The UK was the first and is now is probably the only country in the world which still uses the 30 amp ring system for wiring socket outlets in electrical installations. The ring main circuit has long been the subject of many a heated discussion between electrical experts, but as an electrician you'll no doubt need to test them in the course of your duties. Here is an overview of Ring Main inspection and testing procedures.
Warning- Please note that this is a 'dead' test so the distribution board, consumer unit or circuit must be isolated from the electrical supply. To begin with you'll need to carry out a Safe Isolation Procedure and get permission to turn off the supply.
Here's the testing procedure:
Disconnect all appliances and accessories from the ring circuit and carry out isolation tests between the Line, Neutral and Earth CPC.
End to End Ring Continuity Test
Set your test meter to low resistance range and 'zero' the test leads. Measure the resistance of ring continuity of each of the live, neutral and earth conductor ring loops. Record these end to end statements for future reference and entering onto a test sheet. Note that where the earth conductors are smaller, the resistance will obviously be higher. For example for twin and earth cables 1.5mm will be 1.66 times higher than 2.5mm.
Super Loop Test L & N
Cross connect the live and neutral conductors from opposite ends of the ring and to form a super loop. If at first this sounds confusing, try drawing the live and neutral loops of a ring circuit on a piece of paper with the opposite ends connected. You'll see that it forms one big super loop.
Test resistance between the ends you have just connected together. The reading taken between the super loop ends should be about half that of your previous continuity reading. You can then verify the ring main wiring and connections by going to each socket outlet and testing between live and neutral. Where the socket is on the ring and the connections are 'good', your readings should be within 0.05 of your original super loop reading at the distribution board.
These readings are not normally recorded but will form the basis for your assessment of the ring main circuit connections.
High readings could indicate that the socket is on a spur from the ring, that connections need tightening or that the socket is faulty giving a poor connection to the plug and needs replacing. Where the readings are not consistent this could indicate that there are cross connections and the ring circuit has been bridged.
Super Loop Test L & E
Cross connect the line and earth conductors from opposite ends of the ring and to form a super loop. Test between the two sets of ends you have just connected together. The resistance reading should be about half that of your previous continuity reading. You can then verify the ring main wiring and connections by measuring the resistance at each socket outlet as for Live and Neutral described previously. Testing between live and earth at the sockets also checks for correct polarity.