Earthing a metal Clad workbench

[JohnW2]

When I had to do a workshop with metal coated tops I used 50mm² cable to bond as anything less can melt with welding current.

What did you bond the worktops to? Presumably it would have to be direct to the welder (otherwise some of the current would go through the installation's CPCs) - which may be practical (with 50mm²) for a 'fixed' welder, but presumably not for a DIY 'portable' welder, would it?

Kind Regards, John

 

[Nozzle]

I had not considered putting the installation at risk by having it earthed while doing welding. Though it's not a professional set-up so there is no external earthing terminal other than the one in the plug or the return current clamp - which would then, when attached to the work piece become the same potential. As far as I know, this welder is just a whacking great transformer and rectifier - so "schematically speaking" looks the same as a standard power supply. And in a standard power supply the chassis earth is NOT connected to the 0V rail. I am now in two minds whether connecting mains earth to the metal bench is the right thing to do. Or even if cladding it in steel is the right thing to do!

Nozzle

 

[bernardgreen]

Maybe a detachable sheet of steel put in place when necessary

Personally I would earth the steel and have an RCD on the supply to all sockets in the area. Lights on a separate RCD (or no RCD ) so lights remain on after the accident.

I accept the risk that the earthed metal increases the "strength" of a shock if I am touching the metal at the same time as holding a defective metal tool but trust the RCD will operate. When the defective tool is placed on the earth metal there will be earth leakage and RCD will trip ( and maybe the MCB depending on what the defect is )

 

[ericmark]

I used 50mm² after have to do a re-wire due to earth wires frying in the tray work it was caused by bad earth connection on the welder and also the fact that the type of welder needed an earth on the output most today due to this problem do not have the output earthed. As I said an old Oxford oil filled unit and the welder used a magnetic earth clamp which was not the best way to get a good earth.

I did not want that welder moving around the factory so it was hard wired into workshop and every bench was linked to next bench with 50mm² and to welding set output and to building metal work to ensure no way could the welding current go through standard earth wires again.

Non conductive benches would be far cheaper and also using air cooled welding sets which have insulation on internal wires so don't need the output earthing.

I also had an Oxford welding set dropped off a forklift only a couple of foot and outside it looked fine. But on winching the top off I found the bare conductors had bent with the shock and primary and secondary were connected together.

I wanted to scrap all the oil filled welders but the welder thought they were far better than the new ones so the bonding with 50mm² was the compromise.

Unfortunately it is all too common to forget to move a welders earth and do damage as a result. Before class II drills I have renewed many a drill flex where welding current went through it.

 

[EFLImpudence]

It was not bonding.

 

[ban-all-sheds]

I get suspicious when I cannot see even a single speck of sawdust

And breathe......

 

[JohnW2]

I get suspicious when I cannot see even a single speck of sawdust

And breathe......

I feel far more at home with that

Kind Regards, John

 

[Nozzle]

All this chat about Oxford welders is very insightful and I recognise different set-ups need different configurations, but I don't have the fancy heavy duty oil filled gear, just a portable domestic set-up. I plan to find out if the earth pin is connected to either the current return on the LV side (either full time or switched) but I don't think it will be.

I recognise that earthing the table and touching it with a whilst holding a faulty tool with the other hand might cause a worse shock than if it had earthed through my feet... but in either event I'm relying on the protection to operate anyway. Is there such a thing as an RCD that is more sensitive to operate? Either operates with a smaller current, or operates in a quicker time? If so, and they're available in a double-socket face plate I'll install one in a belt and braces approach. (Only if I find out the earth pin is no connected to anything on the LV side of transformer)

Nozzle

Update, it would appear the earth is not connected to the LV side http://www.machinemart.co.uk/documents/MIG110E-160EN.pdf

 

[JohnW2]

I recognise that earthing the table and touching it with a whilst holding a faulty tool with the other hand might cause a worse shock than if it had earthed through my feet but in either event I'm relying on the protection to operate anyway.

Fair enough. In truth, if you were wearing shoes and/or had a sensible floor (not wet concrete in contact with soil!), the chances are that you wouldn't get a significant shock 'through your feet' if you touched something live.

Is there such a thing as an RCD that is more sensitive to operate? Either operates with a smaller current, or operates in a quicker time?

The literal answer is yes, but it wouldn't really help you. You can get 10mA RCDs which operate at a current of 10mA (or above), rather than the usual 30mA (or above). However, RCDs do not (cannot) limit the current through you - they can only limit the duration of that current. If you experience a shock, the actual current through you (and sensed by an RCD) depends only on the impedance of the path through you to earth - and if it is a 'serious shock', that's likely to be well over 30mA, so a 30mA RCD would trip, anyway. I don't think that RCDs which trip faster than the 'standard (10mA or 30mA) ones are readily (if at all) available.

Kind Regards, John

 

[Nozzle]

Ah that's some great info thanks. I see the reality that if there's something not available off the shelf to do a job, then the engineering work has already been done... and there is a good reason it's not available (i.e there is no benefit to be had).

Nozzle

 

[JohnW2]

Ah that's some great info thanks. I see the reality that if there's something not available off the shelf to do a job, then the engineering work has already been done... and there is a good reason it's not available (i.e there is no benefit to be had).

It may actually be due to physical limits (without incurring ridiculous cost). There would be a potential 'benefit to be had' if RCDs disconnected more rapidly, but getting much faster than current ones (less than 40 milliseconds at high fault/shock currents) might be very difficult and/or expensive (don't forget that just one cycle of 50Hz AC electricity is 20 ms).

Kind Regards, John

 

[ban-all-sheds]

I do wonder where all these faulty portable tools are going to come from.

 

[JohnW2]

I do wonder where all these faulty portable tools are going to come from.

The same sort of places that all other items of electrical equipment which develop faults come from, I suppose.

If you are suggesting that we're talking about very unlikely/rare events, you're obviously right, but consideration of such unlikely occurrences seems to be the name of the game as far as matters of electrical safety are concerned.

Let's face it, if one did away with all protective devices (apart from service fuses), determined cable sizes by rough rules of thumb (without consideration of methods of installation), and also did away with most 'detail' regulations, there probably would not be a catastrophic increase in electricity-related 'incidents' - but the culture and regulations we live with wants to minimise the 'unlikely', and even the 'very unikely'.

Kind Regards, John

 

[Nozzle]

physical limits (without incurring ridiculous cost).

This is what I mean by the engineering work. Something might be possible, but that doesn't make it necessary. I'm confident B-A-S will contest this in some way or other.

Nozzle

 

[JohnW2]

physical limits (without incurring ridiculous cost).

This is what I mean by the engineering work. Something might be possible, but that doesn't make it necessary. I'm confident B-A-S will contest this in some way or other.

There is always room for debate, depending on what one regards as 'necessary', which in turn depends upon one's attitude to risk.

From what you've said, you're fairly enthusiastic about RCDs, but the fact is that, even before RCDs had even been heard of, the number of deaths due to electrocution were trivial compared with, say, deaths due to use of ladders, stairs, or any number of other causes which were 'accepted'.

People probably get confused by things I write about RCDs, since they may appear to some to be contradictory. I am, indeed, far from convinced that RCDs have had any appreciable impact on damage to life and limb. However, my view is that IF one decides one wants to use RCDs, then one should maximise the chances of their providing the intended benefit - and that would include minimising the time they take to operate. However, as I've said, whether it would be practicable (at reasonable cost) to get them much faster than they currently are, I don't know.

Kind Regards, John