ADS

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.

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.