Protecting Service Fuse?

[ondablade]

Since we're on electrics again I thought I'd throw another quickie in too. I've been right through estimating maximum demand for the house and the workshop.

My supply is 80A, but it'll be feeding three consumer units when the dust settles - the house, the original workshop CU, and another I'm installing to hold enough MCBs to power my machines on radial circuits.

This seems to be OK practice, but it has the effect of leaving the situation so that far more power can be drawn than the incoming service fuse can handle - if more circuits are loaded more than the max demand estimate presumes.

Would it make sense in this situation to feed all of the incoming power through an 80A type C breaker to protect the service fuse (HRC fuses it seems can tolerate pretty heavy overloading anyway), and enable easy shut down of all power to the house?

Plan B is just to leave it all alone, and rely on the butch nature of these fuses - it looks from the data sheet that they can handle well over 100A for quite an extended period anyway. Trouble is that if it does let go getting involved with the service provider (an ex state monopoly) tends to be ruinously expensive.

A few related practical questions.

Is meter tail the type of single core cable used to wire inside a consumer unit?

If so should it be colour coded - blue/brown/green yellow as normal? (I've seen some that are not)

What mm2 sizes are required to handle 60A and 80A? - the sizing tool on the TLC doesn't seem to cover this.

How is the earth cable sized - the same mm2 as the L&N conductors?

This new CU will have all of the 16A dedicated machine circuits, the 30A rotary converter, the 30A dust system fan motor inverter (these can't usually be used with an RCD) plus the 13A workshop sockets (split in two ring mains) wired from it - about 9 circuits.

Should it use/not use an RCD on the circuits other than the inverter? (i.e. be a split unit) Will the 5.5kW rotary converter pop RCDs? Nusiance tripping from this and the other circuits is the concern

 

[srp]

I can't answer all your questions, but here goes anyway.
If you total up all the potential loads in the average house it would always exceed the installed supply capacity. The regulations allow for 'diversification' which means that it is extremely unlikely that everything is switched on at once.

The idea of providing mcb's is to protect the cabling downstream (melting insulation, risk of fire or shock). The service fuse is obviously the last line of defence in the case of an overload. I don't see why you couldn't install a single large-capacity mcb between the service fuse and your first consumer unit, but you'd have to check with the latest IEE regs.

If you think that the current you will normally draw will exceed the installed supply, then you have to get it uprated by the supply company. In the old days of regional supply authorities (was yours the NIEB???) this would cost nothing - the electricity board were duty-bound to install the other 2 phases from the street supply (this situation sometimes arose with large houses fitted with electric storage heating systems). That was when electricity supply was a nationalised industry though, and things are very different now.

I've only ever seen single core meter tails, with colour coded insulated pvc and a plain colour (grey usually) pvc outer sheath. The insulation always used to be red and black, but are now brown and blue coded. For a domestic installation 16mm2 is a common size, but 10, 25 and 35mm2 sizes are also used.

A connecting box is often used to split the supply when two consumer units are installed (called a Henley block I seem to remember, with five ways, either sp or dp).

Definitely can't help with any of your other questions (not reliably anyway), and you'll have to check what I've written above with the latest edition of the regs, which I don't have to refer to (esp the bit about diversification).

 

[ondablade]

Thanks SRP. The reason for thinking in terms of protecting the service fuse is simply that it's such a dog and pony show and very expensive to get anything sorted that involves our service (ha!) non provider.

I seem to be OK on 80A in that the normal draw is well below that, but it's very hard to be definite that a problem situation might not at some time arise given the uncertainties of diversity you point out.

I'm not sure I can get 100A at any reasonable cost - if at all. It'll certainly require a meter change, as the present unit is marked 80A.

