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)

wrote:

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)?

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)

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 -

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

or is all this unneccesary and the lack of RCD protection
'grandfathered' in?

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.




--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 21 Sep, 22:47, John Rumm wrote:
wrote:
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 -

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

That makes sense.


or is all this unneccesary and the lack of RCD protection
'grandfathered' in?

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.

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

On 21 Sep, 22:04, wrote:
On 21 Sep, 22:47, John Rumm wrote: wrote:

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 -

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

That makes sense.



or is all this unneccesary and the lack of RCD protection
'grandfathered' in?

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.

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

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

jim

On Sun, 21 Sep 2008 15:03:16 -0700, jim wrote:

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

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.

--
John Stumbles

Fundamentalist agnostic

"John Stumbles" wrote in message
...
On Sun, 21 Sep 2008 15:03:16 -0700, jim wrote:

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

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.

--
John Stumbles

Fundamentalist agnostic

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

wrote:
On 21 Sep, 22:47, John Rumm wrote:
wrote:
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 -
No, since an RCD protected socket does not offer any protection should
you nail through a cable supplying it.

That makes sense.

or is all this unneccesary and the lack of RCD protection
'grandfathered' in?
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.

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

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.

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

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.

Augustine of Hippo - 17th ed. good, but ...not yet!

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.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Sun, 21 Sep 2008 21:47:07 +0100, John Rumm wrote:

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)

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


--
YAPH http://yaph.co.uk

Hypnotising Hypnotists Can Be Tricky

YAPH wrote:
On Sun, 21 Sep 2008 21:47:07 +0100, John Rumm wrote:

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)

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

Yup, also true.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

"John Rumm" wrote

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

Yup, also true.

--
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

"TheScullster" wrote in message
...

"John Rumm" wrote

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

Yup, also true.

--
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.

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.