17th ed and consumer unit replacement - OT query

I have an old consumer unit (Wylex) with rewireable fuses.

If I decide to replace it, will I be required to use RCD/RCBOs on all circuits (either one 'whole house' RCD, or one per way)?

Currently, there's a single ring serving the whole flat (approx 85 sqm) and a recently installed ring serving the white goods in the kitchen (replacing a spur bodged in by a previous occupant). Given a fair number of switch-mode power supplies on the main ring, I'm worried that it'll exceed the leakage budget. Would a better technical solution in that case be to replace all the sockets on the main ring with RCD protected sockets, is that allowed/necessary under 17th ed - or is all this unneccesary and the lack of RCD protection 'grandfathered' in?

With my luck, I should probably have replaced the CU when I had the chance before the 17th ed came into force.

Cheers (or maybe not)

Sid

(Oh, and sorry for being strictly off topic - I won't be replacing the CU personally - I don't have the appropriate test equipment available, and more to the point, even if I hired it, I don't have the knowledge to use it properly)

Reply to
unopened
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Neither probably...

Whole house RCDs are not really acceptable since they offer no discrimination in the event of a fault - one trip and you lose everything. They are also prone to nuisance tripping as a result of excessive combined leakage from all the circuits.

RCBOs on each circuit would certainly be acceptable, but expensive.

A more typical approach is to divide the circuits up between a number of RCDs. Two would be a minimum, and would usually pair downstairs power circuits with upstairs lighting circuits etc. In a flat then perhaps power circuits on one, and lighting on the other would be more appropriate.

Note it is also still allowable to have circuits without RCD protection. However one has to ensure that the cables are not liable to damage. That means either burial at >= 50mm or protection by earthed shielding of some sort (e.g. metal conduit, or via a metal screened cable)

No, since an RCD protected socket does not offer any protection should you nail through a cable supplying it.

If you were close to the leakage budget on your main ring, then the answer would be to split it into two rings, and place each on a different RCD.

Reply to
John Rumm

Would a better technical

That makes sense.

Thanks John. So I should have replaced the CU when I had the chance. Splitting the ring will be the devil's own job. The wiring is in metal conduit, but I'm certain the continuity wont be good enough for it to be classed as adequate, even if earthed. I feel a bit like St Augustine of Hippo - 17th ed. good, but ...not yet!

Best regards,

Sid

Reply to
unopened

Or run the cables on the surface: AIUI it's only for cables buried < 50mm the RCD becomes necessary.

Reply to
YAPH

would earthing the existing metal conduit and thus not needing RCDs be a goer? Or have I missed something....

jim

Reply to
jim

Yup, also true.

Reply to
John Rumm

Perhaps keeping it but providing an additional one (or radial) might be another solution. You will probably find that the leakage issue is a non problem anyway. Its quite common in older 16th edition style installs using a split load CU to have all the power circuits on a single RCD and just the lights and some fixed appliances (immersion heater, cooker etc) on the non RCD side.

Note also that you have already split off some of the load for the separate kitchen ring, and that could easily have its own RCD.

If you have a sparks doing the work for you then he will be able to test its adequacy easily enough. Same goes for splitting a circuit - they are used to getting wires through places you might have thought impossible.

I think you may be worrying unnecessarily, the 17th edition does not really change the fundamentals anyway. Even a 16th edition install would typically have all general purpose power circuits on a RCD anyway.

Reply to
John Rumm

I think you'd have to guarantee that the conduit formed a good earth for its entire length so that when muggins blasts through it with his electric drill and contacts the live conductor it passes enough current to earth to operate the circuit protective device (fuse/MCB/whatever). Unless the conduit is substantial screwed steel that's unlikely to be the case: most conduit I've seen in domestic installations is rolled thin metal sheet joined with similarly flimsy clamps at junctions, none of it guranteed to give a good circuit.

Reply to
John Stumbles

But John, the clamped conduit dates from the 50's & 60's. Normal steel conduit supplied these days is fairly substantial but I admit rarely installed domestically - I've put in quite a bit in semi comercial situations (in my own homeworkshop ( hobby but rather extensive)) - but I always run an earth (sorry CPC) conductor as well.

AWEM

Reply to
Andrew Mawson

"John Rumm" wrote

Can't you simply earth the conduit at the point that it enters each back box by linking to the socket earth? In this way you cover the vertical drop that is likely to be drilled through and continuity of these to the horizontal under-floor runs is not an issue.

Phil

Reply to
TheScullster

Its the old timers.. he is thinking off how they actually ran the protective earth using the conduit. I agree that if the protective earth is in that cable earthing the conduit at either end should comply. Its no different to SWA which only (in some cases must) be earthed at one end.

Reply to
dennis

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