 

[cgw59]

Hi,
I worked for a local electricity supply company for 34 years, the problem with main fuses blowing is a common one and often down to consumers adding additional loads to their properties without telling the company. If you require a larger incoming supply it will not always cost money provided that the supply cable is capable of handling the proposed load they will fit a 100amp main fuse into the cut out for you, do not attempt this your self as there are stiff penalties ( and risks) for interfering with any equipment on the suppliers side of the meter position. There would also be different tariff charges if you were foolish enough to declare any additional load as being for commercial purposes!! . As for your other questions meter tails and cu tails are usually 16mm PVC/PVC .main earth conductor i.e. the one connected to earth should be of an equivalent size of the supply conductor. When you use the term RCD's I am not sure if you mean RCD's or are you referring to MCB's, these are two different pieces of equipment and are designed to work in different ways. RCD ( residual current device) is used for the protection of a complete installation i.e. fitted at the incomers to a consumer unit and protects the whole of the unit and a re commonly rated in milliamps 30 being the most commonly used, MCB ( miniature circuit breakers) are fitted on each individual circuit coming out from a consumer unit and are rated in amps 6, 16, 30,32 etc.
p.m. if you need any advice re supplies.

 

[devonwoody]

Tell us how you shut the power down before the consumer unit?

Is it legally possible to do so?

 

[cgw59]

Hi all new installations should be fitted with a double pole double throw switch on the tails coming from the meter to the consumer unit / units. If you have to break a seal at a main fuse for it to be withdrawn to allow work on an older installation where a switch is not fitted then it unavoidable but it is always a good idea to let the supplier know that you have done it and the reason, they will normally reseal at the next meter reading without any charge, remember that although the main fuse at your premises is rated at 80 / 100 amps the cable that you supply is feed from could be protected at source by any thing upto 200 amps and in some cases even higher. Main fuses and meters are sealed to prevent two things 1. the higher current that would be present at the incoming side of the fuse and 2. protection for the company against fraud.
hope this answers your question.

 

[devonwoody]

I havent got the dp switch, (bungalow 1963) but it would have to be a big one, my fuse is 60amp.

 

[Racers]

Hi,

You used to be able the have a switch between your meter and consumer unit fitted free, who knows now!

Pete

 

[ondablade]

I'm in Ireland Devon so the practice may be a little different to the UK.

The electrician I asked about the size of the service fuse last week having shut down the power demand by removing the fuses in the CU just opened the meter box and popped it out to check. It turns out that the fuse holder is separate from the meter, and that it has no seal on it.

The only time the system was worked on was by the electricians I used when the workshop was built about 12 years ago. They took the new supply to it through a second main fuse in the CU in the house. I'm guessing they may have broken the original seal on the service fuse - which was just a sticker and not the wire type.

Judging by the date it looks like the service provider said nothing and just replaced the sticker with a new one, but placed it so it protects access to the cable terminals in the fuse holder. Conveniently it doesn't prevent removal of the fuse.

Thanks for the info on cable sizes too CG. I meant RCD and not MCB. Inverters (in this case the Omron V-1000 going on the dust system fan) it seems by their nature leak enough to pop any RCD of a rating that would be useful to reduce the risk of shocks.

This means that my supplementary workshop CU will at least have to be a split one if I use RCD protection on the other circuits - all of which will use C type MCBs.

The circuits are as follows - dedicated 16A radials for each of the main machines, the maker's recommended 30A radial for the rotary converter to drive the bandsaw, a couple of 13A socket ring mains, and possibly (for convenience in wiring) a couple of radially wired 13A sockets.

I guess I'm just trying to decide if an RCD is a good idea on this group. I don't know what it might make of the rotary converter, and possibly have a faint recollection to the effect that they can produce a lot of nusiance trips when used with motors.

PS I should say at this point that all but especially Bob (9F) and Roy (Digit) have already been very helpful on my wiring questions. MY apologies if I'm overdoing the number of questions, but it's like pulling a thread on sweater...

 

[devonwoody]

I think my wire seal is just nicely twisted. :wink:

But who do I ask in Devon to replace seal or put in switch, I am with EON at the present time.