I could talk for hours trying to answer your question but in a nutshell
a) All installations done now have to comply to the CURRENT regulations which are the 17th edition of BS7671 2008. so you cant install a 16th edition consumer unit(a 17th ed CU has more RCD's to comply with new requirements)
b) All installations must comply with Part P details of which can be found on this site, which means basically you cant do this work yourself and will need to call in an electrician who is enrolled in one of the competent persons schemes.
c)A HIPS pack will be required to sell your house and certificates will be required for any new wiring installed including all the stuff you've done yourself - complicated
OK let’s start at end. You can do what you like, if you get caught doing something wrong you may be required to correct it or may get fined but it does not stop you doing it in first place. I would assume therefore you would like to do it right first time which means the only difference between you and a qualified electrician is how you deal with the requirements of Part P.
Forgetting about Part P you can find links to that in projects section I will move on to the regulations. Point one has to be they are not law, but can be used in a court of law so may as well be!
The regulations define four classes of people ordinary, instructed, skilled, and competent the latter being the highest qualification. You would most likely be classed as ordinary which means you are not allowed to use re-wireable fuses. Also all cables in walls at less than 50mm depth need protecting which in most cases means using an RCD (Residual Current Device) on all circuits this change came in with 17th Edition.
Also in 17th Edition it was made plain that the requirement to split into circuits to take account of danger that may arise from the failure of a single circuit such as a lighting circuit also included reducing the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation. It does not say you must use two or say what type of consumer unit you should use.
So if you live in a very small house in Conway (Has smallest house in UK) then you may only need one RCD in same way as a single RCD is used in a caravan. But in most houses you will need multiple RCD’s to conform with 17th Edition.
There are as far as houses go two types of RCD’s you can use. The stand alone RCD which then feeds a number of MCB’s (Miniature circuit breakers) or a combined RCD and MCB called a RCBO. The latter can be used in many consumer units designed well before the 17th Edition came out.
RCBO will cost more than a MCB but less than a RCD + MCB and to choose which does have cost implication with an old 4 way Wylex board being replaced with a new board likely the RCBO would be best option using what are often called 16th Edition boards. But where you have 12 fuse/mcb’s to replace then the 17th Edition board with two RCD’s would likely to be cheaper option. But with the RCBO option it is plain change old for new but with duel RCD option more thought is required as to how to split the board and if the split will comply.
Sockets are the big problem as anything can be plugged in and if you use a faulty item it is bad enough getting a shock, which you can still get with a RCD although unlikely to kill you, without also being plunged into darkness. It is common to split up and down stairs so up lights are with down sockets and vice versa. But as to if this complies I can’t comment. Also using battery back-up with lights, and alarms will change what is required.
You can fit yourself by paying the LABC (Local Authority Building Control) in my area £115 and they for that fee should check if it’s all done correctly. Or you can use a registered electrician who can sign their own completion certificate. I don’t know if council can re-charge if you get it wrong!
Fitting RCD’s to an old house can be a nightmare with allsorts of faults becoming apparent once fitted. An electrician should check the house first with his special meters and identify most of the faults before he starts. There will always be one he has missed i.e. the washing machine which is only faulty on spin cycle or the lights which are only faulty when two way switching is in one direction. But the unexpected should be to a minimum. However if you DIY you will not have the meters required and may have real problems. Using RCBO option at least you will be able to isolate the one faulty circuit but if that’s the downstairs ring main you may need to run an extension lead from upstairs for freezer until you can find fault.
I would not recommend changing a consumer unit yourself. Does this answer your question?
So it's not so much a case of fitting the unit that is hard or a problem, it's the fact that if some of the wiring or an appliance is wrong / shorting, then due to the design of the RCD /MCB, I will have no power on said circuit until the fault is rectified - and with no special test equipment this could prove very difficult.
Am I allowed to ask what the cost of employing a professional to replace my consumer unit would be? (I'm near Hampton Court).
With consumer units costing around £100 populated then I would expect between £200 and £500 but so much depends on house really impossible.
As you say inspection and testing is most important on a job like this and the last thing you want is anyone who does not really know what they are doing.
So you need a registered electrician which means they will be a member of one of the Part P systems and be able not only to issue a installation certificate but also a completion certificate.
Since these guys have to pay to issue these completion certificates they are not cheap but with LABC charges it will cost you £200+ to DIY and the guy will likely spend a few hours testing house first so doing more work that you will do so I would expect to spend a day on a change and I get paid over £100 per day so can't see anyone charging less than £300 and if it's too low I would be uneasy as to how good he is.
I have never worked for myself and was commercial rather than domestic so I could be completely out so if Sparx or one of the others who do domestic give a price go by what they say. I think there is a number you can ring to find registered sparks with so luck maybe some one will give you the number on here.
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