Split load CUs - why?

[ban-all-sheds]

Why would you want only some of your circuits to have RCD protection?

Or, to put it another way, why would you want some of your circuits to not have RCD protection?

 

[breezer]

its the lights that have no rcd protection. its to stop the inconveiniance of everything being turned off if a lamp goes (as they do)

 

[ban-all-sheds]

Ah - now there's an interesting thing. My CU is not split load, and whenever any of my bulbs blow, it trips the MCB on that circuit, but not the main breaker - I don't think I've ever had that happen.

But could that be because it's a 1980's vintage 100mA ELCB, and a tad less sensitive than a current (groan) 30mA?

Sometime next year I will probably be replacing the CU with a new one, which will have a 30mA RCD - would a split load one be a better idea? Bulbs do pop with depressing regularity, (especially the ******* recessed RO80s - I'd never have those again), it would be a right royal pain if every time that happened it took out the whole house.

 

[spark]

Have you got a TT system? Just wondered why your whole installation on a 100mA RCD. That is usual for TT systems and if that is the case you'll probably need a 100mA/30mA split load board when you replace it. Lighting etc on the 100mA side and sockets etc on the 30mA side.

 

[il78]

Circuits which are likely to be used for outdoor use should be protected by a 30mA RCD (usual circuits in a domestic premises are downstairs sockets, cooker circuit/13A combined socket, garage sockets etc)

ELCB's and 100mA (domestic) RCD's do not comply with current IEE wiring regulations.

When your lamps blow it doesn't usally cause a fault to earth, more the case of the two filaments to touch causing your MCB to trip.

 

[breezer]

if you think about it where is the erarth in a LAMP, there isn't one, its just that rcd's seem to trip when lamps fail

 

[il78]

Never known of an RCD tripping when a lamp blows.

 

[ban-all-sheds]

Have you got a TT system? Just wondered why your whole installation on a 100mA RCD. That is usual for TT systems and if that is the case you'll probably need a 100mA/30mA split load board when you replace it. Lighting etc on the 100mA side and sockets etc on the 30mA side.

Well - although I've never asked, or thought to ask or check, I wouldn't have thought it's TT - I live in an urban area, it's not an overhead supply, and what I have is an earth connection that disappears off into the ground in the same place that the two cables that go into the fuse that sits before the meter. I've never looked closely to see if I can tell if it connects to the neutral line there, or its armour, or not. I've never seen any local earth rod - if I have one then no part of it is visible in my house.

I've never seen any warnings about PME (but then there's no guarantee that previous occupant(s) haven't removed them), and the earth wire does definitely not go into the service block or the incoming fuse holder, so I'd vote against TN-C-S.

Without more rummaging under the stairs I can't be certain, but if asked to pick an answer from a multiple-choice question I'd say I had a TN-S system.

As for why my CU has a 100mA ELCB - that is almost certainly because that was what was commonly available when I rewired my house in 1987 - I didn't do any research into the types of earth leakage or residual current protection available, or what type of earthing system I had - I just went to a shed and bought an ELCB consumer unit and MCBs to replace the on-off switch and rewirable fuses that were already there.

If this behaviour is significant, then you might be able to tell - connecting live to neutral or earth (I've not done experiments to see which) on a circuit which is switched off trips the ELCB. I found this out the first time I added a new socket - I switched off the appropriate MCB, made sure I had switched it off, plugged a lamp into a socket on the ring to make doubly sure the juice was off, cut through the cable and bing! - everything went out. Being in possession of sufficiently little knowledge to be dangerous I put it down to induced current in the cable as all the cables leaving the CU run in a parallel bunch for a few metres (no, I didn't know about derating calculations either, but in 16 years I've never smelled burning... ).

Now - I know that the sheds aren't the best place to go for qualified advice, and when it comes to electrics everything comes with a sticker saying "If in doubt consult a qualified electrician”, but I would have thought (hoped?) that if the earthing scheme was significant then there would be some sort of warning on CUs about to which sort of system it was appropriate. A DIY chappie like me could well be perfectly competent and confident to pull cables and connect up switches, lights, sockets etc., and do it all safely, but not have a clue that particular types of RCD are not suitable for particular types of earthing arrangements.

Presumably if I ask my supplier they can tell me what I’ve got?

 

[breezer]

i should say taht as you said you have a 100Ma rcd as that was the noem then,but if you ask nicely they may change it foc

 

[spark]

I'm not sure where your information is from il78, but 100mA RCD's certainly are permitted and in some cases are actually required to ensure compliance with the wiring reg's. If you've got your reg's book handy, check out 413-02-19/20. In many cases the sole use of MCB's will not be adequate to ensure disconnection within 5 seconds in TT systems, therefore EEBADS cannot be used as the sole means of indirect shock protection. I'd be happy to run through the calc's if you're still unsure; did you mean to say that 100mA RCD's are not permitted for use as supplemenary protection on sockets reasonably expected for use outdoors?

Circuits which are likely to be used for outdoor use should be protected by a 30mA RCD (usual circuits in a domestic premises are downstairs sockets, cooker circuit/13A combined socket

Do you generally expect that the socket-outlet from the cooker control unit be used outdoors? I don't disagree with you, I too put that circuit on the RCD side, but not because it may be used for equipment outdoors.

Anyway, back to the point. I have come across 100mA RCD's protecting the whole installation and they've been on TT supplies which is why I asked the question. If you have got a TT system, you almost certainly will need an RCD to protect the whole installation, in addition to the 30mA RCD protecting the socket outlets, and the rating of this RCD will depend on the resistance of the earth path. A 100mA unit would be the safest option though higher ratings may be permitted depending on conditions.

 

[spark]

Very unlikely to be TT then ; go for a standard split-load and ignore the 100mA RCD.

 

[il78]

Thats what I said:

Circuits which are likely to be used for outdoor use should be protected by a 30mA RCD (usual circuits in a domestic premises are downstairs sockets, cooker circuit/13A combined socket, garage sockets etc)

I've had many arguments with Mr NICEIC over the RCD thing....only due to commercial premises don't need to be RCD protected......cause there not likely to use equipment for outdoor use....same with schools, we carry out Periodic Inspecting to schools in the area and snag no RCD's present..but if the headmaster sends out memo's to his staff saying not to use equipment outdoors, this will satisfy the NICEIC / regulations. Anything in domestic and its got to be covered

And I should of said 100mA RCDs not permitted to systems other than TT systems........to circuits other than sockets.

Thanks for making dig out the regs though........[/quote]

 

[spark]

oops, clicked twice

 

[spark]

I've dealt with of a few of the NICEIC 'reps' over the years and many of them are just satisfying their own ends by giving themselves a false sense of authority. I would argue that whoever carries out the inspection has the say so about what is and is not safe. I think if you feel it needs RCD protection you are well within your rights to flag it up. I've written items out on the periodic sheets which both my boss and the NICEIC guy has disagreed with but there's not a lot they can say unless they are the ones signing it. In my opinion, a memo to staff would not be sufficient for me to be satisfied that no person would use a socket outlet outdoors. I think if you feel strongly enough you should argue your case, some of them back down if you put up some resistance, especially if you know your reg's book well .

 

[ban-all-sheds]

Never known of an RCD tripping when a lamp blows.

Mine doesn't - I said it's the MCBs which trip (with depressing regularity - why do those poxy 100W RO80s have such a short life?? )