View Full Version : Earthing (grounding) of benches

Philip in China
07-04-2007, 12:02 PM
I have a small blacksmith and machine shop in central China. It has a large tin roof, several steel benches and several hundred kilos of steel stock, tools and anvils. Electrical safety is not the strong suit of the guys who make the power tools we use (welder, pillar drill etc.) and we are also in a region where we get a high level of electrical storms! Add to this the fact that school children will soon be using the facility and you can see why I am worried about the risk of electric shock. I am basically a blacksmith not an electrician so can anybody advise me please?

1. I propose to run a copper earth cable from each bench to a separate ground spike. The spike would be a length of rebar with a forged point driven into the ground. I would flatten the other end, punch an eye which I would tap to M10 and attach the earth strap to that.

2. Most of the tools have metal casings but few if any have an earth lead. I propose to earth these via the 3 core lead to the mains outlet.

3. I think a lightning conductor is an essential. Would anybody argue with that. I also think that that is a job for an expert.

4. My stick welder has a small terminal at the back with an earth symbol next to it. Should this have its own ground spike? It works fine without but I am happy to put one on if that would be safer.

5. I am about to fabricate a steel welding bench. Should this ideally have an earth strap or is the connection to the welder adequate?

Any help you could give to keep the students safe would be appreciated. Would too many really good earths risk attracting lightning??

07-04-2007, 12:10 PM
Direct grounding of benches isn't wise. It ensures a current path close at hand for the user of a faulty power tool. The preferred method is to ground the bench through a resistor that limits the current to a safe value which depends on the local voltage but should be no more than a few milliamps. This is sufficient to trip a ground fault interrupter or in the absence of one to provide a warning tingle instead of a lethal shock.

Paul Alciatore
07-04-2007, 01:30 PM
I would agree that it is unnecessary and perhaps even a possible hazard to ground (earth) every bench. It will provide an instant return path if a person touching it comes in contact with a live conductor. Lineman for the electric companies routinely work on HV lines (direct contact with 50,000 Volts or more) in insulated cages: NO ground = no shock. Too much grounding can be a dangerous thing.

I use a grounded mat on a bench at work. It is for electronic repairs and protects against static discharge. It has a high resistance path to ground as Evan mentioned to limit the current that can pass through it. Perhaps 100K or 1M Ohms.

All metal shelled/cased equipment and power tools should be grounded through their electrical connection. This means the third wire. If they don't have ground connections now, yes, do add them. It adds a grounded shell BETWEEN you and the conductors INSIDE that may accidentally come in contact with that shell/case. Such a mishap will instantly trip the breaker, turning the circuit off. That is how grounding protects the user. You should check local codes as they may vary from those here in the US. But, do not just trust the outlets in the building to be properly installed. CHECK THEM. There are inexpensive, plug in testers that do this in seconds.

http://www.mcmaster.com/ Part # 7018K14 or 7018K15

As for the building, lightning rods may be a good idea, but you may also just run a ground connection from the metal roof to a ground rod. Again, check local codes, they frequently take local conditions into consideration.

07-04-2007, 05:01 PM
If an appropriate mast is installed above your shop for lightning protection with a copper rod on it with heavy copper cable going down to a suitable ground. There will be a cone of protection under it 1 1/2 times the height of the mast. The copper rod and cable will require insulators that stand off from the surface while the cable go down to ground. The rod could even be installed in a tree if the height of the cone of protection would cover your shop.

07-04-2007, 06:00 PM
Installing lightning rods correctly requires a degree of knowledge. There are some considerations that apply that don't come into play at lower voltages and currents. Lightning currents don't turn corners easily in a conductor because of the induced magnetic fields. All wire runs to ground should be kept as straight as possible and if bends are necessary they should be of the largest radius possible.

Lightning rod ground stakes should not be placed near places that people might frequent in a storm. If they must then a set of radial wires leading out from the stake should be buried, at least six or more equally spaced and for a radius of at least several meters. This prevents a deadly step potential from being induced in the ground during a strike.

07-04-2007, 08:04 PM

07-04-2007, 08:28 PM
When you add lighting rods, you are providing a point for lighting to jump to! Do you really want to attact lighting? You might be better off by drawing any lighting strikes to another area. How about adding grounding conductors to near by telephone pole to draw it away from your metal building? If no local phone pole, add a tall pole with grounding so that the lighting will strike the tallest object in the area. Grounding the metal is a good idea but attacting lighting is not a good idea.


07-04-2007, 08:50 PM
When you add lighting rods, you are providing a point for lighting to jump to! Do you really want to attact lighting? You might be better off by drawing any lighting strikes to another area.
That is a well worn myth.

MYTH: Lightning rods attract lightning.

