Consumer Unit configurations

[thomasteve]

Here's a link to a very useful publication from Hager :

http://www.tlc-direct.co.uk/Technical/DataSheets/Hager/Hager_17.pdf

It explains various configurations that overcome the disadvantage of installing a single Safe-t-cut (or similar RCD) which may result in unexpected loss of all power from nuisance tripping etc.

The disadvantage is that either safety is compromised by having some appliances (eg. freezers, lights,alarms ) unprotected by a RCD or it becomes more costly by using 2 or more RCDs or multiple RCBO's.

[Crossy]

Good find Thomasteve, I've added it to the list of useful links in the FAQ thread

Although I (and many others) find the IEE 17th Edition overly onerous on the provision of earth leakage protection, it is an excellent starting point for those wanting a safe installation here.

[thomasteve]

Good find Thomasteve, I've added it to the list of useful links in the FAQ thread

Although I (and many others) find the IEE 17th Edition overly onerous on the provision of earth leakage protection, it is an excellent starting point for those wanting a safe installation here.

Thanks Crossy, page 25 gives a useful description of RCDs and some pictures.

There does seem to be a lot of forum discussions about how to interpret some of those IEE 17th Edition requirements.

Two points I've been trying to find answers for are :

1. Is it allowable to replace the main incoming switch with an RCD or must a main isolator switch be retained ?

2. Is it permissable ( in UK) to have one RCD for all circuits or is that considered as 'inconvenient' as all lights would trip ?

Any thoughts.....?

[Crossy]

Indeed there is, the IET regulations forum is full of it (you don't need to be a member of the IET).

1. Yes, no problem using an RCD or RCBO as a main switch, but see 2.

2. The requirement is that all the lighting is not on a single RCD, so that one does not lose all the lights in the event of a trip.

A single front end RCD is permissible for protection of long meter tails on a TT installation, however, to ensure discrimination it should be a 100mA unit. The installation itself should be protected by 30mA units.

Also note that the UK regs are not law. So, for example, a very small installation with two MCBs and a front end RCBO as a main switch, whilst not complying with the letter of the regs, is still perfectly acceptable.

Adding a front end RCD to an installation that has no protection is also acceptable, as it is improving the safety of the existing system.

Note that an installation made to the 16th or earlier, whilst not complying with the latest regs, does NOT need to be upgraded just to comply.

[thomasteve]

Indeed there is, the IET regulations forum is full of it (you don't need to be a member of the IET).

1. Yes, no problem using an RCD or RCBO as a main switch, but see 2.

2. The requirement is that all the lighting is not on a single RCD, so that one does not lose all the lights in the event of a trip.

A single front end RCD is permissible for protection of long meter tails on a TT installation, however, to ensure discrimination it should be a 100mA unit. The installation itself should be protected by 30mA units.

Also note that the UK regs are not law. So, for example, a very small installation with two MCBs and a front end RCBO as a main switch, whilst not complying with the letter of the regs, is still perfectly acceptable.

Adding a front end RCD to an installation that has no protection is also acceptable, as it is improving the safety of the existing system.

Note that an installation made to the 16th or earlier, whilst not complying with the latest regs, does NOT need to be upgraded just to comply.

That's exactly what I've encountered - just one circuit for lights and one ring main. (All heating and cooking is by gas so loading is low).

The consumer unit appears to be the type sold for a garage - just space for an RCD (30mA) and the two MCBs.

It would be unreasonable to expect owners to modify existing installations to the latest regulations as the supply authority themselves only recently got round to replacing their 70 + year-old ? cast-iron and porcelain head end !!

Surprised they worked on it 'live' with some seriously thick rubber gloves - retained the original underground cable though....

[Forkinhades]

Yes fantastic find. Just shows some of the many configurations you can have to comply.

Personally I like option 3

Can I have your permission to add this to the sticky?

Or maybe a new one?

Sent from my GT-I9300 using Thaivisa Connect Thailand mobile app

[Forkinhades]

^ ah seen Xy has already done it

Sent from my GT-I9300 using Thaivisa Connect Thailand mobile app

[thomasteve]

^ ah seen Xy has already done it

Sent from my GT-I9300 using Thaivisa Connect Thailand mobile app

Also added a link to some useful info on selecting MCB's

[thomasteve]

Some further queries re.consumer units :

1. It seems that although different brands of MCB's, Main Switches and RCD's etc. all comply to the same safety specs. and fit a DIN rail mounting system they do not have to comply to the same dimensional requirements as regards position of connection holes etc.

In practice this means that a busbar strip may not be usable as the connecting holes can be at different heights.

I suppose you can always revert to using some 16mm cable as a substitute to join them together ?

2. What is the best method of extending cables within the consumer unit if they are slightly too short and a new cable length cannot be pulled through.

Wire nuts or twist and tape don't seem to be reliable so would terminal strip connectors ('chocolate blocks') be a better option?

[Crossy]

1. Indeed, it's wise to stick to the same manufacturer for your internals if at all possible. If your incomer is <63A then 10mm² is fine for internal CU wiring and less of a wrestle than 16mm².

2. Wire nuts are actually very good when applied properly, Wago connectors are better. Butt crimps are also very good and compact if you get the ones specifically intended for solid conductors and a proper ratchet type crimp tool. Choc-blocs are also OK, I like to get ones which are big enough that both wires can go under both screws (from opposite ends), means a large contact area between the actual wires and less risk of the block getting warm.

[thomasteve]

1. Indeed, it's wise to stick to the same manufacturer for your internals if at all possible. If your incomer is <63A then 10mm² is fine for internal CU wiring and less of a wrestle than 16mm².

2. Wire nuts are actually very good when applied properly, Wago connectors are better. Butt crimps are also very good and compact if you get the ones specifically intended for solid conductors and a proper ratchet type crimp tool. Choc-blocs are also OK, I like to get ones which are big enough that both wires can go under both screws (from opposite ends), means a large contact area between the actual wires and less risk of the block getting warm.

Thanks Crossy, I will use a choc block of suitable rating.