City & Guilds 2365

Electrical Installation

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Test Method 1

1. Make a temporary link between line and the protective conductor in the consumer unit.

2. Null the test leads.

3. Test the resistance between the line and earth at each outlet.

The value displayed on the meter will be R1 + R2.

R1 is the resistance of the line conductor.

R2 is the resistance of the protective conductor.

Test Method 2

1. 1.One lead of the test instrument is connected to the earth in the consumer via a wandering lead.
   

2. 2.Null the test leads.
   

3. 3.Test the resistance at the earth terminals of each outlet.
   

The value displayed on the meter will be R2.

R2 is the resistance of the protective conductor.

Typical findings

Length = 40m

Cable with 2.5mm cpc and line

Cable resistance = 7.41 μΩ/m

R1 + R2 = 40 x 2 x 7.41 / 1000 = 0.59Ω

If you measure between 0.82Ω to 1.2Ω it’s OK

Typical findings

Length = 40m

Cable with 2.5mm cpc and line

Cable resistance = 7.41 μΩ/m

R1 + R2 = 40 x 7.41 / 1000 = 0.30Ω

If you measure between 0.4Ω to 0.5Ω it’s OK

Step 1

The cables of the ring final circuit under test should be disconnected from the consumer unit and ADS devices (MCB, fuse).

Measure the end-to-end resistances of each conductor.

These tests will give you the values of r1, r2 and rn. rn is the neutral resistance.

If all conductors are of equal CSA then they should have equal resistances. If the CPC has a smaller CSA then it should have a lower resistance than the line conductor. For example if the line conductor is 2.5mm and the CPC is 1.5mm you would expect the resistance of the CPC to be around 1.67 times greater.

Step 2

Cross-connect the line and neutral conductors with a connector block.

Measure the resistance between line and neutral at each outlet.

You should expect the resistances at each outlet to be very similar to each other. The value you measure should be close to (r1+rn) / 4.

Step 3

Cross-connect the line and earth conductors with a connector block.

Measure the resistance between line and earth at each outlet.

You should expect the resistances at each outlet to be very similar to each other. The value you measure should be close to (r1+r2) / 4. It is likely that resistances of outlets connected as spurs will be higher.

The whole consumer unit

The main switch should be off but the circuit-breakers on.

Lamps should be removed.

Switches should be off.

Use a meter to measure the resistance between:

    a. Line and neutral

    b. Line and earth

    c. Neural and earth

A final circuit

The main switch should be off but the circuit-breakers on.

Lamps should be removed.

Switches should be off.

Use a meter to measure the resistance between:

    a. Line and neutral

    b. Line and earth

    c. Neural and earth

The main switch should be off.

All fuses should be removed and circuit breakers off.

Lamps should be removed.

Switches should be off.

Place a temporary link between the line of the circuit to be tested and the earth.

Measure the resistance between the line and earth terminals at each switch or ceiling rose.

Measure the resistance between each of these points in turn:

1. 1.L1 to L2                   
   

2. 2.L1 to L3                       
   

3. 3.L2 to L3
   

4. 4.L1,L2,L3 (connected together) and neutral
   

5. 5.L1,L2,L3 (connected together) and neutral
   

6. 6.Neutral and earth

Record the lowest value as between live conductors.

Record the lowest value as between live conductors and earth.