TRUTH: Lightning rods along with a full lightning protection system (http://javascript%3Cb%3E%3C/b%3E:ld_lightningprotectionsystem%28%29) are designed to intercept a lightning strike that is already occuring to a structure and route it safely to ground, preventing a fire and reducing any damage to wiring, appliances and the building itself.
Lightning rods do not attract nor are they designed to attract lightning. Since the descending stepped leader (http://javascript%3Cb%3E%3C/b%3E:ld_steppedleader%28%29) of a lightning bolt doesn't 'decide what to strike' until it is very close to the ground, lightning will only strike a lightning rod system if it already happens to be in (or very close to) the lightning's path.


J Tiers
07-04-2007, 08:52 PM
Steel benches on concrete are inherently "leaker" grounded (earthed), at least. Concrete on earth is normally somewhat conductive, unless the earth is very dry.

A ground rod that "bakes out" due to dry earth is no longer a ground (earthing) point.

If the tools are metal cased, and have only 2 wire power cables, then the concrete floor is already a hazard, unless by chance the tools are double-insulated, which is doubtful.

Grounding (earthing) other things in a more effective way doesn't do much. The better way is an earthing wire for the cases, and an effective mains earth connection. However, this may not be possible.

It may be that doing nothing is better than trying to do something which may turn out not-so-good.

As for lightning protection, half measures may be worse than nothing. The local situation, tall buildings, trees, etc affect your "threat" of a direct strike.

As for Evan's point, somewhat true.

However, a substantial amount of raised conductors may raise the local "perceived level of terrain" and create a "hill" that is higher and more likely to be struck by a bolt that is coming down somewhere nearby anyhow. Trees tend to be struck, as are any other things that stick up, like golfers raising their club for a drive, etc.

So unless your rods can handle the strike, best not to provide a local "tree" for the bolt to hit. The results of a failure to conduct the bolt may be not good.

07-04-2007, 09:32 PM

Mike W
07-04-2007, 11:03 PM
As far as I know there is extra pains taken that keep any metal parts that could be energized insulated from the operator. I once had a guy that wanted me to put a 3 wire cord on one. I pointed out that they weren't designed for one.

J Tiers
07-04-2007, 11:12 PM
I would appreciate an answer to a question in a previous post.

I noticed on several (at least) tools that have non-metallic/conductive cases that they are "double insulated" and that "earthing" is not required.

Why is that?

What would happen or be the risks if they were "earthed"?

Double insulation is the combination of "basic" and "reinforced" insulation.

You can have "basic" plus earthing of accessible metallic parts, which gives two layers of protection.

Or you can have the "basic" and"reinforced" insulation, again double protection.

This may take the form of two different layers of insulation on a wire, or an additional spacing past that required for "basic" insulation for "creepage" and "clearance" distances. (leakage and arcing distance).

Just two different ways of getting dual protective features.

Normally there is nothing accessible to "earth" if double-insulated. However, for drill motors, for instance, there may be an insulating portion of the chuck shaft, or a coating, etc, if there must be accessible metal extending into a motor etc.

For stereo equipment, the case is simply insulated from the mains wiring by the basic and reinforced insulation.

Added earthing, if you can find anything to earth, would give you a 3rd layer.

The voltage transient withstand is higher for double than for basic insulation, so the "hipot" test may also be at a higher voltage.

Paul Alciatore
07-05-2007, 12:11 AM
Double insulated is just that, "double" insulation. Any and all paths for the Voltage to travel from the conductors inside the device to the operator will have two seperate insulating layers between the two. The insulation on the wires is one, but it may fail if the tool is abused. The second insulating layer is often in the form of a non-conductive plastic case with fasteners (screws) that are deeply recessed to prevent any possibility of contact. An additional layer may be added to a tool like an electric drill to insulate the motor's armature or field coils from the metal parts that must penetrate the case (like the shaft the chuck attaches to).

The basic idea is that a single insulation failure will not lead to a shock. Kind of like the space shuttle with it's redundant systems (dual O rings). Bad example? You be the judge.

07-05-2007, 02:46 AM

07-05-2007, 03:05 AM

07-05-2007, 03:37 AM
What of the situation where a steel bench (all steel or steel-framed) is fixed to the steel frame of a shed -

Then be careful not to touch the bench and live conductors at the same time. It's not illegal or against code, it's just not wise to directly ground the benches if it can be avoided. Personally, I really prefer to avoid relying on the operation of safety devices such as a GFI to stay alive.

07-05-2007, 05:49 AM

Philip in China
07-05-2007, 06:43 AM
I thought it wasn't as simple as it seemed. I was right. I will gaet contractoes in to sort it out. I was prepared to put an extra spike in- spare the rod, spoil the child!

Any thoughts specifically relating to the arc welding bench?

07-05-2007, 08:00 AM
Any thoughts specifically relating to the arc welding bench?

