by ericmark »
Sun Dec 14, 2014 12:57 am
Please realise I am now quoting regulations rather than saying what I would do I will get to that after.
1) All new sockets under 30A with the exception of where marked for a single item of equipment i.e. freezer need RCD protection to 30 mA at 40 ms maximum time. It is unlike a fused 30A supply will be RCD protected to 30 mA may be to 100 mA delayed (S type) but not 30 mA.
2) Any buried cable in the wall will also need 30 mA RCD protection if less than 50 mm deep.
3) The loop impedance for a 30A fuse to BS 3036 is 1.09 ohms for a 0.4 s disconnection time. With a B32 MCB it would be 1.44 ohms.
4) The volt drop if wired to BS7671:2001 is 4% if wired to BS7671:2008 i.e. RCD protected then 5%. The volt drop can be calculated from the loop impedance.
5) Under BS7671:2001 or before it was around 80 meters for the volt drop if under BS7671:2008 then 106 meters is the limit. The limit is because of the volt drop and the formula is rather complex. For volt drop we consider with a B32 MCB that the first 20 amp is drawn centre of ring and the remaining 12 amp is even spread so the design current is taken as being 26 amp when working out volt drop.
So in theroy to add extra sockets to a ring designed before 2008 is rather problematic. So now ways around it.
Clearly you have to know what is already there to extend the ring so earth loop impedance meter will be required. If all the extra wires are either special Alu-tube or surface then you could fit RCD sockets however fitting these is expensive.
The other method is to use RCD FCU (fuse connection unit) which will mean you can add many sockets all protected by a single RCD but the total load is only 13A total.
At the consumer unit we fit passive RCD's this means with a power cut they stay connected so with return of power everything still works. But where the loop impedance is too high to ensure there is enough voltage with a short circuit for the device still to work we use an active RCD this means it fails safe and any power cut the RCD will trip and needs resetting. This could be a problem with for example a freezer.
To know if safe to use a passive type you do need to know the loop impedance.
So to use an active RCD FCU and form a spur is reasonably safe when the loop impedance is unknown. But with any other method you need the readings.
The readings should be taken from the centre socket and often a few sockets will need testing to work out which one is centre.
So question is what would happen if to exceed the loop impedance for either the fuse rating or volt drop. Well that 0.4 second figure will be extended. Not too bad with a fuse maybe 0.6 seconds instead of 0.4 seconds but with a MCB it jumps from 0.01 seconds to 1000 seconds if the magic 1.44 ohm is exceeded so extremely important with MCB protection.
As to volt drop well much depends on the equipment. Anything with a switch mode power supply likely unaffected but non electronic fluorescent electric lights will likely fail to light. Radios will have mains hum, and freezers and fridges may burn out the overloads. Battery charger may fail to fully charge a battery.
It is the centre of the ring most affected so it may not be the garage but some other room in the house and if that happens to be the kitchen then fridge/freezer damage is a real problem.
9 times out of 10 extending a ring without testing works OK. But you are taking a risk. As to if electricians test fully I really don't know. But to become a scheme member they have to have the test equipment so one would hope they would use it. They also have to issue a minor works which should show their results.
Because of the price of test equipment near every DIY guy breaks the rules even to hire costs around £75. There are plug in testers with loop test. But every one I have seen the pass lamp comes on at over 1.44 ohms so although the fail lamps shows there is an error the pass lamp does not mean there is no error.
