Shower with no RCD or supplementary bonding

mapj1: 
 

well. you need to get the human between two conductive parts at different voltages. 

Now  I  have seen this happen with a shower actually, where a leak inside caused water to pass over the live terminals and trickle down the wall. At the same time more or less earthed water came out of the exit hose. However, both were high impedance paths and all that was reported was a bit of a tingle when leaning on the tiles and under the shower.

Amusingly having explained all this to the wide eyed friend who had been the victim,  she said ‘gosh we could have 3 dead bodies in the shower!’ I pointed out that this was unlikely as I for one would not go climbing into a shower cubicle with a corpse already in it without investigating first…

There is very little risk, unless the shower has exposed metal plumbing, and modern ones do not, the modern way is that the hose is actually rubber lined and screws to a plastic fitting, and the incoming water spigot is also plastic.  If the shower develops an internal leak, stop using it.

Mike 

Some showers have a water pressure release valve situated in the exit pipe just before the flexible shower hose to open if the shower head gets blocked. It is sited on a nylon water tube. Recently whilst fault finding I had the experience of getting wet feet. The blinkin thing had opened and water at high pressure issued forth spraying me. To my mind the water could have entered upwards into the electrics as the water outlet is positioned against the wall tiles. I would definitely prefer R.C.D. protection there.

Here is the pressure release valve. The short spur is positioned against the tiled wall and when open spurts water in all directions.

Z.

Zoomup: 
 

mapj1: 
 

well. you need to get the human between two conductive parts at different voltages. 

Now  I  have seen this happen with a shower actually, where a leak inside caused water to pass over the live terminals and trickle down the wall. At the same time more or less earthed water came out of the exit hose. However, both were high impedance paths and all that was reported was a bit of a tingle when leaning on the tiles and under the shower.

Amusingly having explained all this to the wide eyed friend who had been the victim,  she said ‘gosh we could have 3 dead bodies in the shower!’ I pointed out that this was unlikely as I for one would not go climbing into a shower cubicle with a corpse already in it without investigating first…

There is very little risk, unless the shower has exposed metal plumbing, and modern ones do not, the modern way is that the hose is actually rubber lined and screws to a plastic fitting, and the incoming water spigot is also plastic.  If the shower develops an internal leak, stop using it.

Mike 

Some showers have a water pressure release valve situated in the exit pipe just before the flexible shower hose to open if the shower head gets blocked. It is sited on a nylon water tube. Recently whilst fault finding I had the experience of getting wet feet. The blinkin thing had opened and water at high pressure issued forth spraying me. To my mind the water could have entered upwards into the electrics as the water outlet is positioned against the wall tiles. I would definitely prefer R.C.D. protection there. I believe that the makers Aqualiga(?) and Gainsborough use these pressure release devices as do others.

 

Z.

In the 70s I bought a house with an electric shower, 6kW wow! It was running off a 30 Amp hot fuse wire which I changed to a M.C.B. We got no shocks. After a while I when I could afford it, installed a current operated earth leakage circuit beaker rated at 500mA I seem to remember. It was  a Crabtree big brown/black coloured box. 500mA! Anyway it would have tripped much sooner than a hot wire fuse or M.C.B. if water got into the electrics or the heating element shorted or partially shorted to earth.

Edit. Add. Yes I have just confirmed that the Crabtree E.L.C.B. was a model L60 with a rated sensitivity of 500mA. And it had a test button as well. It was built like a tank and would still operate today as it was of such high quality.

Z.

I prefer to see an RCD too, it trips before faults get out of hand, but having also lived with many showers without, both nice ones here and wilder looking ones abroad, including some that look similar to that photo I posted with twist joins and so on, I'm trying to reassure the OP that even when it goes wrong and is not earthed it is not that dangerous to the end user so long as the water is of drinking grade. We tend to assume that a non-compliance with the latest regs == lethal danger, and it is not necessarily so.

And I agree the older RCDS and Vo ELCBs were very well engineered, but in modern money at  £100s  we'd not pay. (and most folk  did not want to pay for them back then either…)

Mike.

Could someone please explain how the R<50/Ia actually affects anything and makes a difference?

My attempt…

Supplementary bonding satisfying the R<50/Ia rule won't guarantee the touch voltage will never ever exceed 50V - clearly if the earth fault current exceeds Ia (which it usually will in TN systems if you're aiming for 0.4s disconnection times) then 50V could quite plausibly be exceeded.

What it does do though is ensure that 50V can't be exceeded if disconnection time takes 5s or more - and given the characteristics of most overcurrent protective devices(*) - that puts you on a curve whereby if the situation meets 50V and 5s, it's going to meet (if the fault current is higher) 0.4s at 115V (or higher again) 0.2s at 230V - i.e. below the ‘safe’ line for electric shock.

So in an installation where 5s disconnection times might be present (and thus on a TN system people might be exposed to 115V or more for up to 5s) - either from large/old circuits within the bathroom itself, or distribution circuits etc outside the bathroom but touch voltages imported through extraneous-conductive-parts or shared c.p.c.s - supplementary bonding within the bathroom gives protection similar to 0.4s disconnection times on everything.

