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Grounding a metal workbench

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keso

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Hi all,

I'm seeking an opinion: Is there any reason that wiring my metal workbench to the "neutral" line in my wall outlet (verified to be neutral and not hot) for ESD is a bad idea in the absence of a true earth ground?

I'm in an apartment and hammering a new spike into the ground or cutting new holes in the wall in search of a better earth ground aren't real options.

The wall outlets in this apartment are ungrounded. However, they are protected by a GFCI, which is required by California law for homes (or at least rentals) without grounded outlets.

I'm wondering if I ground the bench to neutral (which I've already verified not to be hot), am I going to be turning the bench into some sort of huge antenna for broadcasting EMI or something? Or perhaps are random blobs of static charge now going to be capable of tripping the GFCI when I ground myself to my bench?

(oh, and yes I am planning on putting a high-value resistor inline with the ground link for safety reasons in case I accidentaly touch a hot wire)

Any thoughts? Hidden gotchas that would make this a bad idea that I'm overlooking?
 
Hi all,

I'm seeking an opinion: Is there any reason that wiring my metal workbench to the "neutral" line in my wall outlet (verified to be neutral and not hot) for ESD is a bad idea in the absence of a true earth ground?

I'm in an apartment and hammering a new spike into the ground or cutting new holes in the wall in search of a better earth ground aren't real options.

The wall outlets in this apartment are ungrounded. However, they are protected by a GFCI, which is required by California law for homes (or at least rentals) without grounded outlets.

I'm wondering if I ground the bench to neutral (which I've already verified not to be hot), am I going to be turning the bench into some sort of huge antenna for broadcasting EMI or something? Or perhaps are random blobs of static charge now going to be capable of tripping the GFCI when I ground myself to my bench?

(oh, and yes I am planning on putting a high-value resistor inline with the ground link for safety reasons in case I accidentaly touch a hot wire)

Any thoughts? Hidden gotchas that would make this a bad idea that I'm overlooking?


The neutral side of your AC should have a connection to a ground rod in the main service or circuit breaker panel. It would be much better to run a separate wire from your bench to that panel and attach to the ground wire there. Another path would be clamping a wire to a nearby cold water pipe, especially if it's copper. If neither of these are possible and you can't or won't drive a new ground rod, then I recommend you not attempt to ground your work bench at all via the neutral wire of a near by plug.

Rules vary in countries and even within counties and towns here in the US, but I'm sure that what you are proposing is in direct violation of various laws and rules such as NEC in the U.S. Suppose you had some kind of electrical fire in your house and after inspection they found your added wire? Even if it was not the cause it could very well invalidate any property insurance claim, it's just not worth it.

Do it right or not a all ;)

Lefty
 
I would agree with Nigel on avoiding the metal bench for electronics work. One can't argue that if the bench is grounded - that any source above ground will short circuit if in contact with ground. The problem - if you (your body) becomes the path from the source to ground.

My experience with ESD is limited however there is usually a very high value resistance between the charged object and ground - I presume so that the charge can bleed off or not build - and so that any current flow is well below safe levels.

I'd also support the comment about doing it right or not at all. Lots of lessons have been learned - some the hard way. Those lessons are incorporated in the rules or codes.
 
Neutral is not ground in California! If you want ground, check the ground terminal, be sure it is really ground, and use that.

The only grounded workbench I use is for welding. I don't think I would want one otherwise.

John
 
Nigel! Mr. Floating Scope wants an earthed workbench? How odd! ;)

I agree that a connection to neutral is a BAD idea. As mentioned, you'd be risking your insurance coverage, it would be against city code (the NEC would hammer you for that one) and in poor or deteriorating wiring, the neutral can be elevated above ground by several volts.

I don't mind the metal bench except that a metal surface is not very comfortable to work on. And if something goes wrong, it could become "hot". How about slipping a plywood or MDF surface over it, maybe adding a layer of Formica?

Dean
 
Either connect the bench through a 1 megohm resistor to earth from a water pipe or other ground, or leave it floating.

Be careful about any material you place on the bench. Wood isn't bad but many types of plastic materials can generate large values of static charge and are bad for ESD. For the best protection they make ESD workbench pads which are made from a high resistivity but conductive hard foam material.

If you attach a wriststrap between yourself and the bench before you touch any electronics on the bench, it will neutralize any charge difference between you and the bench and prevent ESD damage, even if the bench isn't grounded to earth. The main ESD concern is the charge difference between you and the circuit, not between you and earth ground.
 
Thanks guys, these have been some great responses.

I think the one in particular about possibly invalidating a future property insurance claim on account of code violation is particularly good. I hadn't considered that.

