accidental electromagnet?

[allfloaton]

can someone explain to me the dangers of accidentally creating an electromagnet?

 

When cabling to an overhead lineset, I am told to never wrap the cable around the batten, but it can be coiled up and then attached? But I've seen this done on electrical battens when there are no jumper cables or tie line being used to cable, but rather just the dimmer cable plugged into the light with the cable wrapped around the batten a few times in order to avoid the cables hanging down.

 

What and when is this a danger? What are the consequences?

[richard]

This isn't seen as much of a danger in creating a magnet (and is very unlikely to I think) - more that it could cause the cable to over heat. This is in reality pretty unlikely, however it's seen as bad practice to twist cable around the bar, it looks messy and can cause the cable to twist if done tightly

[smalljoshua]

Electromagnets use DC current, all the cables in common use for entertainment are carrying either one or multiple conductors of an AC current source. As the direction of current is repeatedly reversing in AC, the average field strength will be 0.

 

Josh

[J Pearce]

Unless you are working with single conductors or have a significant earth leakage issue Kirchoff's first law (ignoring any capacitative effects for now) would dictate that the sum of all currents at any one point is equal to zero, so any magnetic flux generated in one direction is cancelled by that generated in the opposite direction.

 

There is however increased likelihood of overheating or mechanical damage when wrapped around a bar.

[timsabre]

Harder to derig as well.

[themadhippy]

Electromagnets use DC current,

Bit of a sweeping statement there josh,I can think of several electromagnets that use AC,the most common being a motor were theirs often 6 of the buggers attracting and repelling each other

[smalljoshua]

Electromagnets use DC current,

Bit of a sweeping statement there josh,I can think of several electromagnets that use AC,the most common being a motor were theirs often 6 of the buggers attracting and repelling each other

Ok, so maybe I should have said "static" electromagnets. For an electromagnet to have a non-zero average field strength over a given period of time, the signal must be alternating with a period of less than that of the measurement period.

 

Josh

[allfloaton]

So, essentially, you're all in agreement that no, cables shouldn't be twisted, but not because of an electromagnet risk? So it's essentially impossible for that to really create an electromagnet? If someone could elaborate in more layman's terms (I'm working to get my physics knowledge up to par) it would be much appreciated.

 

I thought that sounded fishy when I heard that being told -- I'll certainly tell others in the future to not wrap around bars in order to prevent damage to the cables instead of due to an "electromagnet."

[dbuckley]

A single wire with a current flowing through it creates a magnetic field around that wire. Lets assume a DC world for a moment, and go back to school, where you were taught to wrap your right hand around a wire with a current flowing through it (through the wire, not the hand!), with your thumb in the direction of the current flow (+ve to -ve). See the Right Hand Rule on Wikipedia.

 

http://www.magnet.fsu.edu/education/tutorials/java/handrules/images/right-hand-rule.jpg

 

So one wire with a current flowing through it creates a magnetic field.

 

A second wire right next to that first one but carrying the same current in the opposite direction will also create a magnetic field, but in the opposite direction. Because the two wires are really close to each other, their fields cancel out. So the magnetic fiueld observed a distance away from the pair of wires is zero.

 

Now lets go AC as we do in the real world of lights. The direction and magnitude of the current flowing is changing all the time, but the important thing is that any instant the same amount of current is flowing but in opposite directions. So the fields still cancel.

 

This property is important in reverse in balanced audio signals, but as we're in the lighting forum, we'll not kick that sleeping dog.

[ace]

It is also a nightmare for whoever has to strike the cable at the end of the gig. it is a lot quicker to untape the coil of spare cable than it is to unravel it! As for electromagnets - I suspect someone has been pulling someones leg here....

[vinntec]

I always thought this was bad practice because it could induce mains hum onto, for example, a mic cable running along the same bar? By the way, has anyone else noticed how active the Sound forum has become recently? Peter

[paulears]

50Hz electro-magnets are not exactly rare beasts - including degaussing coils on tube TVs, and of course bulk tape demagnetisers. It depends on what you want them to do.

[Jivemaster]

AC and DC magnets exist.

 

However with a typical mains cable having current flowing up one core and down the other core there is NO net magnetic field at any time so no electromagnet can be inadvertently created.

 

However (again!) cable has resistance so current creates heat and tightly coiled cable may get hot enough to be damaged -Yes I have seen PVC cable welded into one lump due to long term overheating. Also wrapping things round the bar makes changes to the rig hard and derigging even harder.

[mark_s]

As has already been alluded to, the most significant danger is quite possibly the crew on the get-out lynching you. It's in the same camp as over-judicious use of tape on looms on truss, over-judicious use of tape in general (you really, really don't need six wraps to hold a cable in place!) and wingnut spanners.

 

:)

[OllieDuff]

There is a theoretical heat issue - if you look at a cassette reel-type mains lead, it'll have two power ratings - one for fully wound, one for fully unwound, because if you actually draw 13A down a tightly coiled cable, heat will build up and melt things.

 

However, in the situation which you're talking about (wrapping around a 48mm barrel, with a bit of space between coils) then the main issue is that you have to unwind it at the get-out, which is very dull compared to just cutting or pulling the tape off and the cable coming away easily.