Is an RCBO not an RCD (and MCB in a single package)?
A bit of bondage!
- Thread starter dingbat
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I may be stating the obvious but you seem to be arguing both ways.
It may be a 'grey area', as such, but only because it is a compromise.
It would be best if electricity were not introduced into houses but because it is and it is dangerous we do what we can to minimise the risk.
In your kitchen sink example -
If the sink is connected to copper pipework throughout the house then we have no choice as it is 'earthed' by the pipe entering the ground.
However, if the sink is isolated, which is more likely to happen?
A live conductor will touch the sink and a person will touch the sink and get a shock and, presumably jump, (true that if the sink were bonded the CPD would have disconnected the supply) or -
A person leaning on the sink picks up something which, for whatever reason, is live (if the sink were bonded the current wold flow through the person straight to earth and the person may not be able to let go).
As we cannot do both we have to choose the likeliest scenario for maximum safety which is not to bond non-extraneous metal parts - whether they be spoons or emotive sounding objects like sinks or baths.
I think - hope - you would agree that the latter is more likely and therefore would also agree that it is advisable not to bond isolated metal objects i.e. those which are not extraneous conductive parts.
It may be a 'grey area', as such, but only because it is a compromise.
It would be best if electricity were not introduced into houses but because it is and it is dangerous we do what we can to minimise the risk.
In your kitchen sink example -
If the sink is connected to copper pipework throughout the house then we have no choice as it is 'earthed' by the pipe entering the ground.
However, if the sink is isolated, which is more likely to happen?
A live conductor will touch the sink and a person will touch the sink and get a shock and, presumably jump, (true that if the sink were bonded the CPD would have disconnected the supply) or -
A person leaning on the sink picks up something which, for whatever reason, is live (if the sink were bonded the current wold flow through the person straight to earth and the person may not be able to let go).
As we cannot do both we have to choose the likeliest scenario for maximum safety which is not to bond non-extraneous metal parts - whether they be spoons or emotive sounding objects like sinks or baths.
I think - hope - you would agree that the latter is more likely and therefore would also agree that it is advisable not to bond isolated metal objects i.e. those which are not extraneous conductive parts.
Absolutely, I agree. Another possible term for grey area might be "requires careful consideration taking into account all pertinent factors"
This requires competent persons to evaluate a existing installations and / or design new electrical installations with a sound understanding of electrical principles. A reasonable move towards ensuring that happens is underway, to be implemented in 2012 as I understand it.
When there are 'swings and roundabouts', such that a compromise is necessary, (s)he who wants to take a balanced approach has to acknowledge the arguments in both directions.
Indeed, that's what one has to do whenever there are swings and roundabouts.
I don't think one can generalise. One has to make a balanced judgment on a case-by-case basis.
Consider the metal bath (and plastic plumbing, so that we have a choice). If there is nothing electrical anywhere near the bath, then the argument for not bonding becomes very weak (probably non-existant). If in addition, there were the possibility that someone in the bath could touch something which was (explicitly or implicitly) bonded (maybe a radiator or {non-electrical} towel rail) the the possibility of some idiot dropping an extension-lead-powered hair dryer into the bath might, although extremely unlikely, actually be more likely than that they would come in contact with anything associated with the electrical fixed wiring - in which case bonding might, on balance, be the statistically most logical/'safer' option - in that particular scenario.
Kind Regards, John
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I'd say the opposite re the bath, if it is fed by plastic pipes and deemed not an extraneous conductive part by any other means then I fail to see how bonding it will make it safer.
Re the extension lead, if someone is daft enough to run one into their bath and drop it then it is their stupid act that caused the problem, not that of the electrical installation.
Re the extension lead, if someone is daft enough to run one into their bath and drop it then it is their stupid act that caused the problem, not that of the electrical installation.
Can you elaborate on this please?
