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Modernising a Domestic Installation - Multiple Consumer Units

John R.

I'm buying a house where the electrical installation is, for want of a better description, a complete mess!

I'm assuming that the existing supply is PME and will check and also measure earth the loop impedance when I get full access.

Currently there's one consumer unit in the downstairs entrance which is old having only fuses and no spare ways.

The plan is to start by renovating upstairs, which will involve re-insulation and will provide great access..
So I think I may as well re-wire upstairs while access is available.

I reckon that the easiest thing to do will be to install an upstairs consumer unit, and at some later time, a new downstairs one.
This will also allow for shorter final circuit cable runs so simplify installation.

Hence to my current deliberations.  I think the following is correct.

(a) The 'main distribution board' needs to house CBs only to protect the cables to the CUs.  (CUs will include RCDs whether required or not)

(b) There's no requirement to have any RCDs in the 'main distribution board'.

(c) There may be merit - or even a requirement - to include SPDs in which case these would best be in the 'main distribution board'.

Does this make sense?  Any comments appreciated.

For background: in my early career I worked as an electrical building services designer for a consultancy.  That was a a long time ago, and that work was on commercial not domestic premises.

Parents

  • (a) Yes, but they may not discriminate well with the final circuit MCBs, so if a 32 A MCB trips, it is likely to take a 63 A MCB protecting the distribution circuit with it. This may be an acceptable risk. Fuses are an alternative option.

    (b) Agreed.

    (c) Yes, or adjacent to it.

  • in reply to Chris Pearson

    Thanks for your response, Chris.

    (a) Yes, I certainly need to look at discrimination since if faults were tripping a 63A MCB and the whole CU, that would be a nuisance.  Would rather avoid fuses though.  Perhaps non-B characteristic MCBs would be an option here.

    (c) My interpretation is that use of a SPD would not be mandatory, so I suppose I need to consider the merits or otherwise of this.  We don't get much in the way of lightening around here, and no hefty surge-inducing loads are expected.

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  • in reply to Chris Pearson

    Thanks for your response, Chris.

    (a) Yes, I certainly need to look at discrimination since if faults were tripping a 63A MCB and the whole CU, that would be a nuisance.  Would rather avoid fuses though.  Perhaps non-B characteristic MCBs would be an option here.

    (c) My interpretation is that use of a SPD would not be mandatory, so I suppose I need to consider the merits or otherwise of this.  We don't get much in the way of lightening around here, and no hefty surge-inducing loads are expected.

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  • in reply to John R.

    (c) In my own home, I decided to install SPD, largely on the basis of discussions in this forum. Like you, we see very little lightning and I do not have much vulnerable equipment. Nothing has been damaged during the past 27 years that we have lived here. It was not exactly expensive, but if installed in the CU it takes up 3 ways as opposed to 1 if adjacent.

    (a) Yes, you may be able to fit a type C in the first DB. In my own installation, the distribution circuit is about 35 m long, so if a fault occurs a few metres downstream of the second DB, there will probably be discrimination.

    The important point is, despite any of the theoretical concerns, I cannot remember when an MCB last tripped. It is not an everyday risk!

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