CU upgrade - which of these issues need sorting ?

[mikej2005]

Hi guys,
I've got an old Wylex CU with rewirable fuses which I'd like to upgrade to a modern CU with circuit breakers and (presumably) RCD protection.

Untitled

• mikej2005
• 4 Jul 2009
• 1

NB. Main fuse is 60A. The two separate 40A RCDs serve two electric showers - 10.8kW via 10mm2 cable & 9.5kW via 6mm2 (the latter of which will be removed when the bathroom is renovated). In the meantime, we never run both showers at the same time !

The house was built in 1972 and it's not been rewired to my knowledge, however the wiring appears to be in good condition. I've replaced all the sockets and have done a number of other jobs round the house (replacing the whole CH system, moving light switches, adding extra sockets etc) which have given me a good insight into how the house is wired and the overall good condition of the wiring, although I obviously don't have the expertise and testing equipment to know for sure.

There are a number of issues I have come across however and I was wondering whether any of these would have to be addressed with a CU upgrade ? These are :

1) Rather than an upstairs/downstairs split, the ring main is split into right-hand side/left-hand side of the house - the upstairs RHS circuit sends spurs to the rooms beneath and the downstairs LHS circuit sends spurs to the rooms above.

2) The lighting circuits are divided into upstairs/downstairs, but the upstairs landing light (which is on the downstairs circuit) has a shared neutral with the upstairs circuit.

3) The double light switch outside the upstairs bathroom (which also controls the landing light) can still be live when the upstairs fuse is pulled (presumably as a consequence of no.2 ?) I've not yet tested whether this is all the time or only when landing light is switched on from downstairs.

4) The kitchen is on the ring main for the side of the house closest to the CU and not on it's own circuit. (currently 3 double sockets but will probably add more in the future). The electric cooker is on it's own circuit, however.

I'm curious to find out which, if any, of these would definitely have to be sorted out in order for the job to meet modern standards (and presumably allow the job to be signed off by the electrician changing the CU).

Thanks in advance for any comments.

 

[securespark]

Sort out the shared neutrals. Otherwise, connecting these circuits to two different RCD's or RCBO's will result in tripping when the associated switches are operated.

Also check whether any of these "spurs" feed more than one outlet.

RF continuity is essential too & continuity of cpc in lighting.

Best to install new circuit for kitchen power, either RF or radial.

 

[rebuke]

Nothing wrong with 1), in fact it can be a good idea, it means if you get a fault on one circuit, you still have power on some parts of both floors until it's fixed...

2) That will need fixing, borrowed neutrals are a pain.

3) Nothing wrong with that, given it controls a light powered by a different circuit - just means you have to isolate both circuits before working on it. I have a similar situation in that a switch in my hall controls both the hall light (on the downstairs lighting cct), and the landing light (on the upstairs lighting cct).

4) It's not against the regs, depending on your estimated total load, but if putting a new CU in and you can run cables easily enough it might be worth putting in a dedicated ring for it...

 

[Taylortwocities]

The house was built in 1972 and it's not been rewired to my knowledge, .

Does the lighting circuit wiring have earth conductor?
Is the main & supp bonding up to scratch?

You should get the tails from the meter upgraded. The red&black tails from the meter to the henley block look undersized.

 

[Spark123]

The two separate 40A RCDs serve two electric showers - 10.8kW via 10mm2 cable

10800w / 240v = 45A
I take it from the pic there is an RCD and an MCB also.

 

[mikej2005]

Thanks for all your replies. I'll try and answer as many of the points raised as I can.

Borrowed neutral - sounds like I need to sort this out.

Spurs - As far as I can tell, the sockets on the LHS and RHS ring mains do only connect to one spur each in the rooms below/above.

RF and CPC continuity - I'm not familiar with these terms, so I'll look into this (FAQs/Wiki ?). Are they something only an electrician with the right diagnostic equipment can check ?

Kitchen circuit - I have access both under the floor in the kitchen itself and from above, as I had many floorboards up when I replaced the CH system and purposely haven't fitted a carpet in the bedroom above yet. I realise that adding a new circuit to the kitchen falls under part P - would fitting the circuit myself, leaving everything exposed and the wiring in place ready for the new CU not be allowed ?

Lighting circuit - all the light fittings and junction boxes are earthed.

Earthing - I upgraded the earth cabling to and around the CU myself, in order to bond to the incoming gas supply pipe using 10mm. The 8-way earthing block is then connected to the main PME block using 10mm, although one or two of the strands had to be snipped in order for it to fit - I guess this may also need addressing ? (Looked to be about 4mm before!)

