Asked to bond a sink (table)

[LeeH]

I have been asked to ‘bond’ some tables or industrial sized wash bins by the engineering supervisor here at work. People stand and work out of these. Simple structure with water that’s filled from a hose.

Q “can you bond these tables, and what do you think?”

Me. “Well, seeing they are not part of the electrical installation, and are very unlikely to be extraneous then No”

I then explained about the 23K rule and how it comes about.

We had a professional chat about it all but couldn’t convince the other party completely and they still think it’s required . Now he’s probably forgotten more then I’ll ever know regarding the machinery here but I have spent long enough in electrical installation and design to be confident. He also cut his teeth back in the 15th which was obvious when I was told you have to bond sinks, ‘cross bonding’ etc.

Anyway I degress.....so I marched off my my AIO tester in hand. Tested the table to a known earth. With the floor wet and the table sat in the water, metal wall panels and with no surprise at 500V test voltage 0.1Mohms.

I’m aware that going around bonding anything metal including the tea bag caddy can increase the risk of shock and I’m trying to prevent this culture from starting.

So let’s discuss either my misunderstanding of electrical theory or how I can science my way out of this hole.

Discuss.

 

[Silly Sausage]

The 23k "rule" is wiring regulations, not PME regulations. Bonding, which is reqd in this situation and creating an equipotential zone ensures the safety of the area. What does create danger is bonding bits of pipe etc and not others, as enviged by the wiring regulations. For example, bonding an oil pipe but not a water pipe "because its got an insulated insert" Regards UKPN, and mind how you go.

If our man tests these metal sinks in question on a hot day with nobody working and nice dry floors, no way will they need bonding.

I'd defer the problem to the summer.

 

[johnduffell]

Bonding a sink that isn't extraneous doesn't do anything to help the work done because it didn't affect the integrity of it or accessible from outside it. If the neutral was lost the whole zone would end up at the same level so that wouldn't be the additional hazard in most cases.
Bond/earth it or not you'd still need a double fault to receive a shock.
The additional hazard would actually be in the case of a double fault situation where live parts were accessible. Touching the bonded sink and the live part would cause a shock.

 

[LeeH]

Well, after all the input I’m still non the wiser tbh.

John, I don’t understand the first half of your post. Cheers.

 

[sam400]

I’ve had this same problem, an insurance inspection in a commercial kitchen recommended the bonding of free standing stainless tables with plastic feet!
I said I was pretty sure it wouldn’t need bonding under the current regs but on testing them they came out as extraneous I can’t recall the exact reading but they were somewhere in the danger zone 1.6k to 22k.
It must have been the grease providing a path.
Anyway I rang my scheme and they told me to bond them all. The site neutral was 240mm I think so it would have meant installing a 50mm
conductor to each table!
I never did suss out s solution as they demo’d The whole building about a month later and built a brand new one!

 

[johnduffell]

John, I don’t understand the first half of your post. Cheers.

Sorry, i should be banned from posting after midnight as i literally didn't proof read it! See corrected version with edits on bold:

Bonding a sink that isn't extraneous doesn't do anything to help the equipotential zone because the sink didn't affect the integrity of it and isn't accessible from outside the zone. If the neutral was lost the whole zone would end up at the same level so that wouldn't be the additional hazard in most cases.
Bond/earth it or not you'd still need a double fault to receive a shock. The zone is there to prevent single fault conditions from being able to cause a shock.
The additional hazard would actually be in the case of a double fault situation where live parts were accessible. Touching the bonded sink and the live part would cause a shock.

 

[johnduffell]

they demo’d The whole building about a month later and built a brand new one!

That's one way around the problem!

If they are within the zone entirely it must have been the solid floor providing the path, although digging it up and installing mesh might have been even more impractical than bonding with 50mm flex