Consumer units, 17th ed & RCDs

[EFLImpudence]

That's life

http://www.screwfix.com/p/mk-sentry-10-way-dual-rcd-board-with-2-rcds-10-mcbs/92791

 

[acmedone]

The cu in your link is specifically for use on a TT supply.

What does TT stand for?

TT is the type of earthing system, it simply means that there are two connections with earth (Latin: Terra); one at the transfomer and one at the origin of the installation. RCDs are usually always required if you have a TT earthing arrangement. Wikipedia has a good article on earthing systems if you are interested.

That is how I thought it was commonly done. Looking at the article, I assume TN-S is actually the preferred configuration?

 

[sparkiemike]

It is not really a preference. The DNO will decide if you are getting TN-S, TN-C-S or nothing (i.e. you provide your own earth - TT).

As a generalisation;
1. Most new builds will be TN-C-S (where the network allows for it)
2. A lot of TT installations can be changed to TNC-S if requested
3. I tend to see a lot more TT installations in rural areas; however have come across TT systems in London
4. Where the TNS system has become inadequate, quite often the DNO will change it to TN-C-S.

 

[RF Lighting]

There are certain installations where TN-C-S is not permitted, such as farms and caravan parks.

 

[acmedone]

There is a massive price difference between say, 1 RCD plus 5 MCBs compared to 5 RCBOs.

Yep; and depending on the the type installation would determine which one you would install. Also would you want a single fault to switch off the entire installation, or would you just want the circuit where the fault has occurred to be switched off?

Lots of houses have systems installed 20, 30, 40, 50 years ago which still work perfectly. It would suggest that, rather than giving protection against creeping faults, someone hammering a nail through a cable is the most likely problem. Why would it matter if all circuits went down?

 

[sparkiemike]

We are not working to regulations that applied to installations that were designed 50 years ago, the latest set of regs came out 2008 with an ammendement last year.

The purest intrepration of reg 314 (division of circuits to minimise inconvenience...etc etc) would be the use of RCBOs, imo due to costs constraints the industry has come up with a half-way house where we have dual RCD boards.

 

[EFLImpudence]

Why would it matter if all circuits went down?

Well, imagine the situation when you decide to hammer a nail in the wall, for whatever reason, when you come home from work.

It's dark outside. You hammer in the nail and it creates a fault between Neutral and Earth.
The single RCD disconnects ALL the power in the house.

In the dark you remove the nail but the fault is still there because you have created a connection between the Neutral and Earth.
The RCD will not reset so you have no power - oven has stopped cooking your dinner.

If you had an RCBO on each circuit only the one damaged circuit would be off and everything else would still be working.
Even with two RCDs half of the lights and sockets would still be working.

The only way to get the RCD to reset is to open the consumer unit and disconnect the neutral conductor of the damaged circuit if you can tell which it is after finding your torch - or dig out the wall and repair the cable - so that you can switch everything else on.

As with everything else, you get what you pay for.
Convenience may seem a small reason to pay extra but should the above happen you probably would be glad that you could ignore the damage and wait until tomorrow, or get an electrician, to repair it.

 

[acmedone]

We are not working to regulations that applied to installations that were designed 50 years ago, the latest set of regs came out 2008 with an ammendement last year.

I realise that the regs changed. But that's rather beside the point (of having separate RCBO circuits) when thousands of older systems continue to work perfectly well.

 

[sparkiemike]

I realise that the regs changed. But that's rather beside the point (of having separate RCBO circuits) when thousands of older systems continue to work perfectly well.

I don't really understand the point you are trying to make! No one is suggesting that all the older systems have to be upgraded, in fact the regs make provision for this, however any new work (on existing or new installations) should comply with current regulations.

 

[ban-all-sheds]

I don't really understand the point you are trying to make!

Would this help?:

I intend to let the flat

I can't be sure enough to say that it is the case, but I would not be surprised to find that the OP is looking for the cheapest way to get the electrics in this flat to a state where he can let it, including looking for areas where he doesn't have to to any remedial work.

 

[SimonH2]

A 'consumer unit' will alway have some form of a main double pole (DP) isolator, so if it says 8 way, its 8 way plus the space for a main DP isolator or a main DP RCD. The cu in your link is specifically for use on a TT supply.

Don't count on the way count excluding the incomer, some (like the Pro Elec ones sold by CPC) count the 2 "ways" taken up by the incomer in the count - so don't expect to fit 9 breakers ina 10 way board I guess it's like a lot of other things, anything to make it sound better.

I realise that the regs changed. But that's rather beside the point (of having separate RCBO circuits) when thousands of older systems continue to work perfectly well.

Well we have different attitudes to risk these days. It's a valid question to ask "do these older systems continue to work well ?", and by current safety standards the answer is no. You don't need to go back very far at all and there is no protection whatsoever against putting a nail through the live conductor of a buried (in plaster) cable and ending up with a live picture hook. I bit one way and you'd have missed the cable, a bit the other and you'd have got a bang from shorting live to earth - but hit it right and your nail is live. Similarly, take an extension lead out into the garden, get a bit of damp in <something> - and shock with no protection.

You are not required to upgrade an existing installation to the current regs - if you have an inspection (PIR) done then it will be assessed against current standards and any deficiencies will be noted, but you are not required to upgrade an existing installation. Two in particular are likely to come up :
Cables buried to less than 50mm without protection in earthed metallic conduit
Circuit(s) in a bathroom without all accessible metalwork being bonded

The latter can be fixed by adding bonding, which may be aesthetically displeasing. Or it can be fixed by making sure that any circuit(s) have RCD protection - which also deals with the buried cables issue as well.

Now, as a landlord I take a slightly different view - with one eye on liability. Regardless of the minimal risk and lack of requirements, I took the view that being fully up to date and with a recent PIR, then I can say that I've taken all reasonable measures to ensure safety of the tenants and their visitors. Everything is up to current standards, and so it would fall on any claimant to prove that that wasn't enough. If I left it at "I'm not required to update" then potentially someone could argue that I'd not taken all reasonable measures.
So in the flat I;ve had a few years, and the house I've recently bought, both have had new CUs and been brought completely up to date (as of when they were done, and I'm not aware of anything significant in the new green book).
As a matter of practicality, I've gone for all RCBOs. As EFLImpudence says, it can be a pain when a fault in one circuit causes others to go off - and deciding how to split your circuits (on a dual-RCD board) can be tricky. So I went all RCBO - helped in part by the low cost they can be had for these days if you are prepared to buy "budget".
If the current CU is spacious enough, then it may be possible to just swap out MCBs with RCBOs - with is non-notifyable for building regs.