Shower with no RCD or supplementary bonding

No it’s not rented out just an old flat with an old wylex board ? 

well if it is to be rented out it would have to be fixed,  as rented rules now require this, but if not then it is nor so serious.

Note I do not do domestic electrics for work ,so  my engineering judgement is based on 1st principles, not napit codebreakers or whatever,  and maybe 3 or 2 depending what else is going on.

Mike.

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. ?

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.

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: 
 

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.

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…

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.