Earthing Stainless Steel Bench?

[SparkB]

I have been asked to 'earth' a metal/stainless steel bench inside a workshop.
The bench is used with hand tools and power tools - there is a socket directly above it where power tools being used are plugged in to.
Also staff are complaining of receiving static shocks when touching the bench, and hopefully the earthing would help with this also.
My thought is to earth the bench via 6mm Green/Yellow to the earth at the socket (metal clad socket).
I have had a few conflicting views on this so just wanted anyone's thoughts?

 

[sean_ork]

Just bond the bench using a 1M ohm resistor, the static will still be discharged when the bench is touched - but it won't be noticed by the user.

 

[SparkB]

Hi Sean
Do you mean bond the table to the earth/casing of the metal clad socket above, with a 1M ohm resistor in series with the bond?

 

[sean_ork]

Yes, that's pretty much all a ESD kit does

 

[VoltzElectrical]

I kept getting one helluva belt off the missus car the other week, but not her, I started closing the door with my foot but one day when we got back, I steadied myself against the wall and got a belt off that! It must have been something building up in my clothing, as it only happens when I am wearing one particular top.

I remember another thread about someone getting a belt from radiators, but it turned out that they were feeling the static discharge TO the radiator that they had built up.

In my opinion, if the feeling is fleeting, but painful and no voltage is apparent during testing, then further earthing the bench will only enhance the sensation?

 

[sean_ork]

The trick is to manage the rate of discharge such that it's not noticeable - hence the series resistor

 

[Lucien Nunes]

I presume the bench is not earthed now i.e. there is a very high resistance between it and earth, such as if it stands on plastic feet or a dry plastic floor. If they are getting shocks when touching it specifically after walking across the floor, it must be the people getting charged and then discharging themselves to the stray capacitance of the bench when they touch it. If that is the case, then neither a resistive connection to earth nor a solid bond will prevent the shocks as the stray capacitance will still be there, it could make them worse as the bench will discharge faster through the bond.

The only situation in which a 1MΩ earth strap on the bench would help is if it is the bench charging up, e.g. if there was compressed air / powder handling equipment on it generating a triboelectric charge, and then shocking the people.

It sounds like the floor needs treatment.

 

[UKMeterman]

Hi,
This stuff should help, Vermason - 220521 STATGUARD FLOOR FINISH 10 L Note it is an ammonia based cure product, so whilst applying it will stink to high heaven...

 

[SparkB]

Thanks for the replies everyone.

Lucien, your theory is how I see it also - however I am not sure how or if the floor could be treated - it's probably not fit for purpose in the environment it is in - I have told them this may be the case.

I have tested the bench etc and have now bonded the steel table leg with 6mm Green/Yellow to the earth at the SWA banjo on the socket above.

Staff say this is immediately better, with reduced static being felt - though not a total solution

 

[telectrix]

before bonding the table, i would have used a voltmeter to determine whether there was a potential wrt earth on the table.

 

[DPG]

Thanks for the replies everyone.

Lucien, your theory is how I see it also - however I am not sure how or if the floor could be treated - it's probably not fit for purpose in the environment it is in - I have told them this may be the case.

I have tested the bench etc and have now bonded the steel table leg with 6mm Green/Yellow to the earth at the SWA banjo on the socket above.

Staff say this is immediately better, with reduced static being felt - though not a total solution

If staff are getting a static charge built up on them from the man-made material of the floor, and then touching the bench and discharging this static charge, then earthing the bench can't improve the situation as far as I can see. The best way to avoid the effect is to touch the bench via a resistance first (as Sean mentions above). There used to be a product available in the form of a resistive self-adhesive pad which was fixed to metal objects (primarily filing cabinets in offices which are carpeted) - the user touches this briefly to discharge the static charge in a controlled way, then they will not feel anything when they touch the metal object. Daz

 

[BroadReach]

Or increase the relative humidity in the space to above 40%, preferably to 50% and the problem will go away

 

[SparkB]

I will look into these discharge pads as a possible suggestion.

As I say staff say the static is less so since I did the connection to earth, strange I know.

Dont think increasing the humidity is an option due to the work they do.

All this due to some new flooring eh!

 

[hasel5]

Change the bench is an option to a plastic one or some other type ?

 

[DPG]

Change the bench is an option to a plastic one or some other type ?

That's OK until they touch something else made of metal like a cupboard or a power tool

This all reminds me of how I always used to get a static shock off car doors when I got out of the car - sometimes quite a powerful one. I never do now though because I know how to avoid it Daz