Singles through metal adaptable box.

[Jono Pashley]

When exiting single cables through metal i understand we have to slot between the holes. My Question being how big does the slot need to be as i took a cover off a adaptable box today and the was only a small slot(maybe 6mm wide) between the 2 20mm KO with Line through one and Neutral through the other.

 

[Riggy]

What's the purpose of the slot?

 

[Pyro]

When taking single-core cables out of a steel box, the important thing is for a slot to be present in the steel box between the holes used by the single core cables. The purpose of the slot is to break the steel link between the cable access holes in the box. The the width of the slot is not a factor. Jono's 6mm slot would allow the slot to be cut and then de-burred with relative ease.

Where you have single-core cables you will have a magnetic field around each cable. Where there is no slot between cores there will be a circulating current in the steel box (or gland plate). This eddy current is most pronounced in ferrous materials because of their magnetic properties. At low cable currents it's not much of a problem, but with high cable currents it will cause heating.

The use of a non-ferrous material as a gland-plate is another way of solving the eddy current problem.

 

[stator-electrical]

The other option of course is to run the line, cpc and corresponding neutral through the same hole or bush.

This prevents the build up eddy currents by the rotating magnetic field of one core cancelling out that of the other as they are (or should be) exactly opposite.

 

[spinlondon]

Hacksaw blade width.

 

[Jono Pashley]

these cables are feeding the main CU in the house 25mm cable through each hole, was just worried this small slot wouldnt be sufficient.

 

[Riggy]

When taking single-core cables out of a steel box, the important thing is for a slot to be present in the steel box between the holes used by the single core cables. The purpose of the slot is to break the steel link between the cable access holes in the box. The the width of the slot is not a factor. Jono's 6mm slot would allow the slot to be cut and then de-burred with relative ease.Where you have single-core cables you will have a magnetic field around each cable. Where there is no slot between cores there will be a circulating current in the steel box (or gland plate). This eddy current is most pronounced in ferrous materials because of their magnetic properties. At low cable currents it's not much of a problem, but with high cable currents it will cause heating.

The use of a non-ferrous material as a gland-plate is another way of solving the eddy current problem.

Thanks for that, never heard of it before (or possible forgot it many years ago!)

I'm thinking we're talking hundreds of amps before it's an issue?

Seen loads of Busbars passing through steel enclosures over the years, don't remember ever seeing a slot - that doesn't mean it wasn't there of course.

 

[Andy™]

Thanks for that, never heard of it before (or possible forgot it many years ago!)I'm thinking we're talking hundreds of amps before it's an issue?

Seen loads of Busbars passing through steel enclosures over the years, don't remember ever seeing a slot - that doesn't mean it wasn't there of course.

even a few amps could cause a problem

 

[batty]

Could deffinatly be a problem on tails in consumer unit.

Batty

 

[Jono Pashley]

would it be possible to get a belt due to this?

 

[spinlondon]

There's always a risk of electrocution when dealing with electrical equipment.

However, I would say the primary risk is that of fire, caused by the over heating of the metal surrounding the cables.

 

[Andy™]

what spin says. its mostly a heat issue

 

[kme]

Hacksaw blade width.

ROTFWL