Welders Yard, major problems!

[apprentice87]

Hi Slipshod,

British standards USED to say that the secondary windings of the set be earthed to the internals of the set, they then changed their mind for this precise reason.

See here for instance

Job knowledge for welders 28: Health, safety and accident prevention: electrical hazards - power source and installation

Note in particular the paragraph;

"In very old designs, the welding circuit was sometimes connected internally to the power source enclosure ( Circuit c). However, the danger is that even with the welding return lead disconnected, and a separate earth connection, welding is possible with current flowing through the earth. Because of the risk of damaging protective earth and other connectors, this type of power source is considered to be obsolete and should not be used"

Trust me, the welding institute know what they are talking about.

The sort of thing you describe was a very common [and well known] problem until the british standards people realised what was happening and changed their minds......

Most of us have done a bit of welding; How often have you tried to strike up, but nothing...a few moments of ehhhh, and then Doh, the welding return has come off.

Think if everytime this happened, the CPCs' in your entire installation got damaged.....

ANY welding set in which the secondary windings are not isolated from the CPC supplying the thing, [thus creating the problem] either;

1, Is obsolete

2, Is faulty

3, Does not comply with british standards.

Industrial welding sets are SERIOUS bits of kit. I have a welder not 20 feet from me as i write this; It is not a particularly big one, but it is rated to churn out 260 Amps, 100% CONTINUOUSLY or, 320 Amps 60% of the time, or 350 Amps 50% of the time..

In welding "duty cycle" terms, this is 5 minutes of welding with a 5 minute break.

This thing requires minimum 50mm csa welding cable... So, unless all the CPC's in your installation are 50mm csa [make your "black and decker" a bit difficult to handle!!] guess what is going to happen the first time you forget to connect the welding return cable...

The sad fact is, the person that wrote the article you quoted from may well be the best electrician in the whole world. The fact is, they know NOTHING of welding machinery.

How can i say this??? It is a question of the very "telling" sentences;

"Inspection of several welder earth clamps found overheated cable ends where the connection to the clamp was made"

and

"In the average welding shop, the earth clamps tend to be tatty, bad or loose connections from the earth lead to the clamp"

No such thing as a "welder earth clamp"... It is a "welding return cable" This is not me being "pedantic", a "welding earth" is a VERY different thing.

Think of this; would you employ an electrician that either did not know the difference between "earthing" and "bonding" or used the two terms "interchangeably" or thought that anyone that did not call "bonding" "earthing" was being pedantic....

john...

 

[SLIPSHOD & SLAPDASH]

Hi John, i dont claim to know the circuitry of welders. My friends workshop, although at home is a professional set up as he welds FIA spec chassis for race & drag cars as well as building fire escapes, security doors etc. I made a mistake earlier in that he's brand new UK bought welder that caused the 'problem' was a Tig not Mig and i think its about the same output as the one you mention. All i was interested in was the 32A single phase supply i had to wire for it. So you are saying this fault should not have been possible on a new kit ?

Richard

Edit, He also explained Tig welding uses DC current and a thought that may have saturated the RCD so it did not trip , but my earlier view of the return welders current path i think still stands.

 

[Sidewinder]

Slips, john,

Please remind me of the fault etc. & I'll dig out the standards if I get a chance and see what they have to say.

 

[SLIPSHOD & SLAPDASH]

Slips, john,Please remind me of the fault etc. & I'll dig out the standards if I get a chance and see what they have to say.

John says that the secondary windings of all welders should be isolated from earth, to stop the return welding current travelling down Cpc's and anything else connected to earth.

 

[Sidewinder]

Yep OK,

What was the fault you had Slips?

 

[SLIPSHOD & SLAPDASH]

Photos would be good of everything

] ] ]

---------- Post Auto-Merged at 23:19 ---------- Previous post was made at 23:17 ----------

Yep OK,What was the fault you had Slips?

Last year i wired a home workshop for a friend. He asked me to pop round and explain a problem he had, he was Mig welding a car and not getting good results. He then noticed he had forgotten to clamp the return lead to the chassis. He then noticed a 240 volt metal cased lead light (which was switched off) and was laying on the engine had had its lead completely melted, no breakers or the RCD had tripped.

 

[Sidewinder]

AH,

Ok thanks, I feel that a read of: BS EN 60974-1:2005 may be in order and see where that gets us!

 

[SLIPSHOD & SLAPDASH]

This is what he was constructing at the time , its part of the cage of a race car. Thats the floor with the prop tunnel being worked on. The ladder type chassis is inside the car to give a smooth underside

 

[apprentice87]

Hi all,

As i said previously, years ago they decided that it would be a good idea to earth the welder secondary to the case of the welder. The "job" was "earthed" too, say to the frame of the shed, [at least it was meant to be] for various reasons that are patently obvious.

Needless to say if the welding transformer secondary is earthed to the frame [and hence the CPC of the set] once the return lead falls off, your welder cpc and the rest of factories earth system now becomes part of the welding circuit and all goes up in smoke!!!! Rcd's will not trip, nor will any MCB's why should they?? They are not part of the welding current circuit.

Anyway, they soon changed their minds!! [in about 1960 i think!!] Now these sets are banned....

Here is a link

Job knowledge for welders 28: Health, safety and accident prevention: electrical hazards - power source and installation

As far as i know, as they explain in the link, they recommend NOT earthing the "job" at all now. They seem to think this is not needed if you have a "double insulated" welding set...!!!

Once again, this is a triumph of brains over sense. What happens now when someone crushes an extension lead or cuts it with a sharp edge... I used to work in a place where we used to make very large fabrications all welded with mig. If there were any repairs to be done they would give you a little "inverter" set on the end of a 400V extension lead. You would be glad the job was earthed if that extension got crushed....

Yet they talk about NOT "earthing" the job [as they put it] to prevent "stray currents"

This was just one of the reasons the job WAS supposed to be earthed, to make sure that if for any reason you DID have any "stray currents" that they would not damage motor bearings and the like.

Ok, say for instance you DO have insulated secondaries, say then, that for some reason you DID still have stray currents and the job is not earthed. What happens when part of the job is supported by, say, a overhead crane. The cranes control circuitry, motors, bearings etc will just love this!!!

While i think of it, here is something the HSE geniuses have not thought of!!!!!!

Picture this....You have a great big welding shop.... There are loads of people all working away, and there are piles of electrical stuff all over; There are drilling machines, presses, croppers, welders, bending rolls, grinders, compressors, all sorts of machinery all over the place. Oh yes, the place is well and truly awash with "exposed conductive parts"

Now, in the middle of the concrete floor, [supported on concreted in steel beams] is a twenty ton fabrication.

Suppose now, I wander along and think; Hmmmmm, i wonder... and I connect my nice shiny new MFT between the fabrication and the MET. Is the reading going to be above 50K ohms [remember people working on this might be all sweaty and damp] I doubt it....

Now, soooooo.... right is the middle of the workshop is a 20 ton "extraneous part" to beat all extraneous parts... and worse, all within touching distance of loads of "exposed conductive" parts. But the HSE say that it is not allowed to bond this 20 ton extraneous part to the frame of the shed or other suitable earthing point.......

john....