Radiator earth bonding

[bernardgreen]

but surely you are doing a Bernard and guarding against the Blue Moon event rather than the more likely events when you would not want them joined.

If a person can touch ( or be in contact ) with two conductive items either or both of which contains electrical circuitry with a supply that is referenced to ground ( Neutral to Ground bond at the substation ) then the Blue Moon is that when one item develops a fault that raises it's potential and the other item has a path to ground ( water pipe etc ) that has some impedance. The person who touches both gets a shock if the items' touchable conductive surfaces are not bonded together.

How severe is the shock ( how many milli-Amps through the body ) and what is the duration of the shock that occurs because the items are not bonded together.

A short severe shock ( time limited by the RCD tripping ) is less harmful than a prolonged mild shock when the RCD does not trip as the impedance to ground ( water pipe etc ) limits the shock current to be less than the RCD's tripping current.

If you doubt a short severe shock can be almost harmless then consider the shock from a defibrillator. The defibrillator gives a short and severe shock that stops the heart fibrillating and then allows it to self restart. The duration of this shock is too short for any significant electro-chemical damage to occur.

 

[ade11]

If metal parts are NOT earthed by contacting other parts which are earthed then that is better and safer.
Connecting them would not be bonding but earthing unnecessarily and introducing a hazard.

Yes,this is something I was made aware of at the age of 10,in the 60s,when my Dad received a hefty belt off one of our new central heating radiators when something made it all live. Probably that sort of situation is why Black and Decker double insulated their metal drills back then, instead of earthing. Safer when drilling through a cable,for others at least,as the drill would become live but not transfer current elsewhere. I have just checked my old Wolf metal bodied double insulated drill and there is 0.3 Ohms between the chuck and the body. Now we have RCDs would it be safer to add an earth so the RCD would trip ? This is a theoretical question as the Wolf is a slow speed high torque non- hammer drill for metal,never used on a wall.

 

[EFLImpudence]

No. Double insulated is considered safer than earthing.

Earthed items become live when faults occur elsewhere.

 

[bernardgreen]

Earthed items become live when faults occur elsewhere.

Earthed items tend to remain at Ground potential

Bonded items can be affected by potentials carried along the bonding connection.

 

[JohnW2]

If a person can touch ( or be in contact ) with two conductive items either or both of which contains electrical circuitry with a supply that is referenced to ground ( Neutral to Ground bond at the substation ) then the Blue Moon is that when one item develops a fault that raises it's potential and the other item has a path to ground ( water pipe etc ) that has some impedance. The person who touches both gets a shock if the items' touchable conductive surfaces are not bonded together.

The 'Blue Moon' arises because not only does such a fault have to develop (itself very rare), but that a person also has to simultaneously touch both items at the very time when a fault arises (again, very unlikely). Multiplying too very low probabilities together results in something approaching a 'Blue Moon'.

However, as I wrote yesterday, some of the regs do seem to exist to 'guard against Blue Moon events', so one has to accept that at least some people regard that as necessary, and therefore cannot dismiss the idea out of hand.

Kind Regards, John

 

[JohnW2]

Earthed items tend to remain at Ground potential .... Bonded items can be affected by potentials carried along the bonding connection.

I think that most of these (recurrent) discussions are pretty moot, given that, in practice, 'bonding' (as practised with electrical installations) virtually always involves at least one 'earthed' part - so that 'bonding' also results 'earthing'.

The only real differences are in relation to 'intent'/purpose and, in some cases, the need to size bonding conductor to be able to carry (under certain fault conditions) much higher currents than would be expected (or necessarily even 'tolerated') in a CPC.

Kind Regards, John

 

[EFLImpudence]

I think that most of these (recurrent) discussions are pretty moot, given that, in practice, 'bonding' (as practised with electrical installations) virtually always involves at least one 'earthed' part - so that 'bonding' also results 'earthing'.

Are there any parts which require bonding that are not already earthed?

The only real differences are in relation to 'intent'/purpose and, in some cases, the need to size bonding conductor to be able to carry (under certain fault conditions) much higher currents than would be expected (or necessarily even 'tolerated') in a CPC.

So - bonding and earthing are different, then?

 

[JohnD]

So - bonding and earthing are different, then?

the research and development lab at a high-tech factory near me was an earth-free zone.

