Top be carried out on radial circuits to ensure the CPC of the circuit is undamaged and connected throughout the circuit

 The highest reading should be obtained from the furthest part of the circuit. If the highest reading appears not to be at the end of the circuit, then further investigation should proceed (It maybe because of how the circuit was installed, although one end may appear to be closer to the CU than other points, it may still be the end of the circuit)

1. Safely isolate supply
2. Cross connect the Line and CPC at one end of the circuit, preferably at the consumer unit
3. At each point of the circuit, a test should been made between Line and CPC
4. The highest reading will determine the R1 + R2 value of the circuit

note: when the Line and CPC conductors are the same size, the total resistance can be simply divided by 2 to find the resistance of just the CPC

In most cases Twin & Earth is used and the size of the CPC is smaller than the Live conductors, therefore this equation should be used to determine the size of the CPC.

eg. Total resistance r1 + r2 = 0.35 ohms

r2= 0.35 x (2.5/(2.5+1.5))

Description of test

Performing this test is to ensure that there is continuity throughout the circuit, that there are no breakages and conductors have been connected correctly. Be sure you have isolated the supply before beginning the test.

Methods

            Test method 1 : Short lead Test

            Test method 2 : Long lead Test

Test Equipment

            Low Ohm Resistance Meter

Note: R1 = Resistance of Line conductor, R2 = Resistance of CPC, Rn = Resistance of Neutral

Test Method 1: Short Lead Test

A Typical example would be to cross connect the Line and Cpc conductors at the distribution board of the circuit going to be tested. At each point of the circuit, at test between Line & Cpc should be performed. Noting that as you perform the tests further away from the cross connected ends at the distribution board, the higher the reading will become. The result recording the highest reading will be the R1 + R2 value on the test sheets. Make sure the test leads have been nulled on the tester or subtracted from the results accordingly

Test Method 2: Long Lead Test

This test is slightly different to method 1, as you are only testing the Cpc. It is known as a long lead test because you may have to use an extra long cable to reach both ends of the Cpc. (Twin+earth can be used to create an extra long test lead). Making sure you null out the test leads before the test.

So basically what you have is your test meter connected to the Cpc at the distribution board and the other (extra long test lead, if necessary) test lead connected to the other end of the circuit. Perform the test, and record the highest result as the R2 value.

An Rcd as a main switch when tripped due to the failure of a circuit or due to nuisance tripping will affect all other circuits wont it

Steel Armoured Cable is protected by a sheath of galvanised steel. This makes it suitable for direct burial, cable ducting or it can be surfaced mounted without any further protection. It can be used for indoors and outdoors.

Steel Armoured Cable (SWA) Cores Colours

2 core = Brown/Blue
3 core = Brown/Black/Grey
4 core = Blue/Brown/Black/Grey
5 core = Green-Yellow/ Blue/Brown/Black/Grey
Alternative core colour - Black numbered

This is a great website which shows you how to make off a steel armoured cable. There are 3 steps shown on this website, very clear and detailed.

Step1:

Step2:

Step3:

Average Prices (taken from www.screwfix.com on 26/10/07)

6943 X 2.5mm 3 core SWA Cable 25m     £31.42

6943 X 4.0mm 3 core SWA Cable 25m     £39.94

6944 X 1.5mm 4 core SWA Cable 25m     £34.97

6944 X 2.5mm 4 core SWA Cable 25m     £44.25

6944 X 4.0mm 4 core SWA Cable 25m     £51.95