Grounding of Portable Welding Machines

[bthomp]

I work in an industrial facility where portable welding machines and light plants are used frequently. The issue of grounding the frames of this equipment comes up frequently, regarding the necessity of this "ground" connection. The equipment comes with a grounding stud for that purpose, but in some cases, the equipment is used in open areas where there is nothing close to bond to, so a ground rod would have to be driven and connected. The NEC discusses portable and vehicle-mounted generators in Article 250.34. This article states that the frame is not required to be grounded when it only supplies equipment mounted on the frame or cord-and-plug equipment supplied by receptacles on the equipment. It seems that portable diesel-powered welding machines and light plants would fall in this category. I don't really see what grounding the frame to a ground rod buys you? I'd like to get thoughts from anyone else with knowledge on this issue.

 

[skiier]

In most countries when you create a source you must gnd it for several reasons - the greatest being personnel safety.
If you are running power cords off the "frame" for any reason than you must ensure that your power source is properly gnded to ensure safety.
When inside you must bond the "frame" to the gnding system in the building or an approved gnding point such as the structural steel in the building. Is the steel in your building properly bonded according to your local code? If not, run a gnd bond of suitable size directly from the "frame" to the main bulding gnd point.
If your "frame" is away from the building at distance less than the length of the longest power cord you can run off the "frame" then you should bond the "frame" to the building gnd. The reason being that workers can extend extension cords into areas of the building that are tied to the building gnd.
If you are away from the building at a distance greater than the length of the longest power cord you can run off the "frame" then you should bond the "frame" to a driven gnd rod or plate. Gnding to the building in this case may create a lengthy and dangerous gnd path. As you stated before when using "cord and Plug" equipment you don't have to gnd the frame. True, but driving a gnd rod at the source improves or increases the odds of a gnd path for faults near the "frame" from cables off the "frame". You are not solely relying on the gnd wire in your extension cord to protect you and as we all know extension cords and power cords get damaged on construction sites.
If absolutely no cord extends off the "frame" then yes I would think you wouldn't have to gnd the "frame".
I would, however, keep an extra eye on your cables if you choose not to gnd the "frame". The thought of damaged/frayed cables in ungnded systems just gives me the creeps.

 

[advidana]

code or no code-not grounding the frame is may cause someone to die.

 

[skiier]

advidana. I thought about this one for a while and it occurred to me it might make sense not to gnd the "frame" under certain circumstances. What about a portable police command post, or a TV news/weather remote in a van? They have there own portable power sources do they not? I am sure there are a ton of other applications where the cables do not extend off the "frame".
If bthomp's "frame" or "light plant" is a deck of some sort - made of metal -and is large enough to contain a welding or work area from which no cable or extension cord extends than he probably wouldn't have to gnd the "frame". The deck that the workers are on becomes earth or gnd to these workers.
The problem occurs with this system, however, when cables and extension cords extend off the frame. Take for example, if a worker is grinding and welding a metal beam off the "frame" on the ground next to the "frame". His old metal case grinder is not of the newer "double insulated" type and has a bad power cord with a broken gnd wire. His grinder has also recently developed an electrical problem in the windings and this has made the metal case of his grinder "hot" or live above gnd. The worker is unaware that the case is "hot" because he is always wearing gloves and these insulate him against shocks. So the worker decides to take a break and join his buddies on the "frame" or deck. He sets his "hot" grinder down on his "ungnded" beam and in turn makes the beam "hot". Let's say the beam is close enough to the "frame" so that is easier for the worker to step up on the beam to reach the metal rail on the stairs or ladder of the "gnded" "frame". So now the worker reaches for the gnded "frame" from the "hot" beam after removing his gloves. If. and I mean if, his feet are dry and his boots are well insulated he may not receive a shock. Maybe, maybe not. I say this not to emphasise the importance of gnding the "frame" but of the importance of maintaining electrical cords properly. A good cord would have protected the worker. A driven gnd rod in this case "might" have saved the worker - depending on the earth "resistivity" - but a good cord would have definitely saved him. So the emphasis must be placed on good cables over driven gnd rods.
Another problem with gnding the frame every time you move is that people will simply not want to drive a gnd rod everytime they move. Some workers may also forget to disconnect the gnd when they move the "frame" thereby constantly breaking and degrading the gnd point. Do you think that gnd connection is going to be repaired properly everytime it is broken? And do you think they are even going to be bothered to repair it everytime it breaks?

 

[pennsy]

I agree with 'bthomb' adding a ground rod accomplishes nothing as far as safty goes.
But to BOND/ground the frame of the welder to the source as it should be, bonded, then the return path to the source will see a very low impedance path(to trip the breaker) and help reduce or prevent electric shock. This of course of dependant how far from the source the welder is. Even attaching to the metal frame of the building accomplishes a little more than a ground rod, ie if there is a fault to the frame of the welder that is grounded(to earth) via ground rod and a fault occurs You touch the frame of the welder you are now another parallel path back to the source the 'hard way' that is through the earth and any other surface that is connected to the earth.
The codes use of the word Ground, grounding and Grounded is much over used to the point where the meaning is almost lost. For example under snap-switches is says they SHALL be grounded, then if you look up the definition of ground it means to go 'to the earth'. Then in the grounding section the minimum ground resistance is 25 ohms, this is not a good ground and will not clear a breaker or blow a fuse. But is to be used as a minimum standard for grounding.In that same article it also says the the EARTH shall NOT be used as the sole return path for a fault.
What they are saying is the switch shall be bonded rather than grounded.
This is the biggest reason that they insist that a Neutral be run to all services whether it will be used on not and that is to clear faults as quickly as possible.

WOC