Myths or facts

[GaryP]

There is a difference between pre-heating a rig and applying a pre-heat.

Pre-heating as you have determined has a limited effect based on how long it was since the rig was pre-heated.

Applying a pre-heat however which is the process of setting the dimmer channels so that they idle at some small percentage of power is very beneficial as David has documented above.

[andy_s]

Anyone else as bored with this myth as I am? can we have a new one please - the post title being pleasingly general, we appear to have become a tad specific.

[Wilf dLampy]

can we have a new one please -

 

"Figure-of-eighting spare socapex/TRS by the dimmer is better than just coiling it. If you just coil it you create a big magnet"

[Guest lightnix]

"Figure-of-eighting spare socapex/TRS by the dimmer is better than just coiling it..."

Sorry Wilf, no myth there, either - it is better to figure-8 your slack cables. If you just coil them, then the collective, alternating electromagnetic fields that surround them can induce eddy currents, which can cause the cables to overheat and (in extreme circumstances) melt :P You can demonstrate this effect on a smaller scale (although I'd strongly advise against it) by taking a d0m35t1c 13A cable drum and running a full 3kW load down it, while leaving the cable coiled on the drum.

 

Inductive effects and the overheating they cause, are also the reason you should never pull more than 2kW down each way of a 1.5mm² multi.

 

Safety aside, from an aesthetic point of view, I never like having large piles of slack cable at the racks and always to run it out, either along the line of the incoming tripe or in another direction. On the rig itself, I'll always run slack along the bar for the sake of neatness; lumps of cable lying on / hanging from a rig look a bit cowboy IMO.

 

Where figure-8ing cables does come in handy, is when you have a cable which need to be quickly run out, mid-show. Coils tend to tangle, fig-8s don't. For this reason, cameramen always fig-8 their cables and some people like to fig-8 their tripes into the cable boxes when touring. Doing it this way takes up more space in the boxes, which means you need more boxes, which take up more space on the truck. On the other hand, it does help keep the weight of the individual boxes down a bit.

[Seano]

If you just coil them, then the collective, alternating electromagnetic fields that surround them can induce eddy currents, which can cause the cables to overheat and (in extreme circumstances) melt

Good one! :P (You are joking, right?)

[Wilf dLampy]

OK (and this does seem a good example of "Myth or Fact" to me :P ) doesn't this mean that if you have a figure of eight rather than a coil you are actually creating two such coils, each of half the length?

[andy_s]

If you just coil them, then the collective, alternating electromagnetic fields that surround them can induce eddy currents, which can cause the cables to overheat and (in extreme circumstances) melt

Good one! :P (You are joking, right?)

 

seen it happen - lost a show because of it! but luckily not the venue, as the system protection worked.

[paulears]

There is a practical reason for a long pre-heat. always a good idea if you need a generic rig to be silent. The first few minutes of getting up to temperature always seem to cause the annoying creaks, and squeaks as housings heat up and expand. Recording orchestral concerts requires the usual no movers, no fans, no motors approach and then a bang from a bit of metal suddenly freeing up is a real pain and spoils the take - preheating largely solves this one.

[Seano]

seen it happen - lost a show because of it! but luckily not the venue, as the system protection worked.

Unless the cables you're talking about were single conductors, and you coiled the lives one way and the neutrals the other, no you haven't. You lost your show because of thermal effects, not electromagnetic ones. It doesn't matter if the cables are coiled, fig-8 or in a big untidy heap - when hot cables are surrounded by other hot cables instead of cool air, they overheat - duh. When you thermally insulate a cable it get hotter than when you dont, and putting it in the middle of a pile is a very effective way of thermally insulating it.

 

Thats why a given size of conductor is rated for less current in the middle of a big fat bundle than it is on its own, and for less in a conduit that surrounded by fresh air. A domestic extension lead will melt if you put a 3kW load on the end while its still coiled on a drum because its only rated for 13A off the drum - on the drum its good for much less.

 

Unless you have a *serious* fault somewhere, the net current flowing through a bit of TRS, socapex or whatever is ZERO.

Every milliamp going out through the live is coming back through the neutral - so whatever shape you coil it in, the current path encloses the space between the neutral and live within the cable itself - approximately no space whatever.

 

When Wilf posted this one I thought he was kidding, because its a no-brainer. Guess I was wrong, I underestimated the power of myth.

 

Sean

x

[dbuckley]

"Figure-of-eighting spare socapex/TRS by the dimmer is better than just coiling it. If you just coil it you create a big magnet"

 

Like this one:

 

http://www.davidbuckley.name/pix/moody_multicores.jpg

 

This is from page 165 of Moody (2nd ed), in which it states:

 

So all the cabling from the lighting trusses to the dimmer was 350 feet of Cranetrol multiline cable ... The cable reels made it easy to move and store such long lengths, ... The reels did not need to be fully unwound. The excess remained on the reel. A short set of feeder cables made the final connection between the reel and the dimmer pack

 

If I've read it right, each multicore carries nine circuits at 4KW, at 110V.

 

So based on what I've seen with extension cables on reels, I would expect the whole thing to catch fire, but theres no mention of conflagrations in the book.....

 

Edited (twice) now for spelling...

[Seano]

I just remembered a cute variation of this myth. When I was doing noise, I did a little University tour.

 

At one, a fresh-faced young student tech told me quite earnestly that I shouldn't coil excess mic cable because I was effectively putting a big inductor in series with the mic and I'd lose all the HF from the sound.

 

Awwww, bless. :P

[Guest lightnix]

If you just coil them, then the collective, alternating electromagnetic fields that surround them can induce eddy currents...

Good one! (You are joking, right?)

No I wasn't - it was what I was taught by people more experienced than me, way back in the dim and distant past.

 

Of course, that doesn't mean they were right... :P

[andy_s]

I'm no physicist, so I'll take Seano's word for my little problem being due to hot cables and not the magnetic inductance thingy. All I know for sure is the cables feeding power to our dimmers were coiled up nice and neatly (not by me, but I'll be honest and say at that time I wouldn't have known any better), they melted, and the fuse went. This was before RCD and ELCB and other sensitive protection, so the problem was quickly solved by use of a 6" nail and a bit of gaffer. (Yes - I am joking....)

[Jamtastic3]

One guy I knew had 2 Pioneer decks, a 4 channel mixer, a lamp and a 1K HEATER! all plugged into a 6 way - