Supplementary bonding sounds dangerous!

[SparkyNinja]

Runners should be ok, but if they have those little plastic wheels, glue some wahsers on them. You should be sorted then

 

[bernardgreen]

That's why I keep asking you what you mean by "exposed". You don't bond conductive parts which are not earthed....

As you beg so nicely for enlightment I will enlighten you as to me opinion.

Exposed metal is to me any metal which can be touched ( including any that is behind removable covers ) and may become electrically live as a result of one or two faults happening.

Metal table legs of a wooden topped desk are exposed ( prudish victorians would cover them ) but very unlikely to become live. Metal legs of a metal topped desk on which electrical equipment may be used could become live along with the top and should be bonded. Some laboratory and other work places insist on such desks being bonded to earth.

I have seen the vertical columns of metal shelf support systems bonded to earth where metal shelfs could be fitted and electrical items used on the shelves.

 

[ban-all-sheds]

As you beg so nicely for enlightment I will enlighten you as to me opinion.

Thank you for your opinion.

All sorts of people have all sorts of odd opinions.

Exposed metal is to me any metal which can be touched ( including any that is behind removable covers ) and may become electrically live as a result of one or two faults happening.

That's a definition with no relevance to the definitions of either exposed-conductive-parts or extraneous-conductive-parts, or the regulations for either main or supplementary equipotential bonding.

Metal legs of a metal topped desk on which electrical equipment may be used could become live along with the top and should be bonded.

Should be?

Who says so?

Why?

Bonded to what/where?

Is such a desk an extraneous-conductive-part?

Some laboratory and other work places insist on such desks being bonded to earth.

That's not bonding, it's earthing.

And does the insistence stem from a genuine understanding of what's going on, and a genuine risk analysis, or from superstition and ignorance?

If a workplace has equipment or flexible cords which are in such poor condition that faults could raise a metal workbench to line potential then I don't think that the HSE would regard them earthing the benches as having fulfilled their obligations under the HASAWA, and EAWR etc.

I have seen the vertical columns of metal shelf support systems bonded to earth where metal shelfs could be fitted and electrical items used on the shelves.

And I've seen helmetless people riding motorcycles whilst wearing shorts, t-shirts and flip-flops.

Oh - and BTW - that's earthing again, not bonding.

 

[bernardgreen]

That's a definition with no relevance to the definitions of either exposed-conductive-parts or extraneous-conductive-parts, or the regulations for either main or supplementary equipotential bonding

True.

Is such a desk an extraneous-conductive-part?

What is your definition of an extraneous-conductive-part ?

Oh - and BTW - that's earthing again, not bonding.

And how do you see the the regulations on bonding that is not earthing coping with the "through the window" scenario and make it safe.

This where someone inside a room hands an item to a person outside in the garden. The person in the garden is on the ground, effectively earthed. The person inside the room is touching a bonded item of metal. The bond is as per regulations connecting that piece of metal to the case of a faulty piece of equipment which due to the fault has a case that is live.

Now to me it seems clear that if "bonding" is always to the true earth directly or in-directly then the "through the window shock" cannot happen.

And of course we have the external water tap bonded to the "earth" which is derived from the neutral which is. due to phase unbalance in the local network, a few tens or maybe a hundred volts away from true earth.. Adds a sparkle to watering the lawn.....

It just seems that bonding that is not an earth bond simple extends the shock hazard from a faulty item on to all other items that are bonded.

 

[AndyPRK]

Metal legs of a metal topped desk on which electrical equipment may be used could become live along with the top and should be bonded. Some laboratory and other work places insist on such desks being bonded to earth.

I have seen the vertical columns of metal shelf support systems bonded to earth where metal shelfs could be fitted and electrical items used on the shelves.

Are you sure this isn't to prevent against static damage to electronic components ?

 

[aptsys]

Are you sure this isn't to prevent against static damage to electronic components ?

Yes it is, and it's connected to earth through a 1 to 10 meg resistor.

 

[bernardgreen]

Are you sure this isn't to prevent against static damage to electronic components ?

