Discuss Lets Talk about Large Earth Currents in the Commercial Electrical Talk area at ElectriciansForums.net

James the Spark1976

electricity is it real or just magic?
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I have a customer who called me in to look at a small electrical fire.

They are a welding and fabrication company and have several welders operating on site.
The circuit pointed out to me that had been on fire was a 3 phase 16A supply for a floor mounted crane.
the crane is able to rotate on a hinge and pass along a beam on rollers, this is fed by a flat catenary cable (4c 2.5mm)
it is evident that a huge current has passed down the earth cable for this crane, as they reported thick smoke coming off it and when I examined it, the cable has melted along its entire length through both the inner and outer sheath.
the circuit breaker for that circuit has not tripped for quite some time in my estimation after looking at the damage to the cables.
I have tested the crane (after replacing the damaged supply cable with a temporary one) the crane has no faults and is in good condition.

None of the phase conductors have got hot and melted so I am assuming that the earth current has come from somewhere other than the crane supply itself.
The obvious culprit is that someone has welded something that is hanging off the crane hook and not correctly attached the earth clip of the welder.

The crane mast and jib (bit that goes from floor to sky and the bit that goes out to the side) is NOT BONDED.
My understanding of welding is that if you don't connect the earth clip to the device being welded then no current will pass, I cant see how this can pass down the earth, but it has.
the obvious solution is to bond the crane to earth, however this is not so simple. I can easily bond the mast, I can put a flex from the mast to the jib but the catenary cable will not support a big enough earth cable to deal with 400 to 600A welding current.
I am considering the following solution as the regs book is not going to provide me with an answer.
10 sq mm earth to the mast direct from main earth terminal
10 sq mm earth (tri rated) from mast to jib
6 sq mm earth (tri rated) along catenary ( 6m ) to chain hoist.

This will provide a big enough return path for any large current that is accidently passed from a welder but only for a relatively short duration.
(10sq mm tri rated is about 90A continuous if memory serves me correctly.)
I would then like to monitor the current on that earth cable and either sound an alarm or cut power to the welders if it were to go above say 10A.
what are your thoughts?
p.s. anyone know a nice off the shelf current monitor that provides a fault output if current sensed goes out of limits?
 

darkwood

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One of the welders had not got his return clamp on properly, check all local circuits to that board as it could have damaged multiple circuits, the welding sets need checking for loose returns and if none identified then the welders need to be taught how to clamp their return down securely...this scenario is very dangerous and has killed many welders over the years.
 

westward10

In echoed steps I walk across an empty dream.
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30 odd years ago was forever being called to a fabrication shop due to burnt out wiring from the welding. Metalwork to be welded was laid onto the cable trunking for support and hey ho burnt out wiring.
 

Ian1981

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I won’t pretend to fully understand welding machines but are we talking stray currents here, returning through the cpc or through to earth via a path of least resistance?
 

Lucien Nunes

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The welder return lead should normally be isolated from earth, so that if the return clamp is not properly attached, welding current will not divert along other earths and CPCs. The workpiece or table can be separately earthed. If welding current has passed along the crane supply cable in the scenario you describe, then most likely the return clamp was attached to something earthed such as the previous job that the welder was doing or some structural steel, or is faulted to earth. Hence, the crane CPC completed the circuit to the job on the crane. This is pure operator error and there is no simple method to completely protect against it, but as you have pointed out it might be possible to protect the particularly vulnerable crane circuit.

Passing the CPC through a current transformer would enable a sensing relay to sound a warning at an adjustable threshold. You would need to establish what effect the impedance of the CT would have on the Zs, and whether precautions would need to be taken against the CT secondary going open-circuit. A Hall-effect sensor might be a better choice.

Perhaps welder manufacturers already addressed this problem and have a product available? Worth asking the experts.
 

darkwood

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I posted a pic up many years ago of a flex that had taken 400amps because the manipulator used to rotate the welding job hadn't been clamped and the return lead was clamped to the buildings structural girder just to keep it out of the way while the heavy job was craned in and sat in place... that simple error of forgetting to pop the return back in place gave the 400amps an alternative path to the girder and then return via the manipulators supply flex.