I installed a GFI on the outlet next to mine. I presume they are available, it was made in China. ;) My double insulated angle grinders (made in China) are plugged into it. :D

Philip in China
07-05-2007, 09:08 AM
You naively assume the chinese tools available in the west to be available in China.

07-05-2007, 09:12 AM
You naively assume the chinese tools available in the west to be available in China.

Not really. We have the same problem here. The best select wood that we produce is only available in Japan and China. We get the stuff with the knots that fall out when you pick up the board. They need to change the lumber grading here to be more like that for plywood, "good one side" etc.

J Tiers
07-05-2007, 10:28 AM
A previous comment re. earth stakes and dry ground resonated with me as the Contractor said the same thing - it has to go way down. I recall asking him if I needed to keep the ground damp. He said it wouldn't hurt but as the stakes were so near the house (one near an external tap, the other abutting the slab to the shed/work-shop, and another to the copper water supply pipe) that it would have to be real, real dry for the ground to get that dry that far down.

It isn't unknown for the rod to be surrounded with some salt-like substance buried. Sodium chloride may be too corrosive, IIRC calcium chloride is used. If of interest I can look it up.

The idea is to raise conductivity, and also to attract whatever water is available.

Do not get the idea that a ground rod, even though required by code, is perfect.

A ground rod may have a resistance to ground of 25 ohms and still be acceptable. Per US electrical code, if the resistance is above 25 ohms, a second rod must be driven. But there is no requirement for the resulting resistance.

In fact, there is probably no way to assure a very low resistance. Earth is variable, and isn't a great conductor in any case.

The best way is a large "network" of ground rods and buried wires, as is done for large radio antennas. But that ends up essentially replacing the earth with conductive metal.

07-05-2007, 10:59 AM
The lightning "attraction" thing is not exactly a myth. Can you pull it from great distances? No. Stick an easy path to ground high enough up in the air that it begins to drain charge and you stand a good chance of eventually draining a large charge in the form of a lightining bolt, however. Its certainly true that you are not likely to pull it from miles away, but a cloud containing a large charge overhead would love a short path to ground.

In a previous life, I worked for a television station in the engineering department. My boss was the chief engineer and did most all of the transmitter work and even some tower work. I worked at the transmitter site from time to time and on one trip he pointed out the giant maybe 8' in diameter expanded metal mesh half-donut laying on the ground. It was one half of a donut shaped unit that went near the top of the tower and provided a ground path. It was designed to drain static charge off passing clouds before it built up enough to discharge in the form of a lighting bolt. Apparently, it didn't get the job done as it was intended because it ended up sucking a rather large charge off and suffered a direct hit which blew it to pieces...it then fell 500' to the ground.

Still, on the original topic, I second the notion of not grounding the work surfaces. This is the worst case scenario....a bit akin to having a hand on a water pipe during a lighting hit. Rubber on the floor would be a better solution.

We belive my younger brother may have been hit by lighting when he was a kid. When he was younger, he was down at my father's barn with my dad. It was clear a storm was coming and when my dad reached for the hydrant to draw water and got a shock, he sent my brother to the house ASAP. My kid brother was on his way back when we heard a crack and looked out and found him on the ground. He didn't know what happened, but said that it felt like someone hit him on the head with a board. He may have been saved by being on crutches at the time. A better path to ground could have carried enough current to kill him.


07-05-2007, 11:14 AM
The myth is that a lightning rod on a structure attracts lightning. It doesn't as it is only slightly higher than the structure itself. The idea is for the lightning to hit the rod if it was going to strike the structure anyway. The fact that the rod is conductive metal doesn't come into play until the structure is being struck. Before that the charge on the structure and the rod are the same so no "attraction" occurs.

In still air on a clear day the potential difference near ground level is several hundred volts per meter. This is the effect of the charge difference between the ionosphere and the ground, two giant plates of a capacitor. During a thunderstorm the potential can climb to many thousands of volts per meter. At these voltages the difference in resistance of a structure vs the lightning rod is negligible for the conduction of electrostatic charges and both are at ground level charge. It isn't until appreciable current begins to flow that the lower resistance of the rod has any effect on the current path. That is when the leader jumps up to meet the down stroke and with a rod it will have the easiest time going through the rod.

07-05-2007, 01:39 PM
Hello Again,
About 55 years ago I worked for Col. Dodge of Dodge Associates in Massachusetts installing lightning protection in trees.
I forgot to mention that we dug down in the area that the ground rods were to be put in and put a lot of peat moss in the ground to retain moisture.
Col. Dodge is the person that came up with the idea of installing lightning protection in trees.

07-05-2007, 05:05 PM
I remember some years ago, an electrician friend of mine telling me about a job he had when working at a very large steel works. He had to check the earth (ground) on a large turbo-alternator set and on looking at the electrical drawing, he noticed that the earth consisted of an old steam locomotive firebox buried in coke breeze. In the UK locomotive fireboxes were usually made from copper.