From Jan 2012 (I think it is Jan) the requirement for qualified supervisor for the competent person schemes is being increased to NVQ3 (or the new equivalent of NVQ level3)
QUOTE:
The industry bodies represented on the Electrotechnical Assessment Specification (EAS) Management Committee have agreed the requirements for new Qualified Supervisors that will be introduced in January 2012. From 1st January 2012, for the first time, a level of technical competence equivalent to level 3 NVQ competences is to be the minimum requirement for all new applications for Qualified Supervisors.
There are five different routes to achieve this standard, including on-site assessment, off-site assessment, a mixture of both and the gaining of appropriate qualifications.
The industry bodies represented on the Electrotechnical Assessment Specification (EAS) Management Committee have agreed the requirements for new Qualified Supervisors that will be introduced in January 2012. From 1st January 2012, for the first time, a level of technical competence equivalent to level 3 NVQ competences is to be the minimum requirement for all new applications for Qualified Supervisors.
There are five different routes to achieve this standard, including on-site assessment, off-site assessment, a mixture of both and the gaining of appropriate qualifications.
True - but, as I said, one needs to make a case-by-case judgement as to which of the approaches is safer. A good electrician should do that on the basis of all considerations (including possible consumer stupidity), and not wash his/her hands of possibilities which are not the fault of the electrical installation.
In the specific bath scenario I described, there is zero risk due to the electrical installation (doesn't get anywherte near the bath). If there is a risk due to user stupidity, no matter how small, that surely shifts the risk-benefit balance in favour of bonding ('extremely small' is still greater than zero) - in that specific situation.
As I keep saying, it is necessary to think about the specific scenario, not just generalise.
Kind Regards, John
P
Paul_C
Therein lies a big problem, I think.
While at this point in the early 21st century we have more regulation than ever before, and the bureaucrats and committees are working on even more restrictions to ensure, supposedly, that those doing the work are competent, it seems to me that we have a fairly substantial proportion of "qualified electricians" now who really have absolutely no grasp of many basic electrical principles at all. They may have a dozen different C&G and other papers, but they don't seem to understand some very basic electrical theory.
I find it rather worrying that with all this extra regulation, talk of initiatives to improve things soon, and so on there are so many people who have gone through "the system" and yet lack an understanding of things such as earthing and bonding principles which were well established and understood half a century ago.
I disagree totally, making something that is electrically floating earthy can make it worse.
There wasn't a requirement to bond baths fed by plastic pipes in the 16th edn - there is even a pic in the on site guide stating this.
The 17th edn OSG simply says supplementary bonding is not required to metallic parts supplied by plastic pipes (4.7)
Consider you have bonded the bath and someone drops their extension lead in it, not enough current flows to pop the 13A fuse and there is no RCD protection. A person leans on the bath and picks the extension lead out, they will have a nice path between their arms and a nice earthy object for current to flow. You can't take into account everyones stupidity when designing an electrical installation - it would drive you insane!
No question at all, your concerns are 100% valid. There is and has been for some years a bias towards BS7671 regulation-centric teaching (for want of a better word) and not enough electrical science foundation. My understanding is that it has not gone unnoticed and it will be factored into new courses. This vast problem of under education (lets call a spade a spade) won't get fixed overnight or in 2 years but I understand the courses will include a lot more scientific training and that in my opinion can only help to improve the situation.
No, that was my point about arguing both ways. You cannot plan for both.
There is hardly any likelihood of a live cable touching the bath (and this is not the purpose of bonding) but it is possible for someone leaning on the bath (while not 'having' a bath) to pick up a faulty appliance or frayed lead, for example.
That is why only extraneous parts shall be bonded or, rather, why non-extraneous parts should not.,
In this particular scenario I would not bond the bath. But BS7671 is not a law. There may be some reason why John (for example) may bond the bath because of a specific, identified risk factor and that, I think, is fine for a specific scenario but not as a general solution. When building a block of identical flats then you may well spec that the bath does not get bonded (because all the plumbing to it is plastic)
In my view it acceptable either way provided it can be justified according to the balance of risk for a specific situation and that the underlying science is well comprehended.
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