by ericmark »
Fri Oct 08, 2021 5:34 am
We wire house sockets using 1.5, 2.5, 4, or 6 mm sq cable, also often 2 x 2.5 mm sq with a ring final, we can protect that cable with 13 amp fuse, or 16 to 32 amp MCB or RCBO, so there is no single answer, you need to find out how supplied before you can alter it.
It is common for a fused connection unit like Mr White shows to be fitted in the home where the cable comes out to the out building, having a twin pole isolator switch means you can switch it all off if water gets in. Inside the home often plastic rather than steel shown.
There are three things which limit the number of sockets, with an unfused spur on a 32 amp supply only permitted one device, but if that device is a FCU then it can feed as many sockets as you want as long as the prospective short circuit current is high enough to open the protective device and the permitted volt drop is not exceeded.
Clearly volts, amps and ohms are related with ohms law, so the prospective short circuit current is related to the loop impedance, as is the volt drop.
With a 13 amp fuse the loop impedance needs to be better than 2.42 ohms, 3 amp fuse 16.4 ohms, but if exceeded it means the device takes just a little longer, but still works.
However with a miniature circuit breaker there are two devices in one, a thermal trip which takes a long time to operate, and a magnetic trip which works in a fraction of a second. The magnetic bit is given a letter, B = 3 to 5 times, C = 5 to 10 times, and D = 10 to 20 times the rating of the thermal cut out.
So with a B32 MCB it needs 160 amp to trip it, or 230/160 = 1.44 add a little 5% for safety so 1.36 ohms is required to open the magnetic part, this is a problem for the DIY man as the cheap plug in testers often show if better than 1.9 ohms, so with a B20 at 2.18 ohm the plug in tester is OK, but not with the ring final with a B32 MCB. And once the figure is exceeded the time for the thermal part to work is very much longer, so unlike the fuse, the loop impedance for a MCB/RCBO is very important. Just a little too much and it does not work.
For the volt drop allowed 5% so if incoming supply 0.35 ohms then 0.94 ohm for the middle socket on the ring final which equates to 106 meters of 2.5 mm sq cable. That is line - neutral where the other is both line - earth and line - neutral which ever is the highest.
0.94 = approx 245 amp, and many loop impedance meters show line - earth in ohms and line - neutral in amps, so prospective short circuit current needs to be over 245 amps.
In real terms we don't really worry too much about volt drop any more, it was a problem with fluorescent lights and refrigeration units, still is for many fridges and freezers, but most other stuff have switched mode power supplies so can work with massive volt drop. So unless putting a freezer in out building or using fluorescent lights not worried too much about volt drop.
As to the earth loop impedance, one is not to rely on the RCD, but in real terms as long as there is an RCD then really your OK with up to 200 ohms earth loop impedance, after that it is considered unstable.
It seems the RCD FCU the RCD does not comply with the BS EN number required so officially you should not rely on them, however I think that is being rather pedantic.
In my home every circuit is protected with type AC curve B RCBO's which is a MCB and RCD combined, but not all homes have that, where many circuits are protected by one RCD you need to remember just a little water in out building can stop half the house working, and turning off the line is not enough, both lives, that's line and neutral need to be switched off.
If not RCD protected your not permitted to add to the circuit, BS 7671 is not retrospective, but when you add to a circuit it needs to comply with current regulations, and with current regulations all circuits need RCD protection, well there are some exceptions, but unlikely to apply to a domestic property.
So DIY wise, if using a switched FCU or RCD FCU if house not RCD protected your reasonably safe, and to be sure you can use a plug in tester with loop test which cost around £50.
To test a ring final or radial supplied with a 32 amp curve B you need a loop impedance meter, which start at around £150, so not really a DIY tool.
You should also use a RCD tester, a 30 mA RCD should not trip at 15 mA and at 30 mA trip within 300 mS and at 5 times so 150 mA trip in 40 mS and I can't measure that on my stop watch, so again a special tester is required.
Personally where the RCD is an addition, not too worried, if it trips with test button then good enough for me, but if working in some one else's home clearly I must test.
The RCD has been talked about a lot lately with testing, we have types AC, A, F and B, and some testers can be set for the different types, but the 40 mS and 300 mS times are only when tested as type AC. So one asks why have the type A, F, and B settings on the tester?
Some times it all seems silly, we don't test tripping times for a MCB we trust the manufacturer, however early RCD's could fail if a strain was put on the casing when torquing up the terminals, so we always tested them, the old non electronic RCD was rather prone to sticking, the new electronic type are far better.
Oh am I glad I have retired. Getting far to complex.
