RCD

[Bill Wright]

We were talking about Christmas tree lights and I said to my sister, "Well if all else fails you've got an RCD. Actually, thinking about it, when did you last test it?" "What do you mean 'test it'?" "Actually, wasn't it fitted when you had the house rewired just after you moved in?" "Yes that's right, I think. It would be about 1978 at a guess." "While you're at it then, see what the rating is. It's wrote on it. It will be a number then 'mA'. Most likely 30 or 100." Later she she rung me from home. "Nothing happens when I press the test button except a click -- what should happen? And it says three zero zero small 'm' capital 'A'. I thought you said it would say 30 or 100? Did you mean 300?" I said, "I think it means you'd better not use that antique washing machine until we get things sorted out."

Bill

[Andrew Gabriel]

Sounds like it is actually an RCD (as opposed to a Voltage Operated ELCB). A 300mA one will have been fitted in the case of an earth rod or other earth connection which possibly has too high an earth resistance to blow the fuse/MCB on a short to earth. It isn't designed to provide electric shock protection (even when it works;-)

In any case, it's not working and needs investigating.

Electrician may insist on fitting a replacement 17th Ed Consumer Unit, which won't be cheap.

[John Rumm]

Might be worth Bill seeing if he can find out what sort of earthing system is in place.

its one of the TN variants, then a knackered RCD is undesirable but you still have some level of functioning protection system. If however its a TT install, then it needs fixing as a matter of priority since without it, she has potentially no mechanism for clearing faults at all.

[Bill Wright]

Thanks John and Andrew Bill

[A.Lee]

Snip no RCD etc.

I came across this last week, TT install, 5.6 ohms earth loop was the best reading I got at a socket. The 30A fuse wouldnt blow if you hold the live cable with one hand and the water pipe with the other. ELCB device at the incomer, how do you test them? You cant, and being as it was 30+ years old, I wouldnt rely on it anyway.

The householder didnt seem too bothered.

Your easiet/quickest/cheapest option is to get a 30mA RCD fitted in place of the 300mA one. These are only £20ish , and can be fitted in a hour easily. The other things the sparky will find can be fixed later. You will have to make sure the gas and water is bonded. Preferably in

10mm cable, but smaller is better than nothing (and could be compliant anyway).

[Andrew Gabriel]

The test procedure is in the 14th edition regs. Can't see mine just now, but IIRC you use 1kW transformer (can't recall secondary voltage, but probably 50V) to generate a high earth current. A pass is caused either by the VOELCB tripping, or by the inability to generate more than a certain voltage (less than the transformer's secondary, 25V?) between the house earth and real earth (which means the earth impedance is low enough that the VOELCB is not actually required, so you won't generate enough voltage different to trip it).

A common misunderstanding is that other connections between the house earth and real earth bypass the trip coil and render the VOELCB protection ineffective. This is not the case - these parallel connections reduce the earth resistance and reduce the need for the VOELCB *providing* the VOELCB's earth rod does not have an overlapping earth resistance region with the other earth connections.

The catch is few electricians nowadays will still have a suitable test transformer (or understand how VOELCB protection is designed to work).

If the test button still trips it, it's *probably* OK, but a modern RCD does provide more protection.

VOELCB's can be vulnerable to false tripping caused by faults outside of the installation (such as a neighbour's earth leakage), and also by lightning strikes.

[A.Lee]

...

Being as I had no interest until the 16th, no wonder I didnt know!

Well, yes, for once, I rang up the Tech helpline to see how to test it, the answer was "dont bother, it is obsolete, recommend a new RCD"

[John Rumm]

There is another difficulty, in that you rarely seem to find one wired up the way it was supposed to be. It can be this that gives rise to the reputation of more operational difficulties than there should be.

If you can find the 14th edition procedure, it might be something interesting to put on the wiki.

[John Rumm]

It would probably not blow if you actually connected the live cable to the water pipe!

Might be doable, but two common snags there are finding a device that fits, and hoping that the earth is actually appropriate for re-use in the first place (i.e. is not a gas pipe for example)

[Graham.]

I noticed that, but decided if he's that tough, discretion > valour.

[Tim Lamb]

In message , John Rumm writes

That'll be appendix 6 section (3) then:-)

I'm too idle to type it out but I'll try scanning when my brain gets to grips with sizing issues.

[John Rumm]

Yes, was not suggesting he try!

In fact relying on any value of MCB for protection from direct contact shocks is at best "hopeful" (unless you literally have balls of steel!) However with working earthing any of the common sizes should give protection from indirect contact.

[Andrew Gabriel]

Sure. Unfortunately, I haven't seen it for a while.

Did I remember the transformer voltages and power ratings correctly?

[John Rumm]

Test voltage not exceeding 45V and a transformer of 750VA

[Robin]

GIYfingersF?

[Tim Lamb]

In message , Robin writes

for historical reasons:-)

Mind, most of the wiring in my outbuildings complies with the 14th!