(*) Clearly the idea originated with fuses so that there was a definite relationship between increased current and decreased disconnection time - so it might seem at first glance that MCBs with their instantaneous operation above a certain fault current wouldn't support this approach, but for (B and C types at least) it does sort of come out in the wash, since if they meet the requirement for 5s (which they'd do in <0.1) then they'd also meet the 0.4s requirement etc (if still <0.1).

The big gotcha is making sure you choose Ia for the worst case device/circuit that could impose a touch voltage in the bathroom - which could be a circuit almost anywhere in the installation - and not just consider the bathroom circuits alone.

   - Andy.

Usually. Unless the water is contaminated for some reason - water out of the tap  in the UK is pretty poor conductor,  a k ohms or few per cm cube (now imagine a pipe full of cm cubes in series, or the higher resistance of those cubes sliced into thinner layers on the tiled wall.)

Water with shampoo or baths salts in is a much better conductor than tap water, as is water with a lot of sweat, blood or other biological fluids. So ‘safe’ levels of touch voltages in the bath are far less than in the dry, and in surgery are even  less again …

Sea water is about as good a conducting liquid as you will find in nature that is not a liquid metal…

MrJack96: 
 

That’s mad!! So in theory what are the hazards of an electrical shower wich is not accessible but no RCD protecte and no supplementry bonding? my guess is it only becomes dangerous if there’s a fault which we are exposed to and we touch another bit of earthed metal work ie a tap which is not as well earthed to keep the touch voltage down? 

Since 17th Edition BS 7671:2008, BS 7671 has conditions for omission of supplementary protective equipotential bonding in bathrooms. So, BS 7671 agrees with you … sort of. The conditions are contained in Regulation 701.415.2:
 

Where the location containing a bath or shower is in a building with a protective equipotential bonding system in accordance with Regulation 411.3.1.2, supplementary protective equipotential bonding may be omitted where all of the following conditions are met:

(iv) All final circuits of the location comply with the requirements for automatic disconnection according to Regulation 411.3.2
(v) All final circuits of the location have additional protection by means of an RCD in accordance with Regulation 415.1.1
(vi) All extraneous-conductive-parts of the location are effectively connected to the protective equipotential bonding according to Regulation 411.3.1.2.

NOTE: The effectiveness of the connection of extraneous-conductive-parts in the location to the main earthing terminal may be assessed, where necessary, by the application of Regulation 415.2.2.

So just to clear this up a shower with no RCD and supplementary bonding is safe until we then have a fault which could end up letting water in. This could the track across to us then we end up touching an earthed pipe for example? 

MrJack96: 
 

So just to clear this up a shower with no RCD and supplementary bonding is safe until we then have a fault …

Yes, in general terms, if all the wiring is sound, it is safe until something goes wrong.

Think of a shallow cable without RCD protection. It is all perfectly safe until somebody bangs in a nail. Even then, it depends upon what you hit first. The danger is pinging the line conductor, so now you have a circuit through L, the nail, finger & thumb, down to ground. That may just give you a nasty belt, but if you happen to be in the bollocky-buff with wet feet, or holding on to something like a radiator, it may be worse.

MrJack96: 
 

So just to clear this up a shower with no RCD and supplementary bonding is safe until we then have a fault which could end up letting water in. This could the track across to us then we end up touching an earthed pipe for example? 

well unless the shower had exposed metal bits itself and there was a failure mode where they came live somehow, then, yes. 

And quite a lot of cubicle designs do not have solidly earthed pipes or door frames to complete the path.

Note that even with a leak, assuming it is the incoming clean water, the column of water connecting victim to the live parts needs to be quite short and fat - in the sense of the cross -section of the water path being a some few square cm, and the path length being not too many cm long.   Otherwise it is just a liquid filled resistor and keeps the current down to ‘tingle’ rather than ‘lethal’ levels.

A thin thread of water few mm in diameter and  several cm long will not do.

Mike

PS units of water purity… 

TDS (mg/L) = k · EC (µS/cm)

Total dissolved solids in milligrams /litre  =how much salty residue stuff (in milligrams) is left behind if you boiled off a litre of water .

k varies a bit but for typical calcium and sodium sized impurities that are singly or double ionized per atom (this relates to how many electrons drop off the ions when they get wet )is between 0.5 and 1.

Electrical Conductivity  in µS/cm is 1 over the resistance of a one cm cube of water in megohms.

Tap water around the world typically has conductivity values ranging from 50 (pure) to 1500 µmho/cm (becoming undrinkable, or at least seriously unpleasant to drink, this upper limit represents around 1 gram or so of salts per litre.)

so 1/50 of a megohm = 20k ohm per cm cubed to more like 1/1500 of a megohm = 700 ohms per cm cube.

If we pretend  that 15mm plastic pipe has an interior cross-section of 1square cm, and I appreciate its not quite,  then a foot of that pipe (30cm) could be (20k*30) = 600k if the local tap water is very pure, or more like (0.7k* 30) , 21 k or so. for the crummy stuff.   Even the latter would give a  ~ 12mA shock from 230V mains, which is not likely to either trip an RCD or kill you. It is quite likely to explore your full vocabulary of disappointment however. ?