I have seen £1000's of pounds worth of damage caused when similar situations occurred and the unlucky wire was a 10mm bonding cable in a trunking system with multiple circuits, well you can imagine what happens to all the circuits when the bonding wire glows red hot and melts through all the insulation of 10 adjacent circuits... all because the welder hadn't checked his clamp was fitted for the weld.

The injuries and deaths caused by this is when usually these things happen and then are not discovered, imagine loosing your earth on circuits supplying equipment your using, as well as the possibility the insulation of the phase conductor(s) may have been damaged as well from the heat.
 

marconi

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London United Kingdom
I would then like to monitor the current on that earth cable and either sound an alarm or cut power to the welders if it were to go above say 10A.

https://www.nktechnologies.com/current-sensing-switches/


https://www.nktechnologies.com/current-sensing-switches/as1-compact-case-current-sensing-switch/

I have used these Hall effect current switches before, and the AS1 in my last application. You could thread the CPC through the AS1 and use the NC output to hold on a relay, etcetera......

Or you could use the clamp types and some of these have adjustable threshold settings.

Whichever you might use check you buy the one with the desired output contacts ie: NC or NO.
 

PEG

Respected Member
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4,870
Location
Manchester
The welding machine's transformer would isolate the return current from earth,as long as no fault was present on the machine,itself.

When you say "crane" do you mean hoist? Has the supply got "loose loop" suspension on the catenary,mentioned?

There should be some provision,of preventing any current,across the hoist/trolley track,there will be at least four bearings,and one thing precision races do not like,is arcing within the race channel.

If a fault exists on the welding machine,bonding the crane can increase the chances of damage,from the above.

Any drive-train,can be wrecked,if any appreciable current finds a path across bearings or gears....like a continuous spark-erosion process....only MUCH more expensive;)
 

SheffSimon

Regular EF Member
Messages
93
Location
Sheffield
I have a customer who called me in to look at a small electrical fire.

They are a welding and fabrication company and have several welders operating on site.
The circuit pointed out to me that had been on fire was a 3 phase 16A supply for a floor mounted crane.
the crane is able to rotate on a hinge and pass along a beam on rollers, this is fed by a flat catenary cable (4c 2.5mm)
it is evident that a huge current has passed down the earth cable for this crane, as they reported thick smoke coming off it and when I examined it, the cable has melted along its entire length through both the inner and outer sheath.
the circuit breaker for that circuit has not tripped for quite some time in my estimation after looking at the damage to the cables.
I have tested the crane (after replacing the damaged supply cable with a temporary one) the crane has no faults and is in good condition.

None of the phase conductors have got hot and melted so I am assuming that the earth current has come from somewhere other than the crane supply itself.
The obvious culprit is that someone has welded something that is hanging off the crane hook and not correctly attached the earth clip of the welder.

The crane mast and jib (bit that goes from floor to sky and the bit that goes out to the side) is NOT BONDED.
My understanding of welding is that if you don't connect the earth clip to the device being welded then no current will pass, I cant see how this can pass down the earth, but it has.
the obvious solution is to bond the crane to earth, however this is not so simple. I can easily bond the mast, I can put a flex from the mast to the jib but the catenary cable will not support a big enough earth cable to deal with 400 to 600A welding current.
I am considering the following solution as the regs book is not going to provide me with an answer.
10 sq mm earth to the mast direct from main earth terminal
10 sq mm earth (tri rated) from mast to jib
6 sq mm earth (tri rated) along catenary ( 6m ) to chain hoist.

This will provide a big enough return path for any large current that is accidently passed from a welder but only for a relatively short duration.
(10sq mm tri rated is about 90A continuous if memory serves me correctly.)
I would then like to monitor the current on that earth cable and either sound an alarm or cut power to the welders if it were to go above say 10A.
what are your thoughts?
p.s. anyone know a nice off the shelf current monitor that provides a fault output if current sensed goes out of limits?
Trirated, by which presumably you mean trirated panel, is not suitable for installing in a festoon. Use some drag chain cable, you can get g/y singles, taped or tywrapped to the other flatforms.
 

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