Earthing a metal Clad workbench

[SFK]

Dear Nozzle,
Apologies to you if this is silly idea, and to others if this is a repeat idea.

But why not also use a "Earth Bonding Plug with a 1Mohm resistor" that plugs into one of your sockets and its wire goes to your metal table surface? http://www.amazon.co.uk/s/ref=nb_sb...th+bonding+plug&rh=i:aps,k:earth+bonding+plug

I have seen these used to ensure that the metal table does not gain a Static charge (which could damage circuit boards being tested on it), but my understanding is that the "1Mohm resistor offers protection in the event the workstation becomes live by limiting the current to less than 0.3mA at 240volts and should the fault current continue the resistor will open circuit" (ref: https://www.vodex.co.uk/production-.../earth-bonding-products/earth-bonding-plugs).

Regards sfk

 

[JohnW2]

But why not also use a "Earth Bonding Plug with a 1Mohm resistor" that plugs into one of your sockets and its wire goes to your metal table surface? I have seen these used to ensure that the metal table does not gain a Static charge (which could damage circuit boards being tested on it) ....

When you say 'also', do you mean in addition to actually earthing the worktop which Nozzle has been contemplating. If so, there would obviously be no point.

... but my understanding is that the "1Mohm resistor offers protection in the event the workstation becomes live by limiting the current to less than 0.3mA at 240volts and should the fault current continue the resistor will open circuit"

Whist I don't dispute the arithmetic, I'm not sure I understand what sort of 'protection' that is meant to offer. Are you perhaps talking about protection of any electronics in contact with the workbench?

In general, I think Nozzle's primary concern was protection of himself, not of equipment he was working on, and a 1 MΩ path from the worksurface to earth wouldn't really help in that regard!

Kind Regards, John

 

[SFK]

Dear JohnW2,
My use of the word "also" was meaning in addition to the RCD protection that others mentioned.

The 1Mohm Bonding was put in place by others better qualified to reduce static charge.

But I always felt 'unsafe/worried' with the idea of a direct (cable) link from the earth terminal on the plug sockets to the metal bench, because I felt there could be a chance that I could touch a live with one hand and the earth bonded bench with the other.

So I was told (the reference link I provided and worked 10mins ago now seems to have stopped working!!) that they used the 1Mohm resistor to offer protection in the event the workstation becomes live by limiting the current to less than 0.3mA" which I read as minimising the current through my body before the RCD trips.

But reading all that again, it looks as if I have got myself into a circle as I seems to be saying that the 1Mohm cable keeps the table bonded until the table becomes live at which point the Resistor blows and the table is then no longer bonded.

....doh So again sorry if I have added confusion.

EDIT: And to add to my retraction I have now noticed that the reference I provided says: "Warning the Earth bonding plug must only be used for ESD control purposes".

sfk

 

[EFLImpudence]

But I always felt 'unsafe/worried' with the idea of a direct (cable) link from the earth terminal on the plug sockets to the metal bench, because I felt there could be a chance that I could touch a live with one hand and the earth bonded bench with the other.

That is the dilemma and which the OP has decided is the lesser evil.

So I was told (the reference link I provided and worked 10mins ago now seems to have stopped working!!) that they used the 1Mohm resistor to offer protection in the event the workstation becomes live by limiting the current to less than 0.3mA" which I read as minimising the current through my body before the RCD trips.

The RCD won't trip, will it?, unless another 27mA flows through you to another earth.
The 1MΩ resistor will have no effect on you if the workstation is live.

If the workstation becomes live you want the OPD to open 'instantly'.

 

[JohnW2]

Dear JohnW2, My use of the word "also" was meaning in addition to the RCD protection that others mentioned.

Yes, but as EFLI has pointed out, with a 1 MΩ resistor 'in the way', an RCD would not operate.

The 1Mohm Bonding was put in place by others better qualified to reduce static charge.

It's fine, and 'standard' for that purpose.

