Ring Main Question

Canucklehead

mikey72 wrote: »

Section 4 of which reference book?

Click on the link in my previous post i.e Source

CG

mikey72

Thanks
link wasn't working on my pc, but it appears to be now.

lisal0u

Afternoon everyone,

Dad is going to pop to the house this afternoon to check the resistance and work out the length of the ring. I highly doubt it will be more than 110m as it is only a small house. He has all the gadgets to check.

I think the split between the RCD's is like this:

1st RCD: Ring, oven, kitchen underfloor heating, livingroom underfloor heating, lighting.

2nd RCD: lighting, bathroom radiator element, boiler, bathroom undefloor heating, garage

I will have to check as I'm not completly sure!

mikey72

It's not the length of the ring that matters. It's the voltage drop and resistance. As these both increase with cable length, the rules of thumb above are developed to give an etimate of the maximum length that will probably pass. At the end of the day, if the test results for the ring are in spec, it will pass, if they are high it will fail. If the cable has a high resistance for any reason, even a short ring will fail until the figures are brought down.
Is the bathroom lighting on the second RCD, or the upstairs if that's were your bathroom is, just to keep it tidy.

ic

I'm no expert, but it seems odd to me to give the boiler (just the system pump and electronics?) and the bathroom radiator (perhaps 800W?) their own circuits - given neither will be particularly demanding. In my house the boiler sits on the same ring as the rest of the floor.

lisal0u

Hi Mikey, yes the upstairs lighting is on the same RCD as the bathroom bits. I'm keeping my fingers crossed that the resistance test is ok! I'll let you know!

Hi IC, we were just playing safe with the boiler, it could easily be changed. The bathroom radiator element has to be on its own circuit, everything in the bathroom has to be as far as I am aware.

Thanks for all your help guys! I've got building regs swimming around in my head! Had to get certificates for the windows, steel beam, supporting stud and chimney support. They also had to give advice on where the new waste pipe from the bathroom would be sited!

Its definately a good thing though as it means I will have a safe house when its finally finished (if it ever gets finished that is!)

Canucklehead

Good afternoon: The Electrical Safety Council has an excellent website and hosts a forum featuring technical guidance concerning the application of the requirements of BS 7671: 2008 ( aka 17th Ed. Wiring Regs).

Wiring regs, Gas Regs, Building Regs Parts A through P are riddled with 'shoulds' rather than 'musts' and are always open to 'interpretation' .;)

HTH

Canucklehead

lisal0u

Hi Everyone,

Just checked the resistance value of the ring main and it was 1.83ohms. I haven't got the 17th edition with me to check if this is ok or not. Can anyone let me know?

The resistance of 2.5mm twin and earth should be 0.00741/m which would mean we have nearly 250m of cable. This is not possible as the house is quite small and would also mean splitting the ring into 3 to get the resistance low enough.

If anyone has got any advice I would be very grateful
Kind regards
Lisa

mikey72

Doesn't sound good.
What type of supply is it?
You need the 17th edition, if your dad is qualified he should have it, or tell us what he last worked to.
You need to get him to go through a full installation inspection/test, and trace the faults, if there are any.