Elecric shower blown fuse

On Sun, 12 Feb 2006 19:58:42 -0000 someone who may be "Andy Pandy"
How is it installed? Is it run through thermal insulation, clipped to a wall, enclosed in conduit? Is it grouped with other cables?

There should be bonding between metal pipes, metal baths and electrical equipment in the bathroom. It can be concealed and hard to find, but if it is not there the installation is dangerous.
If there are several earthed metal pipes running to the bathroom, say hot/cold water and heating flow/return, and these are bonded to the shower, then that is solid enough earthing. However, changes to the plumbing may change this.

A common misconception. Fuses don't have mechanical parts which have to move to open the circuit. Depending on the supply and the fault, a fuse may disconnect a circuit far faster than an RCD.
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A bit of all really! As far as I can tell, it runs through an internal wall with the cold pipe, up into the loft (insulated), then down through another internal wall and ends up in the cellar at the CU, at that end there are a load of cables going into a conduit.

inlet.
I can't find any. Rather than taking floorboards up etc - can the bonding be checked by measuring the resistance? Are there any good on-line guides on how to do this? A simple test confirms electrical connection between the main metal items.

Indeed.
-- Andy
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On Sun, 12 Feb 2006 22:32:38 -0000 someone who may be "Andy Pandy"

Is it above the insulation, below it, or in the middle of it?

How long is this conduit?

Not really, unless you wish to disconnect them from the rest of the house:-)
Is the shower above a metal bath? If so is there a green and yellow wire going to that? Is there any low level electrical equipment in the room, such as a towel rail, if so is there a similar wire going to the equipment or the box from which it is fed.
I suspect this is something in the FAQ, but if not there is probably much information available. Look for supplementary bonding in a search engine.
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David Hansen, Edinburgh
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the
Below - but this is only a short run (maybe 3-4 foot).

a
Well it drops 2 stories from the loft to the cellar. The only other cables coming out at the loft end are lighting cables by the looks of it.

checked
The shower referred to in my OP isn't, it's in a shower cubicle. My other shower, however, is over a metal bath.

I can't find any earth cables anywhere in either bathroom, but I've not had a proper look under the floorboards. Would you expect the hot & cold pipes feeding the sink to be connected above the floor level - because they certainly aren't in either bathroom.

Nope - no low level electrical equipement unless you count the immersion heater which is in the airing cupboard in the bathroom.

Have done.
http://www.tlc-direct.co.uk/Book/5.4.3.htm
looks like a pretty good guide.
Thanks.
-- Andy
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On Tue, 14 Feb 2006 22:33:42 -0000 someone who may be "Andy Pandy"

Then the insulation will need to be considered in your cable sizing calculations. If a cable passes through an insulated wall, such as a cavity insulated wall, then the heat can escape at either end of the insulation, but in this case the distance is too great.

The question is how many cables go into it at the bottom and how heavily loaded they are. It sounds like you will need to take the grouping into consideration in your calculations. Lighting cables can often be ignored in such calculations, if they are lightly loaded.

The book you mention below is a good guide to what needs doing in both cases.

Then you have two options. The first option is to take up the floorboards and have a proper look, with all that this entails. The second option is to add some supplementary bonding yourself. Only you can decide which course to take.
There is a third option, do nothing, but I wouldn't recommend it.

Not necessarily. If it was done then it might well have been done out of sight, which is neater but gives doubts to people who come along later. However, it may not have been done. I don't know the history of the house and its services.
Bonding can be done neatly, by hiding the cables behind pipes and putting the clamps up behind fittings.
One clue would be if you have a metal sink in the kitchen. Is this bonded? If it is then that implies that at some time someone who knew what they were doing was involved with the electrics.
The bonding should have been checked when the showers were installed. At the very least I would expect a green and yellow wire going from the shower unit to the metal bath. If it does not have an earth tab then a bolt can be screwed into one of the feet.

I see from the bottom that it was, "Extracted from The Electricians Guide Fifth Edition by John Whitfield". This is an excellent book that I recommend to anyone who is thinking of doing electrical work themselves. Buy or borrow a copy. The actual Wiring Regulations are in a "harmonised" format, which makes them difficult to follow until one knows them well and so not suitable for many people.
The diagram tells you more than a thousand words. Note that the bonding to the shower tray assumes a metal waste pipe, there is no need to bond little bits of metal such as the strainers in the bottom of shower trays.
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It doesn't go through an insulated wall (it's an internal wall).

into
out
It looks like the cooker cable and a few light cables go in at the bottom. The cooker is almost directly above the CU so the cable won't go very far till it branches out of the conduit.

