The Problem With Ring Final Circuits


The conventional set up in most UK homes is for socket outlets to be supplied by a ring final circuit [RFC]. This came about after the Second World War due to austerity and a National shortage of copper. It became possible to supply a 32 Amp circuit utilising 2.5mm conductors. The earlier standard was 2.5mm² radial circuit, protected by 15 Amp over-current protection device [fuse].

Ring Final Circuit [RFC] outlet missing line conductor

Socket outlet with line conductor missing from termination point.

The picture above shows the main problem with RFCs. The line conductor [red] is missing from one leg of the ring. However this circuit was protected by a 30 Amp fuse, but 2.5mm cable rated at 20 Amps, when NOT in RFC configuration.

The picture below shows the schematic layout of a ring final circuit.

Circuit diagram for Ring Final Circuit RFC

The diagram below shows the basic terminology for circuit layouts.

The damaged circuit would function without any clear problems. Appliances plugged in would still work and no one would be aware of what is going on behind the scenes. The two main problems are imbalanced loading causing overheating and possible failure of protection devices to operate in the correct time to offer fault and/or shock protection.

I prefer to install radial circuits with 4mm² conductors. The radial circuit is quicker to install as there is no need for a return leg. So the actual copper content [material costs] is broadly similar to a RFC installed with 2.5mm² conductors. The circuit will not function if any line conductors are not connected. The testing of radial circuits is much quicker than RFCs due to no need to undertake the three-step ‘continuity of ring final circuit conductors’ tests. The three steps are:

1. End to end test of each conductor, line [r1], neutral [rn] and [earth] circuit protective conductor [r2]. Using a low-resistance Ohmmeter test results must be within 0.05 Ω for conductors of the same size or 1.67 times for 2.5mm²m line and 1.5mm² CPC conductor combinations.

2. Line conductor [outgoing leg] connected to neutral conductor [return leg]  Line conductor [return leg] connected to neutral conductor [outgoing leg]. Using a low-resistance Ohmmeter test results must be taken at every socket outlet, and point of utilisation if appliances connected by double pole switch or fused connection unit. For points of utilisation connected in the ring, the test results should be broadly the same. The approximate calculated value should be (r1 + r n)/4

3. Line conductor [outgoing leg] connected to circuit protective conductor [return leg]  Line conductor [return leg] connected to circuit  protective conductor [outgoing leg]. Using a low-resistance Ohmmeter test results must be taken at every socket outlet, and point of utilisation if appliances connected by double pole switch or fused connection unit. For points of utilisation connected in the ring, the test results should be broadly the same. The approximate measured equivalent to the calculated value should be (r1 + r2)/4. Substantially different values will indicate a spur connected to the ring. Test results rising or falling from the calculated value will indicate a radial circuit or break in the ring following an interconnection within the ring. This is a dangerous condition as  the possibility of fire, overload or failure of shock and/or fault protection is possible. Care should also be taken as socket outlets may give dissimilar results due to dirt or other obstructions increasing resistance readings. If possible visual inspection must be undertaken by examining the electrical connections within the accessory.

The three step test identifies polarity so the remaining ‘dead tests’ are insulation resistance between all conductors [between live conductors and between live conductors to earth] and functional switching.

The reduced testing requirements for radial circuits are:

1. Continuity of conductors [r1 + r2] at furthest point.

2. Polarity at each point of utilisation

3. Insulation resistance of all conductors [between live conductors and between live conductors to earth].

4. Functional switching

The three-step test is not needed for radial circuits.Therefore the time needed to comply with BS7671 testing requirements is greatly reduced.

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All aspects of energy efficiency and help with saving money spent on utility bills. Home visits undertaken to assess what is used and explain what can be done to save money. Central heating control upgrades, central heating system power flushing. Energy saving lighting surveys and installation of low energy lamps and lighting. Every day that action is not taken is an added cost in hard cash terms. Call today to see what can be done. Green Deal Ecologic Ltd, improving your world without costing the earth.

One Response to The Problem With Ring Final Circuits

  1. Pingback: How NOT to Terminate 3 Core SWA Cable « Improving your world without costing the earth

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