M.C.B'S

Mark

Just seen on E-Bay

80 amp RCD in a box... £5 at the moment

# 3188348464

-- Jonathan

Barnes's theorem; for every foolproof device there is a fool greater than the proof.

To reply remove AT

I was assuming a SWA ( steel wire armored ) cable in the ground.

It's an outside run, twin and earth is not realy suitable in most outside applications.

-- Jonathan

Barnes's theorem; for every foolproof device there is a fool greater than the proof.

To reply remove AT

It was using 14 Amps and 480 Watts, power factor=.14 according to the data you have supplied. This is entirely possible.

Mark Rand RTFM

In message , mark writes

A configuration problem?

[snip]

Glad to hear it and hope that is not the problem.

Are you doing all your mail and news reading/replying off-line or on-line?

With the very slow speeds that you report is it possible that you have a 'Trojan' that is high-jacking your bandwidth and causing your ISP to object to what is going out? When was the last time your machine had a thorough scan?

Yes all good advice points taken in. done some checks today, what im really using peek is under 40 amps...so I'm well sticking to the 6mm cable. cable go's up the cavity wall of my house thats since had cavity wall insulation ..so I'm not keen anyway. the problem was the clip on ampmetre giving incorect readings...well readings not what i wanted to see anyhows. phoned up a good friend today an electrical engineer, explained what I had and even he agreed that the 6mm would do the job. he also said I dont need the 80amp breaker ....and stick with what i havce now 63amp. maybe i will put a seperate RCD breaker switch in...... had manweb replace the main fuse with an 80 amp a long time ago. thanks very much guys on helping me out with this problem. all the best..mark

Hi Mark,

From reading what you have written on this subject, there are a couple of other factors that you need to consider.

Firstly, having the cable running through an insulated wall derates it's capacity (I don't have the tables to hand at present, but references follow)

Secondly, with that sort of run and loading, voltage drop also needs to be taken into account. Standard practice is to keep the voltage drop to the far end to a maximum of 2%.

Using the calculator at

suggests that you should be using 10mm cable to keep it on the safe side. Using a 63 amp MCB on your current setup will, in a fault situation, allow the cable to overheat where it goes through the insulation.

Also, if you are exporting the house earth along that sort of run, you may be introducing too much earth loop impedance for the MCB to work properly anyway - the usual way around this is to just use live and neutral out to the workshop, with a local earth rod at the far end, and an RCD in the workshop unit on the incoming side (TT system).

HTH

Gary

I'm a bit confused here - did you mean to refer to both MCB and RCD in the same context?

I'm interested in the earth loop bit for my own situation, I have roughly 200m of 16mmsq 2-core SWA feeding the dry-dock, with an RCD and an earthing rod at the dry-dock end, but the steel wire on the cable is earthed at both ends. Are you suggesting this might be better earthed at one end only?

Cheers Tim

Tim Leech Dutton Dry-Dock

Traditional & Modern canal craft repairs

it's in a tube all the best..mark

You know what I knew someone would come back with this "heating the cable thing" It's only in the insulated cavity for two foot then passes under the floor. it then travels the lengh of my landing and bathroom then exits into a coduit suspended by catenary wire....then over to the workshop. also there is a 16mm earth traveling back to the house ..plus there is a an earth rod stuck in the ground near the workshop with a direct

16mmm earth from the consumer unit there . I worked out my peak and it is less than 8000 watts total. I "had" an RCD in the workshop.63 amp ,30 miliamp trip...it never tripped it always tripped the house one... 100 amp 30 miliamp instead...thats why I'm going for a seperate supply from the tailings with the RCD twin pole breaker there instead. all the best.mark

NOPE IT IS PIPEX'S DOING look in pipex.xtreme.support news group. all the best..mark

Yes this is were I became unstuck.. all the best...mark

Hi Tim,

Yes, I did mean both. The MCB covers the cable along its run, along with the load at the far end. The RCD at the far end is there because a local earth rod is extremely unlikely to have a low enough earth impedance to operate the MCB back at the supply head properly - for properly, I mean within the specifications laid down in the 16th edition. I can't remember exactly the figures, but one that does come to mind is that a 32A type B MCB needs an earth loop impedance (all the way round the circuit, including the run to the substation and back) of about 0.24 ohms. Because you are extremely unlikely to get anywhere near that with an earth rod, an RCD is used at the local end to cover all circuits, as these will trip out with a much lower earth current, usually in the order of 100mA for a TT system RCD.

With regards to the steel wire earthing, I don't believe that it is permissible to use this for earthing purposes in its own right - it should be earthed at the supply end, so that anybody putting a spade through it will short the live conductor out to earth, and trip the protective device at the supply end, hopefully before electrocution occurs!!

As I stated earlier, I don;t have my copy of 16th edition to hand, so for a definitive answer, uk.d-i-y is a very good resource, with people who can quote the regs in much the same way that I can quote the direction to my local pub!!

Gary

I realise that regs change but I had the lecky board fit a new 100 amp per phase 3 phase supply to my shop about 14 years ago. Usual 3 core with steel sheath. Both the shop earth and the shop neutral are connected to the earth sheath on their side of the incoming fuses. This was actual lecky board men doing this and not contractors.

Presumably this isn't allowed today ?

-- Regards,

John Stevenson Nottingham, England.

Neutral and earth are normally connected together at the transformer, so If the transformer is close to the shop it would be quite reasonable to bond them together close to the fuse.

Does one have to pay the crooks a lot more to get a three phase supply run in (It would be simpler than invertors or convertors for my new workshop)

Mark Rand RTFM

As I say it was 14 odd years ago and at that time it was a fixed price of £200 for the installation. They supplied the box for the wall, I had to dig and bury a plastic pipe from the edge of the pavement to the box 5' deep.

They came and threaded a cable thru, dug the pavement up and connected it to the 3 phase ring and fitted the fuses. Later another mob came round, checked the installed wiring for safety and fitted the meter.

Now they do all the work and it's a paid by job price. One guy I know converted some terraced houses to shops and wanted 3 phase from the front door of one house direct onto the pavement, cost was £750.

My Brother is currently arguing with the lecky board about have single phase put in on a small farm he's bought. It's next to a large school with power and he has 33,000v HT lines across his property. So far he's managed to get them down from seven thousand to five thousand :-(

-- Regards,

John Stevenson Nottingham, England.

They wanted £1750 to put three phase into my place . They said the three phase was the other side of the road . They said they would have to dig the road up ...not bore a hole I would have to dig a trench in my drive about twenty feet long and a metre deep also I would have to supply and install the slow bend and the board for the meter......I thought sod it . I hear though,if you have the electricity on telegraph pole near by, it's a lot cheaper. also you can have it charged at domestic rate now...8% vat.....if it's not a business. all the best......mark

Tell him to go & have a look at the nearest gyppo camp, it could save him

5 grand!!

----------------------------------------------------------------------------------------------------- snipped-for-privacy@boltblue.com John Lloyd - Cymru/Wales

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IIRC that's the way that grandfather first got electric for the farm in Devon :-)

Mark Rand RTFM