The point about old or deteriorating wiring possibly elevating neutral up by a couple volts is also something I hadn't considered. This is a pretty old building and I doubt the wiring is that terrific.

As for the safety aspects, I had intended to use a high-value resistor (1M or greater) for the connection to neutral, so electrocution risk should be greatly diminished.

All in all I think I'll skip the whole grounding idea based on this feedback. I'll probably go ahead and wire up all the metal surfaces to each other (bench, metal shelving). The shear surface area of it all by itself should go a long way in dissipating any errant charges just by itself.

Thanks for the input!
 
Neutral is not ground in California! If you want ground, check the ground terminal, be sure it is really ground, and use that.

Really? What do they connect the neutral line to in California?

As for the ground terminal in these outlets, it's floating I believe. Old wiring, unfortunately.
 
Besides the electric side effects, shorts and possible injuries think of the overall negative effect on health.

If you want to ensure safe operation - also mains powered circuits, you should use ground (earth=PE) and a safety fuse which trips if a current between life and and earth and/or neutral and earth of 15mA is exceeded.
(mandatory in German households. Life insurances are extremely expensive if no safety circuit is present, checked by insurance agents.)

Working at a metal work with your ellbows placed on the "cold" surface will probably cause rheumatism.

My workbench is made of wood 2" thick. Even hammering small objects it won't vibrate or kick back.
 
For ESD you want things at ground potential, not tied to ground through a 12 gauge wire. ESD mats themselves have surface resistances in the gigaohm range. They dissipate charge, not provide a short ckt to ground.
 
My workbench is made of wood 2" thick. Even hammering small objects it won't vibrate or kick back.

Years ago, as kids, we used to ride and build old motorbikes round a local farm, and the farmers son had an old chicken shed as a workshop - it was a good big sized shed.

We managed to 'aquire' :D a huge old blacksmiths vice, and built a bench to mount it on. We dug four DEEP holes in the soil floor, and planted railway sleepers in them, we then made the top out of more railway sleepers, and bolted the vice to it.

Now that was what you called a BENCH you could use a sledgehammer to bend metal in the vice, and it never moved!.
 
Don't do it, there could be a break in the neutral which would make it live and the 1M resistor probably isn't rated for the mains voltage.

Connect it to the coper pipe via a 1M resistor.

It sounds odd that you don't have any plug sockets with an earth connection, the ones here in the UK all have an earth connection.
 
It sounds odd that you don't have any plug sockets with an earth connection, the ones here in the UK all have an earth connection.

The rest of the world has inferior electrical systems :D, you can't compare them to the UK.

Go back in time and the UK's electrical systems weren't so hot either, a mis-match of different sockets used, some earthed some not, it's only 'recently' (40+ years?) that it was standardised at it's current high quality.
 
Originally Posted by jpanhalt
Neutral is not ground in California! If you want ground, check the ground terminal, be sure it is really ground, and use that.

Really? What do they connect the neutral line to in California?

Actually, in California (and anywhere the NEC is adopted as law), the neutral line is called the "grounded conductor" and is a current-carrying conductor as the return for the hot line. The ground line is the "grounding conductor" (darn the NEC for making the terms so confusing) and is a safety ground and is not to be used for carrying current except in case of a fault.

Actually, a grounded metal bench would be OK as long as you feed all the power that would be used on or around it through a ground fault circuit interrupter.

Dean
 
The rest of the world has inferior electrical systems :D, you can't compare them to the UK.

admitted with a minor objection: The English system was copied from German safety standards. :D:D
 
The ground line is the "grounding conductor" (darn the NEC for making the terms so confusing) and is a safety ground and is not to be used for carrying current except in case of a fault.

Dean

It would be less confusing if the term 'PE' was used internationally. 'PE' stands for 'protective earth', and that's what it should be.

There is even a safety standard concerning wire connections in a 'Schuko-plug'. PE has to be long enough to be last wire to be ripped off the plug.

Hans
 
Faulty wiring aside, even with rated wiring a short circuit will cause neutral to go 'hot'. I agree 100% with a grounded work bench being a bad idea. Good solid thick wood is best. Having a good solid grounding point nearby is always a good idea though. Try an old iron door knob or basically any substantial piece of metal and wire it with multiple strands of the thickest wire you have to wire rods or sheeting dug into the ground, like out of a basement window. This will provide as close to an 'unbiased' ground as you're going to get and serves as a solid refrence point and static removal point. Regardless of where you live or who did the wiring in your house you can't beat a solid metal pole you dug into the ground yourself.
 
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