ADS

This has started to go around in circles so perhaps a broader view will help Coby.

In an ideal world an electrical installation would comprise RCDs and bonding to various services. You would have protection as far as you can reasonably achieve. However this position has taken many years to evolve.


So I put to you a tragic case, many years ago where a young child was electrocuted at Christmas. Playing with her friend, both 10 years old, like kids do in bare feet. The Christmas tree lights were fed by a two core single insulated flex, as they were in those days. The earth bonding was perfect, water and gas, the heating by radiators. So the integrity of the equipotential zone was intact, PME. However, there was no RCD protection for the socket feeding the flex and the cable, being pulled in and out of the loft for years had become frayed.


The young child stood on the live bare flex and held the earthed steel radiator. 


So, the ideal scenario is RCDs in sensible situations and earth bonding which is a legal requirement when using a DNO PME terminal. 


A couple of caveats, G Kenyon mentioned the stone floor and the baby. While not always practical UK Power networks asks for a steel grid in the ground floor of bathrooms/wet rooms etc which in turn connect to the earth terminal and create a local zone. Failiang that, an RCD is reqd. 


His other comment, regarding the radiators, bonding will cover the pipework in this instance as there are no other services and an insulated floor. BTW, rads themselves are considered extraneous. 


So, there you go Coby, RCDs+bonding. job done.


Regards, UKPN

Listen boys:


This is not a fantasy world, in truth touch voltages are utterly insignificant, there are no magic RCD's that can somehow reduce the magnitude of an electric shock without even disconnecting the supply!


This is the real world where we only have simple physics and that is all, continuity, resistance, logic and common sense.

Therefore we are now, always have been and always will be 100% reliant on EEBADS and loop impedance; and now we simply need to reduce our disconnection times from 0.4seconds to 0.04seconds if we are to achieve the stated claims of the IEC.


Having now spoken to you for quite a while, it has become obvious that the prohibitive price of RCD's was not the only issue back in 1991:

If at least some of us had not mastered the art of insulation resistance testing before their introduction, people like you would have thrown the lot of them out of the window claiming that they were unusable due to nuisance tripping and we never would have been able to progress at all!


P.S. Being considered 'extraneous' obviously makes a radiator an extraneous-conductive-part and therefore, back in our real world, they must be connected to the main earthing point of the installation, therefore providing us a measured/calculated loop impedance and therefore a predictable disconnection time if that radiator should come into contact with a hazardous live conductor! (simple isn't it)?

But a radiator is not necessarily extraneous.


If the radiator comes into contact with a hazardous live conductor, then other means of protection against electric shock according to IEC 61140 (BS EN 61140) have been breached for some reason.


Depending on that breach, it's not necessary, in all cases, for an installation protective device to operate - for example, if a Class II product is destroyed.


That's the function of additional protection. Discuss.

Mr. Kenyon,


If a flex gets damaged and the owner doesn't notice, it is up to you as the experienced man who is paid to know how to do it, to prevent that damaged flex from causing injury!

Post removed by the Moderating team.

An RCD will not prevent you from receiving a shock.


An RCD does not reduce the shock current, but will limit the duration of that shock current to a time based on the residual current detected, as shown in Table 3A of BS 7671.


An RCD is not guaranteed to protect you against physiological effects, but on the statistical balance it will.


The inventor of the RCD actually did expose themselves to shock to test out their theories !


Under some conditions (contact area, what you have on your feet, how wet you are and what's in that water), the shock may be more severe.


It's possible to touch a conductor protected by an RCD, and receive quite a nasty perceptive shock, without the RCD operating. Most electricians will know about this, and be fully aware that the greater risk of exposure to shock is physical injury from secondary effects.


I'm sure everyone that contributes to this forum is aware of these limitations ... and of course no-one would choose to touch hazardous live conductors.


IEC 61140, product standards, and BS 7671, do not guarantee 100 % protection from electric shock, but are intended to meet the essential requirements of the LVD, or Electrical Equipment (Safety) Regulations, along with the Electricity at Work Regulations - many of the requirements are "SFARP".

P.S. Being considered 'extraneous' obviously makes a radiator an extraneous-conductive-part

Not really. As I suggested before you need to read the definition of an extraneous-conductive-part in part 2 of BS 7671 - which reads:

Extraneous-conductive-part. A conductive part liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation.

How do you suggest a radiator can introduce a potential (either into a location or into the installation)?

 

they must be connected to the main earthing point of the installation, therefore providing us a measured/calculated loop impedance and therefore a predictable disconnection time if that radiator should come into contact with a hazardous live conductor!

How do you suggest we maintain a predictable disconnection time when in the real world plumbers modify heating systems using new plastic fittings? Are you suggesting we individually "earth" every radiator and length of pipe? You've got the general concept right, but you've mis-placed it - ADS applies to exposed-conductive-parts, not extraneous-conductive-parts - the former being inherently part of the electrical installation where we can control the loop impedance.

Therefore it is time for you to test your theory and there is only one way to do it; all you need is a calibrated RCD tester and a screwdriver and you can all go home tonight and touch a line conductor.

What exactly are you suggesting as an experiment? During my mis-spent youth I came into contact with live conductors on quite a few a occasions - some resulting in a painful shock, others not. Luckily I survived - but I can't guarantee my overworked guardian angel will always be there so it seems wisest to to push my luck. However if you describe the conditions you are interested in, I'm likely to know from personal experience what the results are likely to be. No religious ceremonies necessary.

 

If a flex gets damaged and the owner doesn't notice, it is up to you as the experienced man who is paid to know how to do it, to prevent that damaged flex from causing injury!

We need to consider a few different permutations of the flex, radiator and victim. If the flex touches the radiator and the victim then touches the radiator then earthing the radiator might help to trigger ADS - perhaps even before the victim touches the radiator. But it's all rather unreliable given the state of modern plumbing technology. A couple of plastic fitting could easily introduce several tens of thousands of Ohms into the earth path - reducing the earth fault current to a level where neither an MCB/fuse or RCD would trip, but the victim could still receive a painful (if unlikely to be fatal) shock.


In another permutation the victim touches the damaged flex and the radiator - in this case earthing the radiator can increase the shock current that flows through the victim. Conventional (fuse/MCB) ADS (which is predicated on the idea of a fault of negligible impedance) won't activate as the victim's body resistance will limit the fault current to a small fraction of an amp. A RCD for additional protection is the best bet in these circumstances - but again there is a region where a painful (but hopefully not fatal) shock can be received without the RCD tripping. Isolating the radiator from earth in this case is actually a more effective way of reducing the shock risk (see the section in the regs about non-conducting locations) - but that's equally hard to make reliable so we don't rely on that approach in domestic and similar settings. All the same there can be benefits from leaving isolated metalwork unearthed.


Yet another permutation with a damaged flex is touching the flex and some other earthed part at the same time (perhaps the case of the class I appliance the flex is supplying) - again conventional ADS can't help and we're reliant on additional protection by 30mA RCD and earthing the radiator neither helps nor hinders.


   - Andy.