance tripping, many on this group for example have advocated
no
protection for light circuits.
Now I prefer to have some protection to prvent fire hazard, and thus have
all light ccts through a 100mA RCD which are much less prone to nuiscance
tripping.
You've obviously mis-read something somewhere. RCD=earth fault
protection, MCB=overload protection.
RCD's work by measuring the amount of current on the live cables and
ensuring they are equal, if they aren't, by an amount determined by
it's rating, then it assumes the current has gone to earth, either
through a faulty appliance or you, and trips.
A light bulb isn't earthed so wouldn't take out an RCD. You're
thinking of MCB's which protect final circuits, light bulbs can easily
take out type B MCB's as the bulb takes a surge of current as it blows
which overloads the circuit but by fitting a type C or a HRC fuse to
the lighting circuits this nuisance tripping can be reduced.
Type C MCB's and HRC fuses react slower so won't always trip\blow.
If that's what you based the 3 classes of protection on then you might
not be the best person to be doing this project.
My question is really pretty simple - do I protect Imeesrion and cooker
or
have them unprotected.
AFAIK the regs do not require protection, but that doen't mean i t's
best
practise.
The immersion doesn't require RCD protection. The cooker doesn't
either, unless the manufacturers instructions say so or if the switch
incorporates a socket that can be reasonably expected to supply
portable equipment outdoors.
Been reading through this thread with some interest.
Correct me if I'm wrong, but I can't see the 100mA RCD offering any useful
protection against fire hazard at all in this installation, and nothing
useful against electrocution.
Rick correctly identifies the need for 30mA RCD as protection against shock
hazard for socket circuits, the rationale of course being that if you touch
a live part then as soon as the current through you to earth reaches 30mA
the protection trips, disconnecting the supply. Fine, so we certainly
install 30mA non-time-delay on all circuits likely to be used for any
external portable appliances, and personal preference dictates whether other
socket circuits (such as upstairs) are similarly protected. Some appliances
within various zones of the bathroom will also require this protection.
This doesn't provide any protection against circuit overload (one of the
causes of fire in fixed wiring), though, because unless it's implemented as
a RCBO then this isn't it's job - the MCB takes care of that. As I see it,
the other likely cause of fires (loose connections in the circuit giving
localised high resistance and therefore heat generation) isn't protected
against by either the MCB or RCD.
So, what's the 100mA RCD for? It can't be against electrocution, because
100mA represents a fatal shock hazard. Whole-installation 100mA time-delay
is used in TT systems due to problems with the earth impedance and
disconnection times for MCBs under fault-load conditions.
I don't see that earth leakage is a significant fire hazard. The most
likely ones are chronic circuit overload (MCB's job to protect against this)
or loose connections (and neither will protect against this, unless the
insulation breaks down to such an extent that there is significant leakage
to earth, and once this happens then it's likely to produce a fault current
that the MCB will protect against).
Not sure about reports of "nuisance tripping" that the OP talks about
either. All the ones I have read have been concerned with blowing bulbs
taking out the MCB due to high fault currents generated. Either that, or
deprecated whole-house 30mA RCDs taking out all the lighting circuits due to
earth leakage from either a portable appliance on a socket circuit, or
mineral insulated heating elements leaking to earth. There's a jolly good
reason why whole-house 30mA protection is deprecated.
Finally, for the OP, there could be a very good reason coming up on 1/1/05
why you shouldn't start introducing unneccessary RCD protection. (as
opposed to "necessary" RCD protection - I'm NOT advocating that they are
always unneccessary !)
My reading of Part P is that RCDs are always "notifiable" devices for the
DIYer. So, if your superfluous 100mA RCD develops a fault, and you're the
type to obey the letter of the law, you've just landed yourself with a bill
for a self-certing spark to change it, or for BCO approval.
--
Richard Sampson
email me at
richard at olifant d-ot co do-t uk