One of the main things you will have to do in your City and Guilds 2395 or 2394 practical is to test the continuity of ring final circuit conductors. It is of extreme importance that that the test is done in the correct manner otherwise you will not be able to detect incorrect connections in the ring.

 

How to Test Continuity of Ring Final Circuit Conductors 


The method for checking the continuity of ring final conductors is described in IEE Guidance Note 3. Provided the test results are recorded methodically the test can be used to confirm: 

  1. The ring is continuous
  2. There are no interconnections ( rings within the ring)
  3. The value of R1+R2
  4. The sockets are correctly connected (provided the readings in steps 2 and 3 have been made using a test lead with a plug plugged into the socket)

The test will also reveal various types of fault. The test must be carried out at initial verification. IEE Guidance Note 3 suggests that 
it need not always be carried out on a periodic inspection if there are adequate records from previous inspections and the ring has not been modified. 

Method 
A low resistance ohmmeter is required. 

Step 1 

Measure and record the end to end resistance of each conductor in the ring: 

  • R11 line conductor resistance
  • Rnn neutral conductor resistance
  • R22 circuit protective conductor resistance

Step 2 

  • Connect the neutral conductor of one end of the ring to the line conductor of the other end of the ring. Connect the remaining line conductor to the remaining neutral.
  • Measure and record the resistance between line and neutral at each socket on the ring (including any spurs ). This is most conveniently done with a test lead which connects the ohmmeter directly into the socket with a plug. ( Some makers supply such a lead.) This also has the benefit that successful test results in Step 2 and Step 3 will confirm that the socket has been correctly wired and the polarity is correct.
  • NB If testing through the front of the socket with a suitable lead an infinite reading will be obtained unless the switch is on. 
  • The result expected at each socket should be equal to one quarter of the sum of R11and Rnn (as measured in Step 1 ). 
  • The readings at any spurs will be a little higher.

Step3 

  • Connect the circuit protective conductor of one end of the ring to the line conductor of the other end of the ring. Connect the remaining line conductor to the remaining circuit protective conductor. 
  • Measure and record the resistance between line and c.p.c. at each socket on the ring (including any spurs ). As before, this is most conveniently done with a test lead which connects the ohmmeter directly into the socket with a plug. 
  • The result expected at each socket should be equal to one quarter of the sum of R11 and R22 ( as measured in Step 1 ). 
  • As before, the readings at any spurs will be a little higher.

Analysis of Ring Continuity Results 


The following end-to-end readings were obtained when a ring was tested:

  • R11 = 0.6Ω
  • Rnn= 0.6Ω
  • R22=1.0

R11 and Rnn should be the same as the cross sectional areas of neutral and line conductors should be the same in a single phase circuit. So the results look OK. 
R22 may be higher than R11 or Rnn as it is common to use a cpc of smaller CSA. In twin and earth cables used in standard ring circuits, line and neutral are 2.5 mm² and the cpc is 1.5 mm². Here we should check that the csa of the cpc is indeed smaller than 
the csa of line and neutral. 

The reading expected in Step 2 is: 

  1. (R11+Rnn)/4
  2. ( 0.6+ 0.6 )/ 4
  3. 0.3 Ω

The reading expected in Step 3 is:

  1. (R11 + R22 ) / 4
  2. (0.6+1.0)/4
  3. 0.40 Ω 

The actual readings obtained in Steps 2 and 3 were recorded in the table below:
 

Socket NoStep 2 Reading (Ω)Step 3 Reading (Ω)
10.310.41
20.30.4
30.320.41

The readings above are very close to the expected values so this ring is OK 

Possible Ring Continuity Faults 


Some possible faults are given in the table below:

Socket NoStep 2 ReadingStep 3 ReadingPossible fault
1Infinite*Infinite*Neutral and cpc reversed at socket
or line not connected
2OKInfinite*Line and neutral reversed at
socket or cpc not connected
3Infinite*OKLine and cpc reversed at
socket or neutral not connected


*Readings will also be infinite if testing through the front of the socket with the switch off. Checking that the reading is infinite with the switch off in Step 2 and Step 3 will verify that the switch is in the line conductor. 

  • A high reading indicates a loose connection.
  • A slightly high reading in both steps suggests that the socket may be on a spur.
  • Where readings are not as expected the cause must be ascertained and the necessary rectification carried out

There you have it.If you follow this method then you should be able to complete a ring continuity test to the required level to pass your C & G 2394 or C & G 2395 practical assessment.