OT:- Trippin mains - Electricity gurus in da house?

No nead to have a tantrum, I replied to the messages as they appeared on my news server, don't blame me if they didn't appear promptly. It was you who claimed 2.5mm cable will handle 30A, apparently confirming we were talking about a 30A fused ring that had broken.

And where did that come from?, I only suggested you should get it fixed!.

Greg

fuses...

Problem now is

Ah but - if it's an old 15 amp radial installation it is probably wired in 7 / .029" or some such ! May even have a nice TRS cover

AWEM

I'm too young to remember that 8-) Greg

On or around Tue, 12 Dec 2006 11:48:05 -0000, "Andrew Mawson" enlightened us thusly:

nah, the wiring's not THAT old. someone has obviously rewired it sometime, most of the visible (and in-use) wires are modern plastic insulation T&E. I suspect, knowing the general construction of the house, that the wiring almost all goes through the space between the first floor and the ground floor ceilings; in theory, it should therefore run down the wall to the sockets. That's "in theory" though. Have to hunt a "wire in wall finder" from someone. Accessing the existing wire will depend on where it runs and how much hassle it is lifting the floor upstairs.

On or around 12 Dec 2006 03:33:56 -0800, "Greg" enlightened us thusly:

OK, OK. mea culpa.

yeah, but the normal reaction from electricians when faced with old places without ring mains and so forth is that it needs rewiring. In an ideal world, *I'd* rewire it, but of course the PTB have put the kibosh on that. Having seen the sort of work some supposedly qualified people produce, I find this somewhat ironic.

Which is not to say that most of them don't do a decent job... but having the bits of paper is no guarantee at all.

Which it won't - 25A is the maximum current rating for 2.5mm TPS

On or around Wed, 13 Dec 2006 08:27:51 +1300, EMB enlightened us thusly:

CSA mm2 Current Amps

so according to that 2x1.0mm² has more than double the capacity of 1x2.5mm², despite being significantly smaller.

which ought not to make sense, or looking at it another way, why don't people use lots of small wires rather than 1 fat one?

There again, the whole lot works on luck and/or a huge safety margin: consider, you put a ring main in the downstairs part of a house, excluding the kitchen 'cos you're clever. The ring is of 2.5mm² and so any one socket on the ring can draw 54A, and since the sockets are individually rated at

Now, if you have overload trips which actually trip on not to much overload, you can restrict the current. how much overload does it take though, and for how long, to trip (say) a 32A MCB?

ratings;

margin:

though, and

Austin,

The current ratings cannot be added because the ratio of current to surface area (and hence ability to lose heat) differ. So to take the

AWEM

It is old (1974), but it's been checked out by a professional sparky, a tool repair shop, and also by an electronics wiz. All concurred that it's a design issue, but NOT a safety issue. The consensus was to use it on circuits that do not have an RCD, which are few & far between these days.

The design issue (apparently) is because it's metal bodied, and is a large motor geared low, the start up process creates a surge that fools the RCD into thinking there's a fault. The RCDs (all the ones it's been used on) are perfectly happy once the drill is up to speed.

It does tend to be used in a drill press, it's a 400 rpm drill that's perfect for drilling core holes in ceramic tiles using TCT hole saws.

I've been assured by all the repair guys I've been to that it's perfectly safe to use. Changing the RCD is not possible as it's carried from site to site, and the drill has passed all required (Australian)site safety tests. It's a quirk, I'm told, not a safety issue, as the drill design pre-dates the modern style of RCD.

AIUI anyone can actually do the install - it's the tester/certifier that needs to be qualified.

/disclaimer :: IANA-sparky

Yes that's the reaction you will almost certainly get if you bring in a sparky, but to be fair they have little choice but to say that as the new regs mean they can't just fix a bit of an installation and ignore the fact that the rest of it is dangerous, they could lose their registration. (we're talking about domestic work here, no chage to industrial, yet)

I'm told that if you're in NICEIC you can't even touch an installation if you find the earthing isn't up to scratch, your only option is to say they have to have it sorted or walk away, apparently that's dinned in when you jump through their (expensive) hoops to get in.

And so do a lot of sparkies, many have left the business rather than jump through the hoops, and a lot of dodgy persons have got the paperwork. Greg

Fair enough Karen, you've obviously taken it seriously and got second opinions. What puzzles me is the fact you can hold an RCD on, it sounds like the ones permitted by your local regs are very different from ours which allways, in my experience, have a mechanism to prevent this. Greg

Current rating is not about the carrying capacity of a bare copper wire which is hugely more than the regs would imply, it's all about the temperature of the insulation. That's why overhead wires are bare, they can carry large currents and get hot because there's nothing in contact with them to be damaged.

Insulated cable on the otherhand has to be rated to keep the (usually PVC) insulation in good condition, PVC goes soft if it gets too warm and the wired can push through it and short, get it even hotter and it goes hard and brittle or even chars. To keep the PVC cool you have to consider how much heat is being put into the cable and how fast it gets out of the cable.

The power put in is inversely proportional to the cross section, doubling the cross section halves the power, but the power out of the cable is dependent on the surface area AND the surroundings. A 2.5mm cable does not have 2.5x the surface area of a 1mm cable so can't loose heat 2.5 times faster for a given surface temperature, hence the ratings are apparently non-sensical.

The cable's surroundings are also hugely important, a cable laid inside insulation can't get rid of heat as well as one 'clipped direct' to a walll so there are huge derates for such situations. That's why people get into bug trouble by expecting a cable to have a single rating, and the DIY sheds only make this worse putting best case numbers on the packs.

In the regs luck is called 'diversity', i.e. the typical use of an installation rather than the worst case use, and yes there are large safety margins, but would you rather there weren't...

This is where diversity comes in, people want many sockets in a room to avoid having to run extensions and use adaptors, in fact the regs now mandate fitting enough sockets to minimise the potentially dangerous use of extensions. This doesn't mean people are going to try and draw

The regs are full of 'tripping curves' and calculations and if you are really interested you can learn how to use them to match a protective device to a cable in any situation, I had to in order to pass the inspector's exams and in industrial work you often need to know such things. But in the everyday word of a domestic sparky you just use standard circuits where someone else had done the calculations, 32A type C MCB for a 2.5mm ring for example. Greg

And of course be sued for huge damages or be prosecuted under the building regs. Greg

And of course be sued for huge damages or be prosecuted under the building regs. Greg

Oopse, that should be type B of course. Greg

Oopse, that should be type B of course. Greg

Oopse, that should be type B of course. Greg