Consumer Unit Change

[imroberts]

I'm thinking of having my CU changed for something a bit more modern. The current consumer unit is an MK unit installed in 1986, it uses MCBs rather than rewirable fuses but has no RCD protection at all.

I've got relatively few circuits (32A ring, 32A cooker circuit, 20A radial (see below) and 6A lighting circuit) so am looking at fitting individual RCBOs for all circuits rather than having an RCD and MCB setup, however have a few questions...

1) I live in a single storey flat so all my lights are on a single circuit. Would it be acceptable to have this put on an MCB rather than an RCBO, even though all the wiring is behind plasterboard at a depth <50mm? It would not be in a worse position than it is at present however I'm aware it wouldn't meet regulations for a new installation.

Before a recent new kitchen my flat had a single ring main which included all the kitchen sockets as well. I took the opportunity to move the kitchen sockets onto a separate 20A breaker to separate them from the rest of the flat.

2) At the same time as fitting a new CU might it be a good idea to split the ring main somewhere, and separate this out into two 20A radials rather than a 32A ring? I know 20A radials are much preferred to 32A rings nowadays (less risk of overloading one side of the ring etc, and in the event of an electrical fault I wouldn't lose power in the entire flat). For the sake of the cost of another RCBO and five minutes ripping out a cable somewhere, would this be a good idea?

Any other advice?

 

[securespark]

You wouldn't need to rip it out.

Just find a socket where you can evenly split the ring and disconnect there. Then go to the next skt along and disconnect the same leg. Split the two legs at the board and you should have two radials feding a similar number of outlets.

Tie the disconnected conductors down to earth for safety.

 

[imroberts]

You wouldn't need to rip it out.

Just find a socket where you can evenly split the ring and disconnect there. Then go to the next skt along and disconnect the same leg. Split the two legs at the board and you should have two radials feding a similar number of outlets.

Tie the disconnected conductors down to earth for safety.

Ah you've hit one of my obsessive compulsive tendencies, can't be leaving unused cables in place!

Otherwise, you've got a valid point

Would you agree it's probably better to have two 20A radials over a single 32A ring?

 

[securespark]

Ah you've hit one of my obsessive compulsive tendencies, can't be leaving unused cables in place!

OK then. Split the ring at the board and at the socket, then insulate the disconnected conductors.

Then there are no disused cables: they are all live!

 

[imroberts]

Ah you've hit one of my obsessive compulsive tendencies, can't be leaving unused cables in place!

OK then. Split the ring at the board and at the socket, then insulate the disconnected conductors.

Then there are no disused cables: they are all live!

 

[sparkwright]

Why do you want the lighting on an mcb rather than it's own rcbo?

Do you think you have overloading on your ring circuit? By keeping it as a ring would mean a possible break in the cpc somewhere would at least still provide each socket with an earth until the fault is found and rectified.

 

[bongos]

1) I live in a single storey flat so all my lights are on a single circuit. Would it be acceptable to have this put on an MCB rather than an RCBO, even though all the wiring is behind plasterboard at a depth <50mm? It would not be in a worse position than it is at present however I'm aware it wouldn't meet regulations for a new installation.

If you're putting the lights on their own RCBO then you're removing the problem of losing the lights if you had a fault on a different circuit. Sometimes a halogen lamp blowing can trip the MCB. If this was to happen then it would trip regardless of it being an MCB or the an RCBO anyway.

The lighting should be fine on a RCBO.

 

[imroberts]

Why do you want the lighting on an mcb rather than it's own rcbo?

I assumed, possibly incorrectly, that it's less likely to trip due to bulbs blowing.

Do you think you have overloading on your ring circuit? By keeping it as a ring would mean a possible break in the cpc somewhere would at least still provide each socket with an earth until the fault is found and rectified.

I doubt I've got any overloading on my ring circuit, the bulk of my power consumption is from electrical appliances which are all in the kitchen and off the ring now. However overloading it is less likely to occur if it was split into two radials, and would have less effect were a circuit to become overloaded and trip out. At the moment I'm thinking in terms of "what if's" rather than requirements

Valid point re: the CPC

 

[imroberts]

If you're putting the lights on their own RCBO then you're removing the problem of losing the lights if you had a fault on a different circuit.

This is the primary reason I'd rather go for a full RCBO setup.

Sometimes a halogen lamp blowing can trip the MCB. If this was to happen then it would trip regardless of it being an MCB or the an RCBO anyway.

The lighting should be fine on a RCBO.

Ok. I've lived in properties in the past where the RCD trips near enough every time a bulb goes, and it drove me insane. This is the reason I want RCBOs all round rather than having shared RCDs.

I presumed the tripping was due to the RCD detecting a L-N fault at the instant the bulb blows, and therefore would not affect an MCB in the same way it would an RCBO. I'm guessing from your comments that I'm wrong?

 

[bongos]

The lighting should be fine on a RCBO
Ok. I've lived in properties in the past where the RCD trips near enough every time a bulb goes, and it drove me insane. This is the reason I want RCBOs all round rather than having shared RCDs..

If anything it should be the MCB that trips not the RCD.

I presumed the tripping was due to the RCD detecting a L-N fault at the instant the bulb blows, and therefore would not affect an MCB in the same way it would an RCBO. I'm guessing from your comments that I'm wrong?

The RCD detects L>E or N>E faults so wouldn't be expected to trip when a bulb blows. You might get a MCB to trip though due to a L>N fault...

 

[AndyPRK]

This is one down side to RCBO's I realised recently.

You don't know whether it is the MCB or RCD function that has caused the trip.

In reality is this ever a big deal ? I guess it maybe for intermittant trips