RCD for shower?

[A.Lee]

I've been renovating the G/Fs bathroom this week, upon going to turn off the shower supply, I found that it was supplied from a really old fuse box, as well as looking like it is a spur from the cooker supply. There are no RCDs anywhere in system.

To make the shower a little safer, I would like to fit a RCD to it. Would something like this be suitable:

I presume, to fit this, I would disconnect the supply to the pull switch, fit the supply to the RCD, then the output to the pull switch?

Ta for any thoughts. Alan.

[fictitiousemail]

Is the cooker still in use? Where does ths cooker cable come from ? A CU? Is the cable in use up to the job iro the shower rating?

[John]

You are in dangerous territory. How old is the installation? What wattage is the shower? what size is the cable?

Whilst it may not be very safe now, you may be opening a can of worms.....!

[George (dicegeorge)]

I'm not an expert but I'm also learning about 21st centruy house electricks.. and in my sister's flat someone had hotwired a shower onto the cooker circuit...

If a cooker needs a 30amp fuse and the shower needs 20 amps and if you want them both on at the same time that would mean you'd use a 50amp fuse which means you could have 30amps over current on the shower before the fuse blows- so that's not much protection, I'd get scared and put in a new wire with RCD for the shower.

I think under part P a competent electrician has to install a new bathroom circuit, but over half the work is the planning and the measuring and drilling holes in the walls and fixing conduit routes which I plan to do myself first.

[John Rumm]

Just looking like, or is it actually on the cooker supply?

Is there a cooker on it as well?

What is the rating of the shower (usually given in kW)

And what size was the fuse?

And what cable is in use? You can find out by measuring it and looking up the size here:

There are no RCDs anywhere in system.

Prior to the 17th edition regs[1], there were plenty of cases when an electric shower could be adequately protected and safe even without a RCD. So don't read too much into there not being any at the moment.

I would be more concerned about checking that the cable and fuse rating are appropriate for the size of shower.

[1] The need to protect cable runs will probably require a RCD anyway now.

In effect - often these are positioned close to the CU rather than close to the shower though.

[A.Lee]

Thinking about it, I was slightly sidetracked by the 'Cooker' on the separate fuse. It is in fact a gas cooker, which obviously replaced the previous electric cooker, so should be no problems with overloading the cable. It is a 30A fuse, with the relevent cable (6mm iirc).

But wouldnt fitting a RCD be much safer than it being protected only by a fuse?

Fitting it at the shower end would be far easier, but yes, I suppose I could replace the separate fuse box for the cooker at the incoming supply end. Ta Alan.

[A.Lee]

See other reply, I was hoodwinked by seeing 'cooker' on the fuse, and a separate 'cooker' switch in the kitchen. Alan.

[fred]

I'd guess that 3/4 of all/old electric shower installations don't have RCDs and I've yet to hear of any Daily Whail exposes on grannies being killed off, so the old concept doesn't seem that flawed to me, provided that the shower is wired and earthed correctly.

Mine is on an MCB only and I am perfectly happy with that, it's even one of the early Dolphin ones with a direct and uninsulated heating element in the water flow.

[John Rumm]

OK, so sounds like you need a label upgrade on the fuse then! ;-)

I think it is one of those "feel good" things - you may think the RCD offers better protection, but if you actually think through the possible fault scenarios it is less clear cut.

Being a fixed appliance the chances of you making direct contact with a live part are very slim (unless you plan on live working maintenance while taking a shower!). Its not as if there are any blades or exposed flexes etc. That then leaves indirect contact. The shower is probably plastic cased, which means no shock from that. So, the final possibility is insulation failure on the heating element creating a fault to earth. If the equipotential bonding in the shower room is up to scratch, that should ensure that you can't be exposed to a dangerous potential for the duration of the fault, and the fuse should open within 5 seconds (and in all probability much quicker).

The times the RCD would certainly help, include situations where the earth fault loop impedance is high - but then again we are only talking about a 30A circuit here - so even a rewireable fuse will open in time with a fault current a tad under 90A. It would also be good where the equipotential bonding is not good enough (or missing)

Might be neater. Note that these separate enclosures include overcurrent protection as well as RCD, and you don't really want to devices cascaded since you are not going to be sure which will trip in the event of a fault.

[Mike Tomlinson]

In article , fred writes

eeeek! That can't be right, can it? It sounds as if you're saying the live element is immersed in the water?

[David Hansen]

On Mon, 24 Nov 2008 13:31:17 +0000 someone who may be Mike Tomlinson wrote this:-

What do you think would be the problem?

Some heaters (not showers) work by passing electricity through the liquid to be heated.

[Sam Farrell]

When I was an apprentice we used to boil a cup of tea by placing two metal plates in either side of a large pint mug and connecting the plates to the mains. It works but don't try this at home children cos if you plug in too many it blows the mains fuse. :-).

[fred]

In article , Mike Tomlinson writes

:-D, I wondered if anyone would do a double take.

On the basis that a picture is worth a 100 . . .

clever little design, transparent plastic case in two parts with a central divider, keeping the two elements apart and forming a narrow channel for the water to pass over the coiled resistance wire elements. Unsurprisingly, the cold water enters on the blue flexible plastic pipe, flows left along the bottom and returns to the right along the top channel, exiting on the red.

The plastic inlet & outlet pipes are a good 50cm long so there's a good long 'insulating' path through the water from live to the heavily earth bonded metal in let & outlet fittings.

[Mike Tomlinson]

In article , fred writes

Looks a fairly modern design (plastic cable ties, brown for live, blue for neutral etc.) but the concept still scares the shit out of me. Reminds me of those toasters from the last century with drop-down sides that exposed the live elements.

[fred]

In article , Mike Tomlinson writes

Installed 1983 the sticker says, so 25 years old.

I see where you are coming from with the scary concept idea but water really isn't that good a conductor. IIRC 6mm is the safe air clearance distance live to earth so 500mm (along the plastic pipe) of water should do just as well.

[Andy Burns]

and a 1971 patent.

[Andy Wade]

It should be perfectly safe without an RCD provided that:

(i) the earth fault loop impedance (Zs) is low enough to ensure that the fuse/MCB will blow/trip in less than 5 seconds in the event of an earth fault, *AND*

(ii) SUPPLEMENTARY BONDING is in place in the bathroom, including bonding between the earth terminal of the shower unit (or nearby isolation switch) and any metal water pipework, particularly pipework feeding the taps of a metal bathtub over which the shower in question is used.

Supplementary bonding can be omitted under the 17th ed. if the relevant conditions are met (RCDs for all bathroom circuits including lights and main bonding in place for the building as a whole). If the shower has no RCD the conditions are not met and the bonding becomes vitally important to keep the shower outlet and your tootsies at more or less the same voltage...

[Mike Tomlinson]

In article , fred writes

Isn't that highly dependent on the composition of the water though? Say if the water became more conductive, oh, as a result of iron sediment contamination from a disturbed mains pipe, wouldn't that be rather dodgy?

I assume such a design would be illegal now.

[Owain]

Or Someone could introduce iron filings or salt into the water pipe before Mrs Someone takes a shower?

Owain

[fred]

In article , Mike Tomlinson writes

That's a good point and I believe it could cause damage to the shower but I think it would still be safe given the following:

1. Cold end is live, hot end is neutral so an increase in conductivity of the water would result in and effective short on the element but the end closest to the user would be still be at neutral potential.
2. The outlet pipe is earthed so that even with a neutral fault and having water with increased conductivity, the output should still be at earth potential.

I'm open to be convinced otherwise though, but please don't try too hard to convince me, I like my little backup shower heater ;-)

That wouldn't surprise me.