I'll have a go at defending the honour of the ring final .....:roll:
The ring final was devised just after the end of WWII as a way of minimising the amount of cable needed in domestic wiring.
The ring final consists of a loop of 2.5mm sq twin & earth (similar to your romex) which starts at a MCB in the breaker box, loops around a number of 13A socket outlets & returns to the same MCB. The phase, neutral & earth conductors are all run in a ring. The ring final is very common in domestic wiring - I'd say every house has it least one such circuit ... my home has three (three story Victorian house, 6 beds, 13 rooms in total). The ring final is also used extensively in commercial work - offices, retail etc.
2.5mm sq cable has a rating of around 21-24A depending upon its installation method and the ring final is normally connected to a 32A MCB, so, while the circuit remains a ring all is well & good .... the cable is well over-rated for the job.
Now, lets consider what happens if the ring becomes broken ..... we now have two radial circuits connected to the same breaker but the cable is now rated at less than the tripping current of the MCB. Obviously not good & not ideal, but, in reality the house probably won't burn. Here's why..... The cable is good for around 24A. Our nominal voltage here is 230V, so 24A equates to 24 x 230 = 5520W. So.... we need to pull more than 5kW from the circuit before it becomes a problem. Realistically no amount of TV's, VCRs, small appliances are going to need this much power.... the only real way pull this much current would be using heaters. All 'fixed' heaters should be wired on their own dedicated circuits as should fixed water heaters ... so they shouldn't contribute to the loading on the damaged ring final. The most likely candidates would be portable heaters (which tend to be 2kW convectors or fan heaters) or laundry equipment (washer, dryer etc). In order to over-heat the 2.5mm sq cable the load would have to be much greater than 24A for a prolonged period ... the laundry equipment tends to cycle once its reached temperature so that probably won't do it .... that only leaves portable heaters. We'd need 3 x 2kW heaters ( approx. 26A) to overload that cable .... even then the overload would only be in the order of 9% ... and how long would 6kW of heat run for before it started to cycle on its thermostats?
So, although, at first glance the ring final does seem to pose a danger of an open circuit which wouldn't be readily detected by the homeowner.... in reality its unlikely that this failure would actually result in a fire. There are also certain other things in our wiring methods which lessen the risk: we don't use wirenuts or pig tails - both conductors will terminate in a tunnel terminal on the receptacle itself (this design clamps down on the conductors ... we don't wrap the conductor around a screw head). Old school electricians will also wire the ring final without actually cutting the cable (they will remove a section of the insulation with a knife, form the conductor into a 'u' shape & terminate that in the terminal, so even if it does come out continuity of the ring is not lost).
There are various tests which have been devised to prove ring circuit continuity & these are a required part of an inspection & test on an electrical installation.
Adrian
