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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?
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?