Sensitive equipment welding.

I have a question regarding safe chassis welding on sensitive production equipment. Some of our new equipment (joining the 80's here) actually has plc's, touch screens, and prox sensors, etc. What if any additonal saftey precautions should I take when having this equipment welded? Removing source AC and saftey grnd has been suggested to prevent possible looping. Any thoughts on this would be appreciated. Schdled project would be involving 300+ amps DC. Thanks very much...

 

[busbar]

That sounds like a very risky proposition. With the few details given, there's no way to assure an undamaged system. It would be a good idea to keep the arc-current return path as small as possible, but I think this is potentially a real can of worms. The weldor has only limited control of the current path, and no control-equipment manufacturer is going to stand behind components subjected to such large electromagnetic and/or electrostatic stresses. I will be interested to hear other opinions.

 

[jlhmaint]

we never weld on a machine unless there is no other option.
take the part off, or the panel off, at all cost try not to weld on the machine. same concept with an over head crane never weld something that is hooked to a sling from an overhead crane.

 

[busbar]

Good point, jlh. Doing that on some conveyors will give you a barrelful of hopelessly wasted bearings. Most don’t ever do it twice.

 

[electricpete]

I have heard (second hand) of bearings being damaged from welding also. What I remember hearing is that most of the damage occurs when the arc is struck. Experienced welder may know tricks to limit the damage potential.

 

[dydt]

hi...
i do work on sensitive production equipment too...a long production line with 4 kinds of touch screen, 3 types of PLC's, a check weigher, induction sealer and communication cables. We do all of our welding OFF the production line...i am not sure if this is possible in your case...mounting of any kind are done by nuts and bolts using a properly designed plates.....

dydt

 

No the part cant be removed, its darn near the whole machine you could say. Its a drawing head mod on a tube/pipe formming machine. Perhaps despite the amount of time it will take we should remove all the elec equip in descending order of $$$.

 

[busbar]

Maybe the "drawing head" manufacturer could be consulted on the matter.

 

[micjk]

D2fxr,
I have been involved in several welding projects involving delicate insturmentation. The best way that we have found is to ground directly to what ever is to be welded if possible using a bolted ground vs a ground clamp. As busbar stated in the begining, this shortens the currents travel path. If it is not possible to ground directly to the item being welded, other mounting methods should be considered. We just had a similar scenario here where an unsupervised contractor "took out" $ 50,000 worth of load cells on one of our vessels.
Good luck,
Micjk

 

[busbar]

Boy Howdy, micjk. The hard-found cashflow perspective can vividly accentuate a 'lessons learned' episode.

 

[electricpete]

There is some discussion in IEEE1095 (vertical motors/generators) paragraph 7.5 which confirms what has been said above. Better to avoid welding if at all possible. But if necessary:

1 - ensure that the part being welded is directly connected to the ground grid
2 - connect the ground calbe from the welder directly to the part being welded and to no other part of the machine.

 

[dydt]

in regular welding method we use a clamp in connecting to ground or chassis...doing welding myself i do notice than if the ground is not properly connected you can see some sparks on the clamp grip...and for a sensitive electronic equipment this spark is "dangerous"...in this situation the current return will try to look for another path and chances are to your sensitive equipment ground...those bunch of green wires on the electrical panels....

...one thing you can do is make sure that the ground is bolted not clamped....free from paint, grease or any non-conduting material...make sure that the welding rod is as close as possible to the ground termination point.....

good luck
dydt

 

[panelman]

worth removing the panel earth?

mike

 

[electricpete]

mike-
In the IEEE document, notice that it was the welding machine ground lead that was not supposed to be connected anywhere other than the part being welded. Normal equipment grounds to ground grid are ok to be connected at other locations and I'm pretty sure they should stay in place.

 

Well we got the job done today without any problems. We first removed 2 touch screens and one easy to remove plc. However we did leave most sensors and control boards in and sorta "winged it" Took your advice and drilled and tapped a 5/8 bolt mount right on main head to be welded. Things are up and running correctly so I guess we got lucky. Thanks for all the advice guys...