Protecting control and instrumentation from welding damage

Hi,
Just wanted some thoughts on protecting control and instrumentation from welding damage.
We had a PLC and drive fail after some welding work was done by an external contractor and want to prevent a repeat.

Does anyone have a policy welders must follow to prevent this sort of thing or any ideas of easy specific things to do.

I was thinking
- welding equipment to be inspected before work, particularly cable and connections
- welder ground connection must be sound and within 30cm of the welding area
- if possible isolate the equipment being worked on (would this accomplish anything if earth is still connected and why?)

Any input appreciated

Was the PLC and drive that failed between the +ve and -ve of the welder? (Between the torch and the ground clamp?)


What was the distance and orientation of the failed devices relative to the welder connections?

Both torch and clamp would have been 2m upstream of control panel, ground could possibly been forgotten to attach at some point which would then cause earth path past the control panel (but would this cause the problem I don't know)

mass89 said:

Both torch and clamp would have been 2m upstream of control panel, ground could possibly been forgotten to attach at some point which would then cause earth path past the control panel (but would this cause the problem I don't know)

Click to expand...

That is likely your problem right there. You probably had a few hundred amps of stray weld current running from the ground of your machine back to the ground of your welder, thus completing the circuit. Depending on what kind of welding you were doing, it would introduce various transient voltages to anything connected to the ground of the machine.



In this case, I would hire more competent people to do your welding


Failing that, you can always electrically isolate any critical components such as PLCs or drives by removing the ground connections temporarily and possibly the power leads as well.

No way of proving it was them though, or if the clamp was not connected. I understand it could take only a moment of either no ground or a bit of a bad ground and sensitive stuff will be toast. I was wondering if there are particular measured you can take to atleast reduce the risk.

Ground clamps are usually dirty and pitted from arcing caused by clamping over rust and dirt. Most times the welder isn't aware that he has two arcs going. That intermittent ground current has to go somewhere and often find it's way thru expensive stuff.

Next time make him file the ground clamp clean or unscrew it and bolt the ground lead to something very close to the weld.
Don't ask how I learned this!

General Answer (not specific to your point)

I'll resist the temptation to preach my whole sermon on isolation. Use devices with isolated serial ports. Fiber optics between buildings, between areas of a building that have separate power sources, and wherever you can get the customer to pay for them. Use lots of separate DC power supplies. See if your PLC has single-point isolated analog input and output modules. Isolation beats suppression every time.

if you're issue truly was caused by a ground not being used, and this is a typical welding station where the ground is placed on a table and not on the workpiece itself. Then maybe the addition of an enable circuit attached to a pressure switch and designate welding ground location. So that when the ground is attached to a point, it allows the welder to be used.

In a former life, I worked in the cement industry. Using a gasoline driven welder (mounted on a trailer) maintenance would run cable for the stinger up a 250 foot tower but clamp to the structural steel for the ground. They did not care how the return current got back to the welder. A large portion of there current probably followed electrical conduit because conduit is designed to provide a "good" electrical connection.

Here are some of my ideas:
1. Provided two return paths like home electrical wiring (ground and neutral). Thus, an alternate path is provided in case the first path is weak or fails.
2. Consider using local grounds to bridge between work items. This will ensure that a "good" return path is provided for all parts that are being welded.
3. Prior to the start of welding us a resistive load to limit current and test the quality of the ground connections on all parts being welded.
4. Consider have electrical personnel present at the start of their welding (each day) to review their connections, equipment and practices.

gas said:

Ground clamps are usually dirty and pitted from arcing caused by clamping over rust and dirt. Most times the welder isn't aware that he has two arcs going. That intermittent ground current has to go somewhere and often find it's way thru expensive stuff.

Next time make him file the ground clamp clean or unscrew it and bolt the ground lead to something very close to the weld.
Don't ask how I learned this!

Click to expand...

Was magic smoke involved?

The first and I feel the most important question is.
Is the welder power supply a generator supply or is it a transformed supply. This may seem like a minor issue but it could be very important. I remember reading a publication over 30 years ago where under water welders would only use generator power welders because stray currents that may be present in transformer supply’s have killed welders, I don’t know if that still true but it’s worth thinking about.
While all current used for the welding process should be contained in the secondary of the transformer sometimes they leak through to the primary or the earth ground or in case of offshore ship board systems it would be the hull of the ship. In that case the potential would be the line voltage.

I have seen referenced here the dirty ground / return connection causing the problem that really not going to be an issue here a lot of maintence welders that use a strong magnet to connect the return lead to the work ( No Clamp) just drop it on the work and go. These are still sold in welder supply shops all the time.
I have worked with many welders for many years servicing them, using them connecting them to automated machines. I actually had one project where we connect 2 800 amp welders in parallel to a common weld puddle on a sub arc application travel using vfd’s all controlled with a PLC without any problems.
All the problems you have described are directly related to the welder getting lazy and not connecting the welder machine return lead close to his work. Usually its because it involves a long lead run. Electricity is a funny thing the electrons will to find a path back to the source, this is true of all electrical systems everywhere. If the resistance of the return path through the plc panel wires is less than the path you want it to take then the high welding current will burn up the plc
In short always connect the return lead as close to the weld as possible and as the weld moves move the return connection.

It is always the location of the ground clamp that causes the issues. I have been working in the Sanitary automation field for 30+ years and always get the TIG welders to put the ground clamp right next to where they are welding.

Nova5,
Yes, there was the expected magic smoke and also smoke pouring out of the bosses ears with profanity.