Garage supply

If the circuit supplies a CU, then it's no longer a ring final circuit by definition.

I don't read the definitions that way - not unless the ring supplies just the CU and no sockets at all. A ring connected to both sockets and a CU is still ‘directly connected to a socket outlet or socket outlets’ so meets the definition for a final circuit … and sensibly still has to meet final circuit requirements (0.4s disconnection time etc). (Same as if a ring supplied a mix of FCUs and sockets). The choice of the word ‘final’ is perhaps a bit unfortunate given its usual menaing in English, but the definitons don't seem encumbered by that.

   - Andy.

My take is that a ring final cct is a final cct in its own right. However if a further fuseway (fuse or breaker for example ) is attached then the downstream circuit is also a cct in its own right and the ring final supplying it is its distribution cct.

So, a ring final circuit is a final circuit and if supplies another (fixed) circuit is is also a distribution circuit.

Some tings are one category some are another and some are both. 

Of course a plugtop plugged into a socket on that ring running say a kettle, lets it run the kettle and is its own circuit but it is not part of the fixed installation.

Well indeed - is a 4 way block extension lead screwed to the wall above the worktop a ‘final sub-circuit’ ? Perhaps by some definition, but you'd still consider the sockets on the ring it plugs into to be part of a final circuit too, not just distribution. I'd strongly suggest not worrying too much about it.

Rather like a biologist wondering which of the entirely man made categories the platypus belongs in - the platypus is not affected and does not care.

Mike.

AJJewsbury: 
 

If the circuit supplies a CU, then it's no longer a ring final circuit by definition.

I don't read the definitions that way - not unless the ring supplies just the CU and no sockets at all. A ring connected to both sockets and a CU is still ‘directly connected to a socket outlet or socket outlets’ so meets the definition for a final circuit … and sensibly still has to meet final circuit requirements (0.4s disconnection time etc). (Same as if a ring supplied a mix of FCUs and sockets). The choice of the word ‘final’ is perhaps a bit unfortunate given its usual menaing in English, but the definitons don't seem encumbered by that.

   - Andy.

This is really tricky. If the garage were a separate building in the premises, see definition of distribution circuit … where the distinction would be far clearer.

The important points in a case like this are:

• the circuit is no longer one of the considered standard final circuit arrangements. I agree that if loads in the garage are typically inconsequential, that's no different to a couple of spurs in a single location. However, if loads might be significant (a couple of potentially heavy-current using appliances in there like tumble dryer and washing machine), it may upset the current balance of the ring.
• we have reduced selectivity that two separate circuits would provide - faults in the garage disconnect the ring final circuit and vice-versa - including a fault in an appliance in the house taking out the light in the garage, which may (or equally may not) be a safety issue.
• the definition for final circuit in BS 7671 doesn't always fit the real world.

This is really tricky. If the garage were a separate building in the premises, see definition of distribution circuit … where the distinction would be far clearer.

I agree with ebee, I think it only seems tricky if you assume that a circuit can't be both a distribution circuit and a final circuit at the same time. If you accept that it can, things seem to slot into place.

In some situations - e.g. temporary event setups - having a DB supplied from a plug & socket would be perfectly normal. Our ring final also supplying a garage CU (or a whole stack of FCUs) is no different in principle. BS 7671 should be able to cope - and the definitions seem to (there's nothing in the definition of a distribution circuit to say it's not a final circuit, or vice versa).

The wording of 411.3.2.3 and 411.3.2.4 might need another look though…

   - Andy.

In one way, that's where I'm going … if one were to decide that a circuit could be both a final circuit and a distribution circuit, then you agree it's not purely a final circuit … which then means it could well be a “circuit not covered by Regulation 411.3.2.2”.

It may also mean that where other regulations concern final circuits, they too could also be either used or ignored at will …