In the situation I refered to it was electrical safety earthing as the shelves were full of operating electrical equipment on soak test and the risk of a fault causing a dangerous voltage to reach a shelf was considered too high to not earth the shelving.

High impedance anti static earth is a different matter.

 

[ban-all-sheds]

If that was the evaluation of the equipment it should have been put in the skip.

 

[bernardgreen]

If that was the evaluation of the equipment it should have been put in the skip.

The equipment was being soak tested immediately after manufacturer and prior to being packaged and sent on to be sold. 99% of failures would occur in the first hour (?) of use so all items were tested for twice that time before being passed as acceptable.

 

[ban-all-sheds]

And how do you see the the regulations on bonding that is not earthing coping with the "through the window" scenario and make it safe.

This where someone inside a room hands an item to a person outside in the garden. The person in the garden is on the ground, effectively earthed. The person inside the room is touching a bonded item of metal. The bond is as per regulations connecting that piece of metal to the case of a faulty piece of equipment which due to the fault has a case that is live.

And what if the metal they are touching is only bonded, and therefore live, because you've been along and connected it to the electrical system when it should not have been because it was not an extraneous-conductive-part?

Now to me it seems clear that if "bonding" is always to the true earth directly or in-directly then the "through the window shock" cannot happen.

Can't it?

What if the person inside is handing the person outside an item of portable equipment which has a live case because the cable damage you described earlier has happened to its cord?

It just seems that bonding that is not an earth bond simple extends the shock hazard from a faulty item on to all other items that are bonded.

And what does connecting something which had nothing to do with the electrical installation, or earth, to the electrical installation do?

 

[ban-all-sheds]

The equipment was being soak tested immediately after manufacturer and prior to being packaged and sent on to be sold. 99% of failures would occur in the first hour (?) of use so all items were tested for twice that time before being passed as acceptable.

If they were so unsure of the quality of their design or manufacturing that they thought there was a risk that within the first hour of the equipment's life it would fail so catastrophically that the case would become live then they should have changed the design or the way they made it.

 

[Spark123]

It depends what it is tbh, some of the EHT stuff I deal with can fail within the first few hours of being installed so isn't put into service until it has soak tested and settled.

 

[bernardgreen]

If they were so unsure of the quality of their design or manufacturing that they thought there was a risk that within the first hour of the equipment's life it would fail so catastrophically that the case would become live then they should have changed the design or the way they made it.

Look up bathtub failure curves. here is a starting point.

http://tinyurl.com/ma6bsg

 

[ban-all-sheds]

It depends what it is tbh, some of the EHT stuff I deal with can fail within the first few hours of being installed so isn't put into service until it has soak tested and settled.

I'm sure, but I don't think that policies on bonding extraneous-conductive-parts (or even just conductive ones) in LV domestic installations should be guided by what's needed in EHT equipment testing.

Look up bathtub failure curves.

I don't need to, thanks, I know about them.

If they were so unsure of the quality of their design or manufacturing that they thought there was a risk that within the first hour of the equipment's life it would fail so catastrophically that the case would become live then they should have changed the design or the way they made it.

 

[Space cat]

I've read through this thread and I don't think the following scenario has been mentioned yet:

You are in the bathroom (or kitchen) with one hand on a piece of Class 1 electrical equipment and the other on a metal tap that's connected to true earth through the pipework. At that precise moment, the electrical item fails with a live-to-case short. During the time it takes for the fuse/breaker to cut the power, the voltage on the case will be half the supply voltage or more. This is not a good time for you to have your other hand on that tap!

Bonding the tap to the wiring earth doesn't entirely eliminate the problem but it does help because the fault current now has a second route to true earth through the bond wire and the tap. If the tap has a good solid earth, the voltage on the case will be lower, more being lost in the live wiring. Alternatively, if the tap earth is poor, the tap voltage will rise to more closely match the case voltage. Either way round, you are better off because of that bonding wire.

When you look at it this way, all the arguments about desk legs become irrelevant. You only need to bond to earthed metal.