New electrical supply to oven

[David Hansen]

On Sun, 04 Jun 2006 20:50:52 +0100 someone who may be F wrote this:-

Ah. That would be another reason for having the 45A fuse separate.

With the main switch off unscrew the bit of green coloured plastic that is presumably under the carrier and through which the pins go. Does a cable go out from either of the terminals, probably the top ones which are the only ones with screws in? If not then it is spare and can be used for a new circuit.

The white, blue and red bits of plastic that the fuse or MCB fits into are usually interchangeable. With the main switch off unscrew some of them (make a note of which one goes where) and you will see that the terminals they plug into are wide enough to take a carrier of any size. The plastic bits just present different sized holes as a means of guarding against the wrong size protective device being inserted.

Under the green plastic of the 45A fuse you should find terminals of similar size. If there are then you can get a smaller fuse or MCB, screw in the bit of plastic that comes with it and that's that. If you want to replace the 45A fuse with a 45A MCB then that should just plug in to the green plastic bit.

However, I don't recommend replacing a cartridge fuse with an MCB unless you are able to measure or calculate whether the MCB is able to break a fault on the circuit. The plug in type of MCB are particularly unable to break fault currents. If they are unable to break a fault the results can be spectacular, though usually the "electricity board" fuse will act before the damage is too bad. Replacing rewirable fuses with MCBs is always a good idea, but cartridge fuses are a different matter.

[Andrew Gabriel]

In particular, the push button breaker (which is plugged in upside down) has a very low breaking capacity. I can't recall for sure, but I think it's only 1000A. The toggle ones are probably 3000A, which is also a bit on the low side without checking the prospective short circuit current of the supply.

[meow2222]

I guess we'll never know, at least not from death figures. Other concurrent factors, such as gradual elimination of the worst historic installs, changing attitudes towards use of electrical tools in the rain (thanks to rcds), changing appliance standards, and changes in other areas of installation standards.

NT

[meow2222]

Short on substance.

In reality older installs are often not the way they you're saying they should be, and thats one reason why problems occur. Situations that dont preent a real world risk are objected to by the new rcd. This is not news to people that fit the things.

I just was cautioning the OP that there could be significantly more to the job than he expected if he chose this path.

NT

[F]

I was hoping it would get the meter to run backwards...

[F]

I'm not sure I understand - sorry.

Are the breakers less safe than the re-wireable fuses they replaced?

[Dave Plowman (News)]

The difference in price between a 'minimal' and a 'more than you'll ever need' number of ways CU is pennies - and you can fit blanking plates while deciding.

[John Rumm]

In some circumstances yes. When you get a hard fault (say a live to neutral fault), the fault current that flows can obviously be very high

- 100s or 1000s of amps. The idea of the protective device is that in the event of a fault like this, it opens pretty sharpish and cuts the current off. Alas with MCBs there comes a point where the fault current gets too high for them, and they simply arc over when they open and continue conducting as they melt into a gooey mess. Most bog standard MCBs can break at least 6000 amp, and some ranges will do much more (10,000 or even 16,000). The replacement ones you have only do 3000.

The cartridge fuses (often called High Rupture Capacity nee High Breaking Capacity), are much better in this respect and can safely disconnect much bigger fault currents.

To know if you have a potential problem you would need to calculate your supply impedance. You can either measure this with suitable test equipment (many earth fault loop impedance testers can do this), or you can do an empirical set of observations, switching in different loads and measuring the current drawn and voltage (even a simple test with a volt meter and a known load like a 3kW kettle would give you a reasonable indication). This will let you work out a figure for the supply impedance in ohms, and hence work out how much current can flow from live to neutral were you to join them together.

So for example, in our place (overhead supply on not too thick wire), I get a supply impedance of 0.35 ohms. This means the prospective fault current is "only" 685A or thereabouts. Hence your breakers would cope without any difficulty. In a big town, or close to a substation however the supply may be much "stiffer".

[F]

Something of a pity, then, that the likes of B&Q, Focus and Homebase sell them as fuse 'replacements' without any cautions.

[Dave Plowman (News)]

In what way? They're a total bodge and not to be tolerated. Any electrician who does this should be struck off. ;-)

I'd only fit a split load CU these days, and things like the immersion and cooker would go on the non RCD protected side. Unless of course earthing or supply arrangements require a whole house RCD, but then you'd likely be in problems regardless.

If something like an immersion is leaking such as to take out an RCD it's not long for this world anyway.