Not sure if there is any bonding in the bathrooms, however I will be able to address these when they're both refurbished.

Not sure about RCDs/MCBs, however I would want the shower circuit incorporated into the main CU anyway, so I'd be losing both mini CUs. The 'ELCB' for the 10.8kW shower is actually 80A, not 40A as I thought.

I'm happy to let the electrician who fits the CU address any issues with the undersized tails, 60A-rated Henley block etc. I was just keen do fix as many of the other issues myself as I can comfortably (and legally!) manage.

Would an increase from a 60A to 100A main fuse be recommended ?

Thanks again guys - please keep the comments coming, as they're most appreciated.

 

[Taylortwocities]

The main earth conductor (from the PME cable head to the earth terminal) should be 16mm, and use all of the cores!

You can ask your DNO to upgrade the fuse to 100A due to heavy demand - ask them to fit an isolator switch so you can mess around without having to break THEIR seal and pull THEIR fuse.

 

[GaryMo]

Would an increase from a 60A to 100A main fuse be recommended ?

Has the current 60A fuse caused you any problems?

I see many supplies with a 60A main fuse and this has never been a problem due to diversity.

 

[securespark]

RF and CPC continuity - I'm not familiar with these terms, so I'll look into this (FAQs/Wiki ?). Are they something only an electrician with the right diagnostic equipment can check ?

RF = Ring Final

CPC = Circuit Protective Conductor (earth conductor)

Continuity is a dead test, using a low range ohm meter. It is important to check (amongst other things) end-to-end continuity to make sure the ring is complete.

If not it should be rectified or an alternative is to trace and isolate the dodgy leg, creating 2 radial circuits. This can only be done if the fuse or breaker is down-rated to 20A and the areas served by the newly-created circuits does not exceed 50m².

 

[ColJack]

while, as rebuke says, there's nothing technically wrong with the rings being split left and right rather than up and down, it does have a safety issue where to do work on one circuit you need to turn both rings off in case it's not on the one you think it is..

while most people will, and you always should, test that a circuit is dead before commencing work, the average DIY'er ( no offence ) will probably just turn the breaker off for the circuit that they think the socket is on..

not a big problem if it's a downstairs socket and your board is labled "downstairs sockets" and "upstairs sockets"..

 

[mikej2005]

Has the current 60A fuse caused you any problems?

No, but I was wondering what would happen if the 10.8kW and 9.5kW electric showers and the electric fan oven were all running at the same time ?

Unlikely I know, but don't you have to take these theoretical situations into account ?

 

[mikej2005]

The main earth conductor (from the PME cable head to the earth terminal) should be 16mm, and use all of the cores!

Is it possible that a PME cable head won't have enough room for a 16mm earth conductor ? If I had trouble inserting 10mm, then I'm guessing that 16mm will be tricky.

I'll have another look at the head in the meantime - the cables from head to meter did make the earth terminal hard to get at IIRC.

 

[mikej2005]

the average DIY'er ( no offence ) will probably just turn the breaker off for the circuit that they think the socket is on.. not a big problem if it's a downstairs socket and your board is labled "downstairs sockets" and "upstairs sockets"..

Having checked every socket in the house, I now have a laminated chart next to the CU with an accurate and clear list of which fuse isolates which rooms

 

[GaryMo]

Has the current 60A fuse caused you any problems?

No, but I was wondering what would happen if the 10.8kW and 9.5kW electric showers and the electric fan oven were all running at the same time ?

Unlikely I know, but don't you have to take these theoretical situations into account ?

I based my reply on this paragraph from your original post:

NB. Main fuse is 60A. The two separate 40A RCDs serve two electric showers - 10.8kW via 10mm2 cable & 9.5kW via 6mm2 (the latter of which will be removed when the bathroom is renovated). In the meantime, we never run both showers at the same time !

 

[ColJack]

No, but I was wondering what would happen if the 10.8kW and 9.5kW electric showers and the electric fan oven were all running at the same time ?

Unlikely I know, but don't you have to take these theoretical situations into account ?

theoretically at about 110A ( 46A for 10.8KW, 41A for 9.5KW and the rest for the oven ), a 60A BS1361 ( are houses BS1361's? ) will last for 60 hours.. ( where the graph flatlines, is this when they stopped counting? )

it will get warm mind, heating up during those 60 hours to the point where the fuse element inside the fuse will melt..