by ericmark »
Mon May 03, 2010 7:43 am
Thanks a lot for the reply - what do you mean by "automatic disconnection type" - do you mean RCD?
Automatic disconnection could refer to fuse, MCB, or RCD that is why I used that term. The big red book we all use has a list of parameters we have to comply with. For example 1.5mm 70 deg flat twin and earth cable Reference Method 103# (in a stud wall with thermal insulation with cable not touching the inner wall surface) has a maximum current rating of 10A. Since ceiling roses are used as junction boxes and they are rated at 6A then in order to ensure no over load of the ceiling rose the lighting supply is normally limited to 6A. If ceiling roses were not used then we could go up to 16A for lighting in theory but in practice we stick to 6A as although bulbs should be internally fused often they are not and most bulb holders are rated 2A and any bulb blowing and drawing 16A will likely fuse the contact pins to bulb and mean you have to replace the whole bulb holder rather than just bulb when it blows.
Now a MCB has two tripping devices one thermal and one magnetic the latter is shown by a letter so a B6 MCB will need 5 x 6A to trip within the milliseconds allowed a C6 will need 10 x 6A and a D6 20 x 6A so if we consider the B6 then 30A has to flow to make it trip. Using Ohms Law 230v / 30A = 7.6 ohms so we must ensure the circuit is less than 7.6 ohms we will normally read direct with a earth loop impedance meter but before power is connected we have to do it with maths and measure the resistance of the cable we refer to this as R1 + R2 then add to this the supply resistance normally given as 0.35 ohms. Where the earth to line current is limited by a RCD then we have to consider Line to neutral current as limiting factor. In practice with Line - Neutral resistance the volt drop limit of 3% will be reached before the point where it will fail to trip so we then calculate the volt drop. And 1.5mm cable will drop 29 mV/A/m and 3% of 230v = 6.9v so at 6A you can use nearly 40 meters. There is a temperature correction but unlikely one would exceed 40 meters so not really worth worrying about.
So even if I get an electrician, the council still have to come and inspect the installation?
No if the electrician is a member of a scheme then he can self certify which is why I said registered electrician not just electrician. He still pays but a fixed yearly fee plus small amount per job so he could end up charging you less than the councils fee.
On a different note - how would they (council) know that anything has been done to the bathroom?
If anything goes wrong i.e. someone is injured then they would know. Or if you come to sell the house they will know. This is one of the gripes in that people will not ask an electrician to test their work for fear of being reported. Also electricians are not responsible for contacting the LABC when not registered that falls to the owner. Although like any builder they will normally sort it out for you they like the builder are not duty bound that falls to you the owner. So there has been many cases when when the owner wants to sell the property they are asked for completion certificates which they don't have. For the council to issue a completion certificate post work being carried out they need to view the installation certificate which can only be filled in by the person doing the work. If you don't have the installation certificate sometimes the council will accept an inspection report but they could insist its ripped out and re-done. This is at the discretion of the inspector.
The installation certificate can be in theory filled in by any person with the skill required to do the work. So you could down load the forms from the IET web site and fill them in yourself. However the forms ask for readings to be taken and entered and to get the readings required you would need what is loosely referred to as a 17th Edition test set which will cost around £750. You may be able to hire one but using one unless trained could be dangerous and you are unlikely without doing a lot of reading to know how it should be used.
When fitting a RCD for example one must test it. 6 tests in all. Three pairs of tests one to show it will not trip when it shouldn't. One to show it will trip when it should. And one measuring the time it takes to trip at 5 times rated current. This has to be less than 40ms and that is not something one can measure with a stop watch so you have to use the special RCD tester.
The test button on a RCD is only the test the mechanics and show it has not sized up.
Every house should be inspected and tested on change of occupant or every 10 years which ever is shorter. Landlords will do this to protect themselves so if anything does go wrong they can show they fulfilled their duty of care. Estate agents and solicitors will will advise you check this paperwork but often will leave it to the buyer to decide if they will buy without it or not. It is often used as a delaying tactic.
If you do the job as the Part P allows for DIY work and apply to LABC first they are responsible for ensuring the work is safe. They will issue a completion certificate once complete. But they do not have to issue the installation certificate that is up to you. They do have to inspect and test at their expense but they do not need to follow the latest regulations and they could in theory pass work which does not comply with BS7671:2008. Only the person doing the work can issue a installation certificate.
To comply it really means any work under £300 which needs a Part P completions certificate is just not worth attempting as DIY. Even for me as a qualified electrician.
