Supplementary earth Bonding query

[RF Lighting]

What if the flex to your double insulted hand held food mixer has become damaged. You accidentally touch it whilst your other hand is touching the sink. You now receive a much worse shock.

but no worse than if you touch the tap, toaster or any other bonded metal or any appliance with an earthed metal case.

Yes but much worse than if the sink was isolated.

If you're stupid enough to try and fish a live appliance out of a sink full of water without unplugging it first, well it's Dawinism at work.

True, but the chance of touching the sink and something that is at the equipotential zone potential is also significant. A metal fronted socket for example when turning the mixer off and resting a hand on the sink.

How many times have you dropped a live mixer in your sink? I haven't ever.

In fact if standing on a dry floor there would probably be no sensation when touching the sink or water thrown out onto the work surface but still in contact with the sink.

So why do you think it needs earthing then if there's going to be no shock anyway?

That could result in resting one hand on the half live ( 120 volt ) sink while pulling the plug.

But you've just said they won't be a shock so it doesn't matter.

 

[bernardgreen]

In fact if standing on a dry floor there would probably be no sensation when touching the sink or water thrown out onto the work surface but still in contact with the sink.

So why do you think it needs earthing then if there's going to be no shock anyway?

That could result in resting one hand on the half live ( 120 volt ) sink while pulling the plug.

But you've just said they won't be a shock so it doesn't matter.

There is no shock is there is only one point of contact. ( or maybe a tingle due to capacitive coupling creating a high impedance second "point" of contact ) If one hand is resting on the half live sink and no other point of contact exists then no shock. But as soon as the other hand or other body part touches something metal or otherwise conductive to the CPC or true ground then a second point of contact is formed and shock can occur.

 

[EFLImpudence]

Bernard - It depends which you think more likely.

If you are in the habit of dropping live conductors into your isolated metal sink (whether full of water or not) then it would probably be wise to earth it - so that the cpd will operate and disconnect the supply.

If you think it more likely that, whilst leaning on the sink with one hand, the other hand should come into contact with a faulty appliance then you would be better off (less of a shock across the heart) not earthing the sink.


If the sink is not isolated but connected to The Earth by the water pipe which will main bonded then we have no choice.

In this case further supplementary bonding may be required (or not these days subject to conditions) - to equalise the potential between the sink and simultaneously accessible parts in the event of a fault.

To hypothesise about it the other way round by suggesting that touching a live sink (still live because it is not earthed) with one hand and then touching an earthed socket with the other hand is just thinking of reasons why the 'once in a blue moon happening' could suggest that something is wrong.

In this unlikely scenario it would be preferable if the socket were not earthed but it has to be because it contains live parts - the sink does not.

 

[bernardgreen]

To hypothesise about it the other way round by suggesting that touching a live sink (still live because it is not earthed) with one hand and then touching an earthed socket with the other hand is just thinking of reasons why the 'once in a blue moon happening' could suggest that something is wrong.

It is often the "once in a blue moon" incidents which get massive publicity and result in an "improvement" in safety regulations. Sometimes the "improvement" is knee jerk reaction which has unforseen implications.

In this unlikely scenario it would be preferable if the socket were not earthed but it has to be because it contains live parts - the sink does not.

What is needed is double insulated sockets.

I personally think the risks from a live sink are very high and believe that that situation should be prevented.

A metal sink in a typical MDF work top is going to have a significant connection to the adjacent taps mounting in the worktop via the inevitable moisture in the work top so a person in contact with a live conductor or appliance casing is going to feel something from mild to severe if they touch the sink as well.

 

[RF Lighting]

I've never heard of a single incident involving a live sink in all my professional career. Not even anecdotally. Can you cite any incidents?

If we start EARTHING kitchen sinks, then where do we stop? Do we need to start EARTHING every single piece of metal in the house that could come live in extraordinary circumstances?

A metal sink in a typical MDF work top is going to have mounting in the worktop via the inevitable moisture in the work top so a person in contact with a live conductor or appliance casing is going to feel something from mild to severe if they touch the sink as well.

So why do we need to earth the sink if it already has a significant connection to the adjacent taps which may or may not be at earth potential.

 

[ban-all-sheds]

Do we need to start EARTHING every single piece of metal in the house that could come live in extraordinary circumstances?

There used to be a chap here who advocated that, or thought he did.

David somebody-or-other....

 

[bernardgreen]

Can you cite any incidents?

None with any back up documentation. An aquaintance did get tingles from the sink in a newly installed kitchen. Kitchen company blamed a faulty built in appliance and replaced that appliance but that did not solve the problem. Before I could get there with test meter the kitchen company arrived un-expectedly and re-placed ( or just re-fitted ) the sink. When I arrived a few days later the sink was bonded to the water pipes.

Do we need to start EARTHING every single piece of metal in the house that could come live in extraordinary circumstances?

Risk assessement is needed together with out of the box and lateral thinking. Normally door handles would not need bonding unless the door was steel and has a door release mechanism operated by electrical power.

There is the very unlikely event that a screw in the door latch plate has pierced a live cable. ( as found in a poorly maintained house in Wales 1979 )

So why do we need to earth the sink if it already has a significant connection to the adjacent taps which may or may not be at earth potential.

Because the significant connection may not be enough to ensure the 30 mA of leakage required to trip an RCD.