The kitchen and utility rooms are bonded - there is bonding between the hot & cold pipes and the sink, and the bonding cable then goes into the plaster so it presumably bonds with the earth wire of the electrical socket ring.
I guess (as per fig 5.15 in the link below), this means that bonding the metal cases of appliances like the washing machine and the dishwasher is not required.
These rooms are in an extended part of the house (built c1994), but the bathroom is ancient (I think it's the original 1930's cast iron bath!). The shower room (en suite) is in the old part of the house but is a converted bedroom.
I would imagine the wiring was done correctly for the extension, but the conversion of the en suite and installation of the showers was a separate project. The fact that two electric showers are installed when the supply fuse is only 60A and the meter is only 40A does not inspire confidence.

Nope - there appears to be none unless it is very strangely routed.
Of course it may be that the resistance of the bath to earth is greater than 50kOhms - thus not requiring bonding as per the link below. The pipes entering the bath are actually lead - about 2 foot of lead connects the copper pipes of the cold and hot water supplies.

Yes, will do. Thanks.

Is bonding the copper pipe just as it enters the shower sufficient, or should the bonding be attached to the actual shower metalwork/earth terminal?
-- Andy
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On Wed, 15 Feb 2006 17:25:13 -0000 someone who may be "Andy Pandy"
It was just an illustration. You will need to consider the rating of the cable when it has thermal insulation on one side.

Unless it is somewhat less than a metre you will need to consider the grouping factor, assuming that the cooker cable feeds an electric cooker.

It is not required. Where equipment is connected by short cables this is enough.

Quite possibly.

That depends on the assessed maximum demand and diversity, but it is certainly not ideal. Do you have an overhead line or cable supplying the house?

That depends:-)
With bonding one needs to understand what is required and what is not. People not understanding come up with horrors from time to time; such as metal window frames, radiators and the metal in a sink strainer all being bonded with little green and yellow wires that will soon be broken off. Fortunately such things are pointless anyway.
If pipes are made out of metal and there is reliable metal to metal contact between pipes and fittings then little is gained by bonding. The pipes and fittings are more conductive than any bonding cable one will see in a house anyway.
One reason for supplementary bonding is to ensure that things people are liable to be in contact with are at the same potential. Taps are turned off with wet hands, lowering the insulation resistance of dry hands. People touch towel rails with wet hands to get a towel. If the metalwork of a shower was to become live then it is a good idea if the bath is at the same potential.
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is
A cable. The main earth terminal is a clamp around the incoming cable - so if I've understood the FAQ correctly it's a TN-S system.

Bonding metal window frames can be dangerous AIUI, as a fault could electrocute a window cleaner outside.
Radiators surely need bonding? (assuming the resistance to earth is less than 50 kOhms).

Exactly. I'd have thought a test would be to measure the impedance between metal objects which are connected via pipes, and if this is greater than a certain value bonding is required. After all bonding simply reduces that impedance. Obviously this would need to be redone after any plumbing work is carried out.
What am I missing here?
-- Andy
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On Fri, 17 Feb 2006 09:38:14 -0000 someone who may be "Andy Pandy"

I'm surprised there is only a 60A fuse on it then. Perhaps it is fairly elderly.
The electricity supply system assumes what seems to those who do not know about it a remarkably high level of diversity, but it still works most of the time. Even if both showers were running together they would only be on for a relatively short period of time.

That's likely.

Consider water taps. Are they bonded? Assuming metal pipes, there is reliable metal to metal contact at the joint between the bonded pipe and the tap. If such joints were not reliable then every length of pipe and tap would need to be bonded.
A radiator is a similar consideration. In a bathroom if the central heating pipes are bonded in the bathroom (and they should also be bonded by main bonding conductors elsewhere) does adding another bonding cable to the radiator achieve anything?
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David Hansen, Edinburgh
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Well it's in a bakelite box with 60A written on it - unless the fuse was upgraded but the box not changed/labelled? Are such things known?

Sorry - that's what I meant - I didn't mean the radiator itself necessarily needs bonding, but the pipes to it.
-- Andy
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On Fri, 17 Feb 2006 12:53:54 -0000 someone who may be "Andy Pandy"

No. One might be able to fit a fuse of a smaller rating, as it is physically smaller, but not a fuse of larger rating.
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On Fri, 17 Feb 2006 09:38:14 +0000, Andy Pandy wrote:

No but their supply pipes do if they are more than 0.5m of metal pipe.
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Thats a bit over the top and a very sweeping statement.Have you seen the bathroom in question ?.The potential is there for it to be dangerous under certain circumstances true, but there are thousands of bathrooms up and down the country which have no bonding. And I have been to some where the bonding is infact incorrect. I don't think it was very wise to put the fear of god into people that they are taking their lives in their hands just having a shower.
Dave
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On Mon, 13 Feb 2006 05:57:08 +0000 someone who may be Dave Stanton

It is a correct statement, when there is a shower in the bathroom where water passes over an uninsulated heating element and one is relying on there not being a large amount of leakage.
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