That is not their purpose. What would happen if the cable from the house to the garage were shorted, and as you are suggesting, the only protection were the electricity supplier main fuse.
They do not protect the supply cable against a short before the MCBs. Their purpose is to protect downstream...
But not a fault in the cable or CU.
Your advice sure is.
That is normal and the average domestic installation. What protects the cable from the meter to the CU is the main fuse.
I haven't given any advise. The only thing I said was take an MCB off the main CU and run three single cores through a conduit to a garage CU. Quite normal.
The supplier's cutout protects a couple of metres of 25mm² tails. Suppliers don't allow their cutout to protect any more than around 3 metres of tails.
which is not at all what you're suggesting in articles news: news: news: news: news:
In the case of the main house CU, yes.
In the case of a garage one some metres away additional protection at an appropriate and lower current rating is required.
SWA is more appropriate and you have been doling out all kinds of misinformation as usual.
You seem to have a fundamental misconception of the purpose of the overcurrent devices (MCBs). They are for protection of the downstream circuits *only*, explicitly *not* the upstream supply. It is the function of a protective device at the *origin* of the supply to be responsible for the protection of this sub main.
Hence overcurrent devices at the substation protect the cable runs to the houses. The main fuse protects the cable runs to the CU and guards against faults in the CUs themselves (unless they are more than two meters from the fuse in which case an additional switchfuse should be inserted). The MCBs in the CUs protects the cables in the house wiring, and also performs the function of disconnection in the case of certain fault conditions.
So in the same way that your 100A main supply fuse can not protect the main cable (which will be supplying several properties), your MCBs can not protect the meter tails since the maximum potential load of all circuits combined will exceed the main fuse capacity. Also you may have more than one CU.
You are missing the point. Consider these situations:
Detached garage with two circuits: 6A lighting, and 32A sockets. Sub main cable to garage CU rated for 40A nominal capacity. Cable fed directly from a junction box that splits the tails from the meter. Supply rated for 100A, TN-C earthing.
Your garage catches fire. It melts the garage CU and results in a L-N short on the sub main cable. What happens?
Since there is no suitably sized protection for the sub main cable at its origin, the cable may burst into flames. What was a fire in your garage is now a fire in the house as well.
You are digging in the garden and manage to stick a spade into the cable. You cause a phase earth short. Lets say that the cable does not burst into flames this time, but the main electricity companies fuse blows taking out all circuits in the entire property and technically requiring the assistance of the electricity company to come and fix the fuse - which won't do because your knackered cable and spade are still wired in, and there is no isolation for them.
[snip more dangerous advice from a drunken Drivel]
You've just quoted one of your posts.
[snip dangerous advice from Drivel]
Best bit of advice.
Ignore dangerous advice from Drivel.
[snip dangerous advice from a drunken Evil]
This pillock want the supply cable to be too small. Some mothers.....
A dangerous man.
This man is dangerous, take no notice of him.
The Hibernian has come home drunk after tossing the odd electric caber.
Can you cite this please. The breakers on the garage CU woud protect it.
No misinformation whatsoever. Conduit is also used. SWA looks naff, conduit looks neat. You are confused.
I am not suggesting anything other than what I just said. That is making a point with a man who doesn't know very much at all.
"Back to the CU with its own MCB" is a very different proposition.
Plenty to choose from:
314-01-01 & 314-01-02 tripping of the house socket circuit RCD, or opening of the FCU fuse will result in loss of supply to unrelated circuits. I.e. lights in the garage (both cases), and socket circuit in the house (second case).You need to stay on the right side of 413-02-09, and 413-02-10 and meet the required disconnection times. This requires attention to the total earth fault loop impeadance in the outbuilding. Depending on the distance it may not be possible. (on a 13A fuse you have about 2.5 ohms to play with and under 1 ohm on a 40A feed - including the suppliers earth impedance which might already exceed the allowable amount). If you can't manage that then you *must* use a TT style setup and regs
412-02-18 to 413-02-20 now apply.If you are exporting the house earth on a TN-C-S system then all of
547-03 (supplimentary bonding) applies. This may not be possible in some types og garage - and is likely to be a PITA in most.We have the requirement to not include fixed point loads on general purpose ring final circuits, the general requirement to not do things that are contrary to standard practice etc (CBA to find the numbers for those).
All the above from the 16th edition.
Then that is what should be done. You only have to build it once - you have to live with it every day.
Note here that the 15A fuse is actually larger than the 13A head end fuse - hence no discrimination - this is just sloppy design. It also renders the local CUs fuse pointless.
If you are going to stoop to this level, you may as well skip having the garage CU altogether, wire the incoming feed into a socket, and add another FCU to take off the lighting feed.
What would be the point of that? Or do you in fact mean it had an incomer switch rated at 100A?
MCBs and RCDs do very different things. An RCD will not trip on overcurrent. A MCB will not trip on earth leakage.
You think?
So what are you suggesting for a 100A main supply? 35mm sq singles out to the garage? At over five quid a meter each, just to save having a switch fuse at the head end.
No.
If you don't know very much at all about something, as has been pointed out (so glad you realise the error of your ways), why do you persist in peddling incorrect and dangerous information?
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