[Andrew Gabriel]

Fuses and MCBs have a number of operating parameters, and one of these is the current flow which can be safely interrupted. If a short circuit happens, the current could reach 1000 or more amps before the MCB operates. If the current reaches a higher level than the MCB can safely break, it might not be able to stop the current flowing, and/or it might destroy itself in the process of trying to do so. When this happens, the next protective device upline will operate, probably your main fuse, but it might not operate before the downsteam wiring is damaged or destroyed by the high current.

The breaking capacity of an MCB is usually written on it e.g.

3000, 6000, or preceded by the letter M for 1000's of amps, e.g. M3, M6. Modern domestic MCB's are usually M5 or M6, and MCB's for commercial/industrial premises are M10.

Probably not, although if that push button one really is only

1000 amps breaking, I would be inclined to buy one of the newer toggle ones to replace it.

[John Rumm]

I would go with the 6mm^2 (4mm will hack the current in this case, but still has the spindly little earth wire). That way you (or future owners) can have an electric hob on the same circuit if required.

If you have 45A cabling throughout, then a switch and then just the connector:

[F]

Printed on its side: 'Type B

5A M1 15A-45A M2'.

Looks like I'm going shopping in the morning.

All the others are M3.

Thanks for the advice and (earlier) explanation.

[<me9]

I have recollections of a fault that resulted in the 80A incoming fuse failing and also a 100A fuse (in the adjacent substation) failing at teh same time. When the REC engineer replaced the fuse the air turned blue (in more ways than one) as the replacement fuse blew in his hand. He wouldn't replace the fuse until I'd isolated the fault. It was a dead short (caused by lightning) to earth on a sub circuit protected by a 6A mcb which hadn't tripped. Subsequent tests revealed that the 6A mcb was working correctly, but wouldn't trip with the short circuit current available from the supply before the 100A and 80A fuses (BS88) would blow.

I have been very wary of MCBs since, but in most circumstances they are OK, but that needs to be confirmed in each situation. The operating time should be less at all currents below the PFC than upstream devices, and it should be capable of breaking the PFC.

[John Rumm]

Indeed. People are being driven by convenience - much simpler to flick a switch than replace a fuse. In a way it makes people less likely to ask the more important question: "why does the thing keep tripping in the first place?"

(I suppose you could argue that explaining to a public that has enough trouble working out what a fuse does, all about prospective fault currents and breaking capacities is asking too much)

[David Hansen]

On Sun, 04 Jun 2006 21:13:44 +0100 someone who may be F wrote this:-

Not necessarily. It depends on the rating of the protective device and what the cable passes through on the way to the oven.

Cooker switches with socket outlets are seldom a good idea.

There are several options.

1) The cable between the consumer unit and kitchen could be sized for the oven and a suitable protective device installed at the consumer unit. In the kitchen there would then be a switch and a cable outlet by the oven. This is slightly cheaper, but does not provide flexibility.

2) The cable between consumer unit and kitchen could be sized for a cooker, but the protective device installed at the consumer unit be suitable for the oven. Things in the kitchen would be the same.

3) The cable could be as above, but with a protective device sized for a cooker. In this case a local protective device is necessary in the kitchen.

Depending on your kitchen I would do 2) or perhaps 3). The bits for each are slightly different.

[David Hansen]

On Sun, 04 Jun 2006 22:38:22 +0100 someone who may be F wrote this:-

Many people have the impression that circuit breakers are safer than fuses because they operate more quickly. However, that impression is wrong.

Protective devices must do a number of things. The upshot of all this is that circuit breakers are just about always more safe than rewirable fuses, though there is the odd exception. However, one cannot say that circuit breakers are always more safe than cartridge fuses as it depends on the circumstances.

However, circuit breakers do have an advantage for those who find even a cartridge fuse to be a problem.

[meow2222]

We were talking about fitting an RCD to an existing old installation. In reality a percentage of such installations do have borrowed neutrals, for various reasons. Hence fitting an RCD does sometimes involve significantly more work.

How exactly are you going to do this in an install with a 4 way fiusebox, where all the fixed appliances are on the rings? Again mixing old installs with new RCDs does bring gotchas.

Well, mine ran for over 2 years split open. But however long it will or wont last, fiting an RCD to such a thing will mean more work here and now. Also applies to cookers, on which leakage is no rarity.

NT

[F]

That was one of the reasons I replaced them: easier to flick a switch than find the fuse wire. I also thought, and this was the main reason, that they would offer another level of safety above fuses.

Ours weren't blowing (rapidly finds wood to touch!), I had simply become aware of a device that (I thought) would lessen the chance of being electrocuted.

[F]

That was my impression.

Understood! A little learning...