But I always felt 'unsafe/worried' with the idea of a direct (cable) link from the earth terminal on the plug sockets to the metal bench, because I felt there could be a chance that I could touch a live with one hand and the earth bonded bench with the other.

Exactly - that's the very dilemma (with standard earthing) that has been the basis of most of the discussion in this thread.

Kind Regards, John

 

[JohnW2]

...by limiting the current to less than 0.3mA" which I read as minimising the current through my body before the RCD trips.

The RCD won't trip, will it?, unless another 27mA flows through you to another earth.

I take it that "27 mA" is your code for 29.7 mA? (I didn't miss that one )

Kind Regards, John

 

[SFK]

Dear EFLImpudence and Johnw2,
Yep - as said as I tried to highlight my reasoning I could see my point falling apart --- hence the doh.

So with thanks for that --- always good learn.

SFK

 

[JohnW2]

Dear EFLImpudence and Johnw2, Yep - as said as I tried to highlight my reasoning I could see my point falling apart --- hence the doh.

Fret not - we've all been there

Kind Regards, John

 

[EFLImpudence]

I take it that "27 mA" is your code for 29.7 mA? (I didn't miss that one )

No the RCD was previously ramp tested and was tripping at 27.3mA.

Sorry, just testing.

 

[SimonH2]

Not quite what SFK had in mind, but how about bonding the table through a (high power, min 12W) resistor - say around 4k7 ?
That would pass enough current to trip the RCD, while also limiting the current in the "user grabs something live and touches the earthed top" scenario.

If you are ultra-paranoid, then connect all such bonding connections via a single point and pass the cable through the current sense hole of a suitable RCD trip relay that will cut the power separately to the supply RCD.
EDIT - assuming that current sense relay trips all power, not just the RCD protected circuits. Otherwise, what could be live without tripping the RCD ?

 

[JohnW2]

Not quite what SFK had in mind, but how about bonding the table through a (high power, min 12W) resistor - say around 4k7 ? That would pass enough current to trip the RCD, while also limiting the current in the "user grabs something live and touches the earthed top" scenario.

I suppose so - but then one would be totally reliant on an RCD (which some people regard as fairly 'unreliable') in the scenario (which is what the OP was primarily concerned about) of something live touching the metal table top - since the presence of the resistor would prevent the circuit's OPD from operating in response to the 'fault' in the normal way. Perhaps worse, it would potentially (in some circumstances) allow a 'touch voltage' of up to about 140V (relative to earth) to exist on the work top without the RCD (or any other device) operating.

There is also the possibility that the resistor combined with the victims impedance would reduce the shock current (in the latter scenario you mentioned) to a little under 30mA, so that the RCD would not operate - so that the victim might suffer a nasty (albeit probably not lethal) persistent shock, whereas without the resistor the shock current would have been high enough to result in 'immediate' operation of the RCD.

This is obviously all very-much 'swings and roundabouts' stuff.

Kind Regards, John

 

[bernardgreen]

deleted

 

[Nozzle]

Well, the deed is done.

8'x3'x1.2mm steel folded over each edge and welded at the corners. Sparingly screwed to the ply in a few placed and earthed via a tag and curly earth cable to an existing JB. The bench is plugged in to a wall socket that has now been replaced by a unit with built in RCD. I'm half expecting the photo of the electrics might draw some flack from the brow-beaters.

View media item 80538View media item 80539Nozzle

 

[securespark]

Flack:

No strain relief on or any support to, cables entering JB.

Signed: B Beater.

 

[Nozzle]

Ah, your flack/feedback is relevant and welcome. When I installed the electrics a few years ago I was met with the problem that these JBs don't have a cable grip as a plug or appliance would have - (if you ask me, the layout of the terminals also leaves a lot to be desired). So Internal to the JB I have fashioned strain relief, by way of a pair of tight cable grips on each flex to stop it pulling out. I say "stop", it wouldn't do really but it offers considerably more resistance to being tugged out. I'll go away and tack in some cable supports this afternoon in fact...

Nozzle