weld currents in a grounded 24 VDC system

[EEAutomEng]

We use 24 volt switching power supplies (115 vac to +24 volts DC) to feed servo drive control circuits and have the (-) terminal of the power supply connected to earth ground (requirement of the drive manufacturer). In my past experiences, I have seen ground wires burned because maintenence people do not put the ground clamp near the work when performing welding. Is there a device I can use to protect the 24 volt system from damage due to weld currents?

 

[busbar]

The same carelessness will destroy bunches of ball bearings, or cause liquidtight flex and hose with internal metal braid/helical wire to deteriorate or catch fire.

Run the idiot off that damages stuff, even if he isn’t a forklift operator.

 

[EEAutomEng]

Busbar,
Unfortunately we can not run the idiots off who do this because they are our customers. Believe me, idiots are a dime a dozen in stamping plants. They are not just spread out one to each village. Any advice on how I could protect the system?

-John

 

[MikeHalloran]

Insist on installation of an actual ground rod for your equipment. Provide a clamp for attachment to the rod. Preterminate cables from your chassis ground and your drive ground to the clamp. Do not connect your drive ground to the chassis anywhere else; only at the ground rod clamp. Fully insulate both ground wires.

DO NOT refer to them as ground wires; call them 'Special High Voltage Drains', or something else that implies exotic lethality.

If your system comes preterminated, then millwrights can install it by themselves, with no help from the electricians, and they might even folllow your directions. Get the plant's electricians involved, and your wiring will be done according to plant custom, e.g., wrong.



Mike Halloran

http://www.deangelomarine.com

 

[busbar]

There’s probably no easy solution given detrimental overcurrent may be orders-of-magnitude greater than electrical components were intended for—rougly equivalent to lightning damage.

Include “fair warning” in service bulletins and maintenance manuals, excluding coverage for repairs for such damage in maintenance contracts or flat-rate, per-unit repair or warrantee agreements. Prepare for damage occurring and set repair fees to fund readiness to restore equipment operation.

It is a fairly universal practice, but in your case is ‘zero-impedance’ bonding of the negative rail of your 24V system(s) needed for operation? Added resistance in the ground-bonding jumper sized to discharge inherent (parasitic) capacitance may inhibit misoperation from transient-voltage incidents, while limiting I²t damage from arc welding. In AC electrical-distribution systems, it is termed high-resistance grounding and has well-understood application in AC systems.

 

[EEAutomEng]

Busbar,
Based on experience, I know that these servo drives won't operate properly without bonding the 'negative rail' to ground. To minimize damage, I could isolate the 24 volt supply to the drive electronics from other control circuits that do not require zero-impedance bonding of the negative rail by adding additional power supplies. I agree with Mike about insisting on an actual ground rod for the equipment.

 

[rbulsara]

Separate ground rod for individual 115V equipment is a blatant Code violation and against the recommendations of IEEE/ANSI 1100 and good engineering practice.

If your chassis one good 'equipotential' block, I am not sure how welding currents affect the 24V DC system?

What is needed is effectively bonding your chassis to a good ground bar which is turn effectively conncected to the system grounding electrode so that welding currents go to the grounding electrode even if they venture near your 'ground'.

If these are long circuits you need to effectively "bond" the equipment at both end of the circuits.

Also is it required by Code to ground a 24VDC circuit or just a requirement of the mfr?

 

[EEAutomEng]

rbulsara,
It is a requirement of mgfr. The control circuits in the drives do not function properly unless this is done.

 

[MikeHalloran]

Oops on me.

Rbulsara is correct. Dedicated ground rods would be a Code violation, because chassis grounds must be tied to the system ground, which must be earthed at one and only one point, etc. ... unless the 24VDC circuits are completely isolated.

But the reported behavior of the drives implies that their control circuits are _not_ completely isolated.

All that I'm left with is using welding cable for grounds...

;-)

-Mike-



Mike Halloran

http://www.deangelomarine.com

 

[peebee]

A clarification:

Dedicated ground rods are not a code violation -- isolating them from the rest of the ground system is where you get into code problems.

Maybe you could provide some conveniently located ground busses for grounding of the welder? Are there some common typical locations where the welding occurs? Can you provide insulating covers over the equipment chasses locations they are currently clamping to to make it harder for them to use your equipment as a welder ground?

In general, I'd recommend making it as easy as possible for them to do things the right way, and as tedious as possible to do things the wrong way. Beyond that, not much you could do besides setting up and enforcing some Standard Procedures.

Well -- you could replace all your small-gauge equipment grounds with big beefy conductors, sized to handle the welder currents. But that sounds like it'd be pretty expensive, and detrimental to equipment operation. I'd look into the welder ground busses first. . . . .

 

[advidana]

just charge your clients more money for repairing the damage ground wires and your problems will go away.

 

[EEAutomEng]

Charging more money will not prevent the damage. When the equipment gets damaged, they call us in to fix it and then, without knowing that they caused damage by welding (they'll never admit it), they ask us why and then the threats start to fly (back charge for lost production, ect.).

 

[snoogie]

I have encountered this with customers in the past who managed to destroy a number of PLC processors before their big boss came down and strictly forbid them to weld on the machine whilst under power (though I guess component damage is still possible with the power off but it seemed to do the trick in this case).We were lucky as we were there when they did the damage so they could not give us the ususal bull story of "it just stopped working".
My method of prevention is make them do equipotental bonding and make sure all the site connections include a ground cable as part of the installation instructions its not a sure fire solution but i have only ever had the problem once so far.