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Old 5th May 2021, 4:52 pm   #49
Lucien Nunes
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Join Date: Apr 2008
Location: London, UK.
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Default Re: When were ring mains introduced in domestic housing?

Quote:
The ring was under the upstairs floor to the locations of sockets on the ground and first floors, then round junction boxes were used under the floor for a single cable to each socket.
This was a popular system in the earlier days of rings that still triggers lively debate about its merits and demerits. It does not affect the load distribution on a ring, nor are any cables unprotected against overload, therefore one can ignore the fact that each point is spur-like in construction. It is easier to chase in one cable to the point and easier to terminate.

A technical advantage historically given some importance was the ability, when wiring via traditional circular junction boxes with slotted pillar terminals, to run the ring itself as a single unbroken cable with the sheath and insulation removed mid-span at each JB. This is somewhat nullified today by the requirement for all junctions to be accessible for inspection unless certified maintenance-free to BS5733. Any JB under nailed floorboards must therefore be MF, but there are no MF accessories that accommodate an unbroken cable run, so one might as well make the connection in the socket-outlet.

Another aspect (more applicable to commercial than domestic) is leakage current. Historically, before the widespread adoption of SMPSUs, operational leakage current was minimal. Today, the aggregate current from many small Class I electronic loads within the rating of a single 32A final can exceed 10mA, at which level a high-integrity CPC is required. This can be satisfied with a ring CPC using dual-terminal accessories, so that any one terminal can be open-circuited without loss of CPC connectivity to the MET. The unbroken cable of a spiky ring via round JBs also satisfies this. With a limit of 3.5mA per 13A socket and one double socket per spur the single cable is acceptable too (although not if serving one hard-wired FCU point leaking >10mA; that would need to be on the ring proper)

Disadvantages include not being able to extend a cable from a socket to a second point, since the spur cable is not then protected against overload; one must take it direct from the ring. Plus a greater number of accessories, which even if MF, are a potential point of failure, and do take time to wire.
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