Basic Testing for ring main extension

[PhilipKS]

Hi all,

I'm hoping someone can give me some advice. I'm extending a ring circuit in my house with 3 new sockets (and a couple of fused spurs).

I have done the new wiring, but before I connect into the circuit I want to test the continuity of N, L and CPC (and then again after I connect up). The circuit is 30 mA RCD protected, and its a c. 4 year old full rewire, so fairly up to date.

Could someone please suggest a few step-by-step pointers as to how this can be done? I have a multimeter. It can measure resistance but not low ohms needed to do an accurate r1 r2 readings, I assume it can still be used to check the continuity?

I have a basic socket tester so can confirm polarity etc with that but would like to learn a slightly more 'robust' method of testing the circuit.

Many thanks
Philip

 

[17thman]

you cannot test it properly then without a low ohms resistance meter

 

[ericmark]

17thman is correct but I think I should explain further. When we fit a ring main we measure the loop line to line, neutral to neutral and earth to earth and one reason is to see it is a ring main. But far more important is to ensure if something goes wrong the MCB will open, and to lesser extent that the volt drop is not too great.
For a B32 MCB to open within a few milliseconds the magnet part of the device has to operate. The B in front of the 32 tells us that 5 times the rated current needs to flow to be sure the magnet part will work so 160 amp has to flow. Here we use Ohm's law and 230/160 = 1.44 ohms in many houses we need less than that because there is also the resistance of main supply cables to take into account so we subtract 0.35 ohms so 1.09 ohms is often the upper limit there are of course two cables Line and Neutral and also both are doubled up so each cable needs to measure less than 1 ohm.
As a for a meter to measure with enough reliability these low values it should use between 4V and 24V and pass a current of 200ma regulation number 612.2.1 and of course normally it will include a means to zero the reading to allow for cables used and be calibrated to show 0.01 of an ohm increments. Most multi-meters will not do this.
Even if they did the next test is with a insulation tester and here we need a 500 volt test voltage then just to completely stump you the RCD needs testing at 15ma, 30ma, and 150ma and the last test must be timed to see it does it in less than 40ms and I just can't see you operating a stop watch that quick.
You can hire the test kit but in real terms better to get someone who also knows how to use it.

 

[ban-all-sheds]

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http://www.tlc-direct.co.uk/Book/8.3.1.htm
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[PhilipKS]

Thanks for the tips. I have decided to complete the install of the 3 new sockets, and then will get the whole downstairs ring inspected as planning to get my sparks in anyway soon to update some external garden lighting (SWA armour not connected to earth...not my doing, some unknown contractor (prehouse purchase) done since full rewire )

I have a martindale ez150 so can do basic check on earth loop impedance and correct wiring in the interim and I've checked and double checked all connections on short extension to ring, so am confident that it is at least as safe as when I started.

Thanks Philip