Shower with no RCD or supplementary bonding

If the c.p.c.s/bonding collectively satisfy the 50V/Ia rule, I think we can say the the requirements for supplementary bonding are met. That's not to say that touch voltages will always remain low (or below 50V) during all possible L-PE faults - but that the duration and touch voltages will be co-ordinated to limit harm, so you'd seem to have reasonable protection from the classic 'fault of negligible impedance'. Without 30mA RCDs there might be some small risk from ‘direct contact’ type problems - e.g. damaged equipment with exposed live parts or faults to a victim not involving the the PE system - e.g. due to a build up of damp/corrosion products - but that shouldn't really happen if things are decently maintained.

   - Andy.

Thank you Andy. 

so when it comes to the 50/ia rule this is between say the water pipe and the CPC of the shower what else should be considered is it simultaneously accessible parts? 

when you say faults not involving the PE system do you mean water ingress that could end up in voltage tacking across it? 
 

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

geoffsd: 
 

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

It's intended to keep the voltage between simultaneously-accessible exposed-conductive-parts at less than 50 V for faults that last up to 5 seconds (with Type B and Type C mcb's and RCBO's however, it's 50 V for faults above 0.1 seconds), so that if you are unfortunate enough to be touching the two parts when a fault occurs it is, to all intents and purposes less of a shock (pun certainly intended).

It won't make a difference to someone in Zone 0 of a bath, but in general it provides a reasonable level of protection, effectively limiting the touch voltage for long-duration faults to ELV level (higher current, quicker operating time, but still well below the 230 V level so reasonable).

In medical locations of Group 1 or Group 2, the resistance is modified to  0.2 ohms - but see also Regulation 710.411.3.2.5 where the voltage available between simultaneously accessible exposed- and/or extraneous-conductive-parts from a fault in a Group 1 or Group 2 medical location must be limited for ALL faults to 25 V AC or 60 V DC.

Yes, but how?

Can you please explain, for instance, the current path of a fault and how the 50V between an exposed-c-p and an extraneous-c-p is calculated or achieved.

If the extraneous conductive parts are caught by the MPB then during a fault of negligible impedance, the fault current is considered to be confined to the cpc and the fault current magnitude being the current required to disconnect the CPD in 5s or +0.1s in the case of mcbs. The actual current may be more but the trade off is that disconnection will be quicker. The touch voltage experienced can therefore be assessed to be the voltage drop along the cpc referenced to the MET.

Similarly when supplementary bonding is in place, then all of the fault current required to disconnect the CPD is considered to be confined to the supplementary bond. The Ut being apparent across it.

Can you please explain, for instance, the current path of a fault and how the 50V between an exposed-c-p and an extraneous-c-p is calculated or achieved.

Calculation (current path and assumptions)

Step 1 decide the maximum current that may flow to earth (live to CPC )without operating ADS quickly.

(if say  breaker is 40A perhaps a 50A current) 

Step 2 measure the resistance between the CPC and the non-bonded part.

(let us assume the non bonded part is connected to the MET by some moderate resistance)

Step 3 Assume (an this is the biggy)  that in the worst case fault, all the current in step 1 flows in the resistance of step 2, and multiply both numbers to get a maximum voltage.

perhaps the cpc is 1 ohm from fault point to the MET and the point of fault rises 50V relative to MET during fault.

less than 50 ?

more than 50  ?

 In reality this over estimates the problem, as the unbonded part may not be connected to the MET at the other end of the CPC, so the resistance may not be the full R2..

Mike

Is the shower made of metal ? many modern ones seem to be entirely plastic, and while they do connect the element body to ground, to trip an RCD if the element body corrodes, it is not an accessible part and given the water path length between the element and the user, it could probably be a lot more live, and not pose a hazard.

In less fussy parts of the world, it is common to heat water with an element in a way that the live hot metal is directly immersed. So long as the water stays drinkably pure, there is negligible shock risk, despite visitors calling them suicide showers..

Big clive took one to bits a while back…

The wiring is often a bit rough by UK standards, this is typical. It is not dangerous to the natives, nor to the careful visitor.

I suggest the original poster  need not worry to much, unless it looks worse than this. 

Mike

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? 

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