How can I tell what type of circuit I have?

[charliefortune]

I want to add some more sockets into a room. I have read some good stuff about breaking into the ring, or adding spurs from existing sockets and am ready to get stuck in.

But how can I tell what kind of circuit I have (i.e. ring, radial etc.)? I need to know this to decide how to add my extra sockets right?

 

[rjm2k]

An initial indicator of a ring is a 32amp MCB , a radial is more likely to have a 20 amp.

To check for a ring at the socket you would power down the circuit, verify the socket is off with a lamp and and use a multimeter to check for continuity between the 2 lives, neutrals and earths a pair at a time (ie attach 1 live to 1 meter lead and the other live to the other meter lead) if continuity is there the meter will show if you have it set to measure resistance.

a radial socket may have 1 or more cables, but there would be no continuity if you seperate and test the wires

a spur socket should just have 1 cable

a ring socket will have 2 or 3 (if already spurred) cables, once the wires are seperated there should only be continuity between the actual ring legs, not the spur.

to work out a radial vs a spur you would need to know what the rest of the circuit is set up as and the mcb size to be sure

to test properly is a bit more involved as you need to measure the actual values rather than just testing for continuity

 

[ericmark]

Even electricians get it wrong from time to time I found a 32A radial circuit using 4mm cable where an electrician (as industrial and non electricians would not have been allowed to work on it) had added with 2.5mm extra sockets around the room splitting the radial to do it. Clearly he hadn't tested.

The tests already given I see no flaw in. The only thing I would point out is new sockets or buried cable needs RCD protection. Also there is a limit to how large a ring could be. Historically, limit of 100 m² has been adopted. In real terms it's the volt drop and loop impedance which limits the length 1.44 ohms for a B32 MCB measured in the centre of the ring.

In theroy you can calculate the volt drop from the prospective short circuit current. One should measure loop impedance both line - neutral and line - earth and for loop impedance and PSC the higher figure is important one but since V/R = A when impedance goes up PSC goes down really opposite.

In practice the normal DIY guy just not have the instrumentation required to test and has to take a chance. As long as verified as a ring and protected with RCD there is unlikely to be a problem. However doing any electrical work there is a risk and the big risk is some one before you have touched the system has made an error.

If when you occupied the house a PIR or EICR was done then unlikely to be a problem. If not then how long since one was done, and how much does it look that previous occupant has done?

 

[PrenticeBoyofDerry]

And to add to that, even if you have an 30mA RCD, do you know it actual functions to it's specification!

 

[charliefortune]

This is really helpful, thanks.

I will try and work out the amperage of my mcb first, so I know what to expect. I also found that each circuit on the box has a sticker, saying 'upstairs main ring', 'downstairs main ring' etc. which should indicate ring circuit I guess. However, they are printed stickers stuck on by the electrician, so it's no guarantee.