Probably another one for John (sorry!!) What connectors do I use to connect the new consumer unit to the original one using the meter tails? the two CU's will be "back to back" each side of a cavity wall, so not too much of a distance about 500mm each tail.
this might also be a dumb question but when I take the main fuse out ( I know its supposed to be done by the supplier!!) this in my eyes only isolates the positive side of the sine wave leaving the negative connected and therefore half mains potential of 115v..
The neutral is not "negative", it is zero (and in effect earth). The live (or phase) swings between +240V and -240V with respect to neutral.
However, the neutral may not be at exactly the same potential as your local earth, and you should treat it with the same respect as the live, if for no other reason than someone might have connected your tails the wrong way round and it actually *is* live. :-(
Not the "John" you are expecting - but I believe that the red carries the sine wave and cycles positive and negative at 50 times per second. Neutral is just what it says - it is bonded to earth at the substation.
removing the main fuse? now answered and all is well. Wiring two ring mains two lighting circuits and two radials for cooker and hob should not be beyond me. If I am uncertain of something I stop and ask someone more knowlegable than me, this I think is the safe thing to do. Stupidity would be to carry on regarless without asking.
But there are other issues, eg you seem to be adding a new consumer unit to the old but haven't mentioned by how much the load will increase, what the rating of the main cutout is or what size are the existing meter tails which you will presumably be joining on to with a Henley block. I know you are trying to get it right before you leap and that is laudable but this really is a job you need to be careful of.
cooker and hob will be removed from the existing CU and moved to the new. load added would be the two ring mains and two lighting ccts both lighting ccts will be no more than 500watts combined, 5 double sockets on upstairs ring six on the kitchen ring so not exessive there either. By main cutout are you implying the main fuse?
Yes. You suggest that the heavy loads (cooker and hob) will be moved between CUs rather than added - eg if you were moving from gas cooking to electric - and that the things that are added won't amount to much. So far so good but we didn't have that info to start with.
Sorry I didn't think of the relevance of loading at the time :-( I was trying to find a suitable connetor to "tee" off the meter tails to feed the second CU but screwfix (got to love em!!) dont seem to have anything greater than 30A
and by the looks of the posts I should have googled the answer to my question about isolating the supply !!!
Yes! Sorry, but you're nowhere near ready to go near that side of the wiring.
It is not "safe" to only remove the fuse on one side. However it is safer (note the relative term) than having two fuses, which is why we do it.
Only "live" should be fused. This carries both positive and negative sides of the current alternately. Forget about positive and negative, even though the tails are probably coloured red and black - this is AC we're dealing with here.
Neutral also carries both polarities, and just as much current as live. If you stick yourself between live and neutral, you will ruin your day. You'll probably acquire burns (i.e. it's unlikely you won't) and you have a fair chance of dying.
So what's this "earth" thing about, and what's the difference between live and neutral?
Somewhere in the system (it varies) neutral and earth may both be connected to the ground (literally - a big metal spike). This has two results:
- there's little or no voltage difference between neutral and the ground that your house or your hot tub are standing on.
- there's a big voltage between live and the ground. If live gets to (faultily) contact anything connected to the earth conductor (such as the case of an appliance) then there's a big current and a bang as the fuse blows, isolating the appliance.
Now note that the "earth conductor" and "the ground" aren't the same thing. They're connected (distantly) and they should be at the same voltage, but the impedance of the circuit is higher for the ground than for a conductor. This is why touching a live conductor is usually painful, but rarely fatal -- unless you're unlucky and also either touching the earth conductor, or are unusually well connected to the ground, such as having wet hands and being in the bath. Either way, it's somewhat less risky than a full-blown live-neutral shock.
If you contact the neutral conductor, then you shouldn't (I said shouldn't) receive a painful shock. Neutral should be at close to the earth voltage and thus the voltage of the ground. If all goes well, then it's safe to handle a connected neutral.
So, if you remove the fuse on the live side, we should remove the two worst sorts of accident, including the likely fatal ones.
_HOWEVER_ this assumes a lot of things. It assumes that live is where it ought to be (no one has swapped them). It also assumes that neutral is "benign", if not actually "safe", and that no-one has connected that somewhere it shouldn't be either. Now as both of them have already travelled a great distance in close proximity inside the same cable, then there's all manner of real "It doesn't happen often, but it does turn up sometimes" accidents with drills, jackhammer and carbonised squirrels in substations that can put volts where you weren't expecting them. The question you've got to ask is, Do you feel lucky, punk?
Now this is why we have isolator switches, and double pole isolator switches at that. Single pole fuses are not a reliable way of isolating circuits. Double pole switches are much better. You can still be killed by a faulty or mis-wired isolator, but you have to have a potential accident in the same room as you, and that's just a lot less likely than any potential accident somewhere, anywhere, between you and Battersea. Commercial isolator switches have key locks on them too.
When you work downstream of the consumer unit, you have an isolator to use. When you work upstream of it, you're relying on a fuse. Don't trust the fuse to isolate things until you've _checked_ that the neutral and the ironmongery is safe to work around first!
So why don't we use douple pole fuses, like we do with switches? Well we used to, some decades ago. Now as the other function of a fuse is to blow with over-current (or just randomly, to keep electricians employed) then these fuses used to blow. If the _neutral_ fuse decided it was its turn to be a firework, then you now had a system that was as live and lethal to work on as ever it was, and it even played dead to trick you into handling it. People regularly got killed that way.
PS - Meter tails carry a lot of current too, which means potential for getting hot and starting fires. Not only might you kill yourself doing it, but doing it badly will have a really good try at burning your house down for you.
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