Extraneous conductive parts

My lack of actual experience is clearly telling. Thank you for spending some time elucidating matters for me.

Obviously in my diagram the circuit protective conductor (earth wire) in the cable supplying the faulty immersion heater should have stopped the cylinder, pipes and tap becoming live, but we do not rely on that happening, hence bonding or RCDs and so on.


Andy B.

Obviously in my diagram the circuit protective conductor (earth wire) in the cable supplying the faulty immersion heater should have stopped the cylinder, pipes and tap becoming live

Even with an intact c.p.c. the immersion circuit (or any of many other circuits outside the bathroom) could still pose a hazard to the bathroom as it's disconnection times were longer than 0.4s - and existing installations correctly done to earlier editions of the regulations could have up to 5s disconnection times for a wide variety of circuits.


 

the touch voltage during a fault is reduced to a maximum of 50 Volts

Technically the 50V condition need only be met when the fault current is only sufficient to achieve a 5s disconnection time - higher fault currents may result in more than 50V. But meeting the 5s/50V requirement with normal overcurrent protective devices keeps you below the 'curve' that (trading increasing voltage with decreasing disconnection times) ensures reasonabe safety from fatal electric shock. If you continued along the same curve you should arrive at the 0.4s/115V point.


   - Andy.

411.3.1.2 Protective equipotential bonding

...main protective bonding conductors... shall connect to the main earthing terminal extraneous-conductive-parts including the following:

(i)Water installation pipes

(ii) Gas installation pipes

(iii) Other installation pipe work and ducting

(iv) Central heating and air conditioning systems

(v) Exposed metallic structural parts of the building

 

Don't misread that like a lot of people do.


The list only contains any of those thing when they ARE extraneous-conductive-parts.

It's useful to keep in mind that the fundamental purposes of cpcs and bonding are different, even thought they use the same G/Y cables. Cpcs connect exposed conductive parts to the MET and thus to the means of earthing, so that if a fault to an exp-c-p occurs, enough current flows to trip the ADS in good time. Bonding connects exp-c-p and extraneous-conductive-parts together in such a way that during a fault, the voltage between simultaneously accessible parts is kept as low as possible until the ADS has time to kick in (or indefinitely it doesn't).


Supplementary bonding is sometimes used in addition to main bonding in locations where the risk is higher. and in situations where the ADS is less effective.

But luckily the beat of the human heart is of order 1 to 2 Hz depending on your physical condition, and electric shocks that are short compared to the beat period are much less serious than those that are longer, and there is a smooth curve of increasing probability of fibrillation, as the shock current or time is increased.

So if we can get the disconnection times down, we can increase the shock current we consider acceptable,  and in turn the voltage for a given state of conductivity. Dry clothed, wet naked, and then direct contact to internal tissues,  are the hierarchy of lowering contact resistance we normally consider, so tighter rules in bathrooms than in living rooms, and tighter again in operating theatres and the like.

I am somewhat disappointed in myself, I must confess. Effectively, in my mind, I have conflated and muddled the two ideas of Supplementary Bonding and Main Bonding. 


I have dusted out a few cobwebs and clarified a few thoughts, ahem...


Supplementary Bonding is no longer required, provided that all circuits are protected by RCDs and that all copper pipes (and any other extraneous-conductive-parts, for that matter), are ‘effectively connected by main bonding conductors to the earth terminal of the installation’ (page 93 of the On-Site Guide). Whether those pipes are ‘effectively connected’ can be tested by performing a ‘continuity test’ (and rectifying matters, should anyone happen to have replaced a section of copper pipe with plastic, without taking necessary precautions to protect the ‘main bonding’...). The operation of an RCD when an earth  fault occurs depends upon all extraneous-conductive-parts being effectively connected to earth via the main bonding conductor (10mm2 for TN-C-S, see Table 4.4(ii), page 48). Apart from being required to be connected to extraneous-conductive-parts within 600mm of the entry point to the dwelling (eg. gas and water pipes), the only other requirement is for extraneous-conductive-parts to a location containing a bath or shower to be ‘effectively connected’ by main protective conductors to the main earthing terminal of the installation. Any additional connections would need to be convenient for inspection and testing purposes.


Yes, if I were thinking about the Supplementary Bonding of a bathroom (because I did not have RCDs installed on all circuits...), I would need to correctly connect any extraneous-conductive-parts entering the location to one another (i.e. a supplementary conductor strapped to the copper pipes...), and to the exposed-conductive-parts (i.e. the earth terminals of any lights, switches, or other permitted electrical parts...). See Table 4.6 on page 50 (e.g. cpc of 10mm2 t&e for a shower would be 4mm2, therefore 4mm2 would be the effective maximum, but - again, showing my lack of real world experience - it might be a lot easier to use the smaller size cables permitted, to link some of the exposed-conductive-parts (eg. lighting and switches...).


Well, that’s that sorted...