Thread: Welding earthed objects

I agree with what you say as I have been there and seen it a few times.

Firstly as a 1st year apprentice 40 years ago.I placed the earth on the track grouser tips of a cat D6 to weld on the body.Fortunately the tradesman intervened before damage was done to to the dozer bearings.

Secondly ,working in a shed while some else was welding on the frame of the building and made the 240V GPO go Poof!.

Thirdly ,now clued up up good earthing after a welding institute lecture, am told by the foreman to support workpiece with the 7 tonne overhead gantry."What about stray earths? " ,I ask

"Do as you are blankety blank told"
Being a good trades person I did as instructed and the crane and bearings were fritzed by the stray current that conducted straight up the cables through the bearings and electric motor.

cheers
Grahame

There are many real issues regarding "earthed metal objects" and where the earth clamp is placed.

Consider that most welding is done either directly on the ground (or slab) or on a metal bench resting on the ground or the metal object being welded rests directly on the ground.

So we, for safety reasons, that unless specifc measures are taken to ensure otherwise we assume all metal objects may earthed.

This is fundamental to welding electrical safety.....where it may cause an issue all metal items are considered possibly earthed.

BUT

We must also assume that any given metal object or part may not be earthed or not very well... or what it may be earthed to, is uncertain or unreliable...or it may not be earthed at all.

Remember we too generaly rest on the ground, or on a metal object that is connected to ground somehow.

Awareness of the ground presented by the greater mass of earth and that connected to the electrical mains system and most metal structures and how that relates to our welding earth is the single most unavoidable safety issue with electrical welding.

Every welder at some time will fail in this awareness, hopefully it will be nothing more that a tingle on your sweaty bum while changing a rod...... but make no mistake a bad failure in this awareness can be costy and deadly.

A great deal of damage from a number of causes can be done by carelessly welding on electricaly connected items.

Safest to
Completly disconnect any electrical item prior to welding.

Place your earth clamp as close as possible to the weld

Never..ever let a loaded rod holder rest on anything metal or conductive.

Resting it in you mask in the old and reliable way... but these days with expensive, electronic helmets ya probaly don't want to do that.

A rubber mat is a good thing, a square of dry plywood isn't bad either.



BTW... we have not been allowed to use plumbing pipes for electrical earths for decades, though they will mostly still be earthed items.

cheers

I have to accept the wisdom of my betters (and maybe elders), but having been welding fairly regularly on and off for more than 40 years this has never happened (or maybe I have left a trail of melted wiring insulation that I have been oblivious to?). It seems that this can't be a big or common risk or we would see and hear of many many more negative outcomes?

Yeh the cenario first presented is real enough......if you get a good solid welding earth to a well bonded item then strike arc onto an earthed appliance or another bonded item that is not connected to the welding earth you can very reasonably fry the earth cables.
What will probaly fry first is the earth wire in the welders' mains cord.

so the welder is set to 80 amps ish......the working earth cable will be fine, the the earth bond from the switch board to the stake will be at least 4mm2 ( 40 amps continuous should hold 80 amps well enough for a while), the earth wire to any 10 or 15 amp outlet will be at least 2.5mm2..( 20 amps continuous ( not rated) and should hold 40 amps well enough for a whole and 80 amps for a short time).
the mains cord to a small welder will be 15 amps flex 1.5mm2...that will fuse first.

Welders with blown earth wires are far from uncommon.

blokes putting a live electrode down on the top of the welder is a common cause... I was warned about this one when I did a welding course a long time ago.

If you strike arc to an item connected by a 10 amp cord......that will go first.

BUT.. if you are a well trained welder, that does not do sloppy things, this should never be a problem.

another practice that can cause the problem, is storing the earth clamp by clipping it to the case or handle of the welder, then placing a live rod holder on top of the welder with the power still turned on.

As I say... if you have basic good welding practice, the issue should never occur......the biggest is never put a welding handpiece down on a conductive surface.

cheers

Originally Posted by Bloss

It seems that this can't be a big or common risk or we would see and hear of many many more negative outcomes?

It will be highly dependent upon local factors too. From memory, the earth impedance can be up to 0.5 ohm to the outlet, and a further 0.5 ohm in the appliance (welder) itself. If the cable run is short, the impedance might be considerably lower.

We are also talking relatively low voltages here too. So, if the earth loop impedance was 1 ohm and welder delivered 40 volts, the current will only be about 40A - it'll warm the cable but it'll take a while. This isn't 'blow your cables apart' current. If the cable run is short and the impedance is low - and the rod is left lying on the welder for an extended period, the cable can quite easily melt the insulation (but I don't think it'll fuse the copper - but I could be wrong).

I was mainly posting the information (with diagrams) to help others gain a better understanding of how these loops can be formed.

Also, another reason was to point out that rather worrying about someone plugging a 15A welder into a 10A outlet, there are real problems that they ought to worry about.

Those earth impedances are maximums permisable......to get .5 ohms in 15 amp cable you are talking about 50 meters or more.
I have a 50 meter 1.5mm2 extention cord and it meets spec.

The figure is about the same from the furthest point of an installation all the way to the earth stake

In practical terms a total earth impedance from the case of the welder back to the earth common at the board could easily be as low as .1 of an ohm that changes things a bit.

the terminal voltage of common or garden transformer welders could be as high as 70 volts, but most likley 50 volts

the fuse rating of 1.5mm2 bare copper is arround 80 amps at 25c, it should burn at arround 140 amps...plastic insulation will melt long before that.
derate for heat and for being insuated.

so the issue is definitely plausable.

So CIG Trasarc easy welder ( common domestic welder) cranked up flat chat 140 amps on a nice solid outlet close to the switchboard.
plausable to fuse the earth wire in the flexible cord in pretty short order.
Especially if the machine has a bit of age and the wire going into the earth terminal is a bit dodgy.

remember too there is resistance at the plug terminal.... that means a possible point of failure.......I have a collection of melted plugtops that have been removed from items that draw less that the plug top is rated for.

put 80 or so amps the the earth pin of a plugtop or power point, there is a good chance of failure.

the issue remains plausable, and probable.

cheers