Garage supply

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.

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.