But conductive parts were bonded for safety.

So, not the same.

 

[JohnW2]

Are there any parts which require bonding that are not already earthed?

Probably not (at least in a standard domestic installation), but I think we have to be be careful about this word "earthed". If it had an absolute meaning (i.e. 'connected to true earth', hence 'at true earth potential') then there would, for example, presumably be no need to have any main bonding in a TT installation, since one would merely be connecting together two things which were already at the same potential. The reality is that there are 'earths and earths'.

So - bonding and earthing are different, then?

As I said, different in intent/purpose. However, mechanistically, '(main) bonding something' and 'earthing something' are (in a 'standard domestic installation') are achieved in exactly the same way - by using a conductor to connect (directly or indirectly) the 'something' to the installation's MET - so, in that sense, they are the same.

However, as JohnD has pointed out, and as with my workshop benches, there are situations in which (although not required by BS7671), one may install 'equipotential bonding' in the absence of any consideration of (or even presence of) 'earth' or 'earthed'.

Kind Regards, John

 

[EFLImpudence]

I don't know why you are being so awkward.

 

[ade11]

No. Double insulated is considered safer than earthing.

Earthed items become live when faults occur elsewhere.

But then the RCD would cut the power wouldn't it ? If someone hit the live leg of a cable with their vintage metal B &D DI drill,not tripping the MCB,they would have no protection, surely .

 

[JohnW2]

I don't know why you are being so awkward.

I'm not really sure that I am, and I certainly do not intend to be 'awkward'. It is not me who makes an issue about people muddling/confusing/mis-describing earthing and bonding.

I don't think the terminology is necessarily as straightforward as you imply. If I connect a CPC to the exposed-c-p of some item, you will presumably call that 'earthing', but if I connect it to, say, a pipe, you will call it Supplementary Bonding. The purpose is different, but the latter doesn't do any less 'earthing' than does the former, since the conductor does not know that it has been 'given a different name'.

Kind Regards, John

 

[EFLImpudence]

But then the RCD would cut the power wouldn't it ? If someone hit the live leg of a cable with their vintage metal B &D DI drill,not tripping the MCB,they would have no protection, surely .

Yes, but that is not the reason appliances are earthed.

Everything is a compromise.
There is always the chance of something happening where not applying safety measures would have been better.

 

[JohnW2]

Yes, but that is not the reason appliances are earthed. Everything is a compromise.
There is always the chance of something happening where not applying safety measures would have been better.

All true, but I do take ade's point that maybe a corded electric drill (or similar) should perhaps be regarded as a 'special case'.

The usual consideration is to protect people from live parts within an item - and, as you have said, 'double insulation' is probably considered to be a better way of achieving that than earthing its (assumed metal) touchable body. However, with a corded drill, there is a specific (and probably much greater) further risk of the drill bit (and hence a metal body, if in electrical continuity with the chuck) becoming live as a result of penetration of a cable external to the tool.

In the specific case of a drill (or similar) there might therefore be a case for a belt-and-braces approach, with 'double insulation' to protect the user from live parts within the drill, but also with earthing of the chuck and body - since, as has been said,, that would result in operation of an RCD if a live cable were penetrated - some might say that one could forget the 'double insulation' if the body were earthed.. Electrically insulating the chuck from the body (e.g. with nylon gears) would be another approach - but, if not earthed, that could leave the chuck live (and potentially touchable) if a cable had been penetrated.

Kind Regards, John

 

[EFLImpudence]

I'm not really sure that I am, and I certainly do not intend to be 'awkward'. It is not me who makes an issue about people muddling/confusing/mis-describing earthing and bonding.

It's called education.

I don't think the terminology is necessarily as straightforward as you imply.

Yet it is.

If I connect a CPC to the exposed-c-p of some item, you will presumably call that 'earthing',

Of course, that's what it is and why you call them a CPC and an exposed-c-p.

but if I connect it to, say, a pipe, you will call it Supplementary Bonding.

Of course, that's what it is.
Wouldn't you call it Supplementary Bonding? If not, why did you install it?

The purpose is different,

Exactly.

but the latter doesn't do any less 'earthing' than does the former, since the conductor does not know that it has been 'given a different name'.

It is a different conductor with, as you say, a different purpose.
It is installed for bonding purposes because both parts are already earthed.