On 21/07/2019 12:11, Kevin H wrote:
Just following up on all the useful help and advice that was given in
relation to swapping my CU.
Thanks for the update Kevin.
I completed this task yesterday. Some notes and feedback which I hope
may be useful for others considering doing this themselves.
I bought a BG 10 way CU (just 100A isolator provided) and a set of
RCBOs to match the current MCBs, plus some spare MCBs as advised (see
later) to help with problem circuits. These were bought from TLC
direct who delivered everything next working day (nearest store 20
miles away, and it was free delivery anyway). Total cost £205.
There was a time a "normal" CU and MCBs could have costed that - so not
a bad deal.
Saurday morning. Checked my meter was working by measuring the
voltage in the CU. Then turned off each MCB before turning off at the
outside isolator switch (no need to cut the seal and pull the fuse).
Went back and checked that there was no voltage coming into the CU
and then off it came. One thing I was surprised with (perhaps I
shouldn't have been) was how loose some of the neutral and earth
connections were. I've never checked the tightness of these
connections in the 20 years we've been in this house. Perhaps that
needs to be an annual check?
Yup I have noticed that on old CUs a number of times. Hence the comments
last time to go back in a few weeks and tighten again after the copper
has had a chance to "relax".
As part of the re-fit I checked there was continuity between the the
two halves of the ring mains.
Given the smaller than expected size of the CU the cabling didn't go
in as neatly as I was hoping. I could have shortended the length of
some of the cables but decided at this stage the main job was to get
power back on the house. A tidy up could wait for another day.
Yup I said that to myself when I did the main one here ten years ago... :-)
(in reality I did need to do a bit of tidyup some time later as I wanted
to move the kitchen onto its own RCBO - and that forced a bit of cable
trimming to get it all fitted in there).
After a few issues with fitting the cables (see below) I had
everything re-instated. Final check of tightness of every screw and a
tug test on each cable then powered back up. All circuits came up
apart from one of the combined lighting circuits which kept tripping.
By trying each circuit in turn isolated the circuit with the fault
and for now connected it to a 6A MCB to allow time for fault
investigation. The lighting circuits were not on the RCB protected
side of the old CU so this fault must have been present for some
time. Will need to go around each of the lights to see if I can see
where the problem is.
Yup its a common issue - especially if the lighting circuits are old.
Good move having the 6A MCB to hand.
In total, with the fault investigation, it took 90 mins with a brief
pause for a cold drink. I still need to fill any gaps with
intumescent sealant which I'll do when I finally connect up the hob
(cable has been run from the kitchen to the CU - what a chore that
was - cutting holes in the ceiling at every joist which of course
were perpendicular to the direction the cable needed to go). When I
fit the cable next month I'll re-tighten all screws.
The CU has now been in for 24h. No nusance trips so far...
Some observations 1. The cable receptacles on the BG
MCBs/RCBOs/Isolator switch are pretty shallow. I had to trim back
every cable inc the tails down to 10mm 2. It is too easy to insert
the cable behind the cable receptacle which meant the cable wasn't
clamped. I had to remove 3 or 4 RCBOs to push back the clamp. 3. The
On many, just undoing the screw on the MCB "all the way" will force the
back of the cage to the back of the recess. Even if creating an opening
far wider than required for the wires in question, it is a way of
ensuring they all go in it, and not behind it.
CU case has a number of round "punch outs". These were recessed but
there was no way (for me) that these were going to come out in any
way other than by being cut out. Perhaps I needed a larger hammer and
a more suitable drift?
20mm hole saw is often easier.
4. Why are the neutral tails on the RCBOs so
long?
Not all CUs are small - so they have to give you enough slack to cope
with most situations. Its far less annoying to have to trim them down,
than needing to extend them. If trimming it makes sense to keep them
long enough that they can still fit in multiple positions in the CU - so
cut a bit longer than the "right" length in most cases.
I didn't want to cut them (read somewhere that it was wasn't
recommended to cut them). Perhaps if I had the appropriate crimps
they could have been shortened but it was just a load of cable to
tuck away behind the rail and which would make it much harder to
replace a RCBO by having to unpick the neural wire.
Thanks again for the feedback and suggestions. My only regret is the
cabling isn't neater. The only additional tools I purchased was a
quality long nosed pilers and heavy duty cutters.
--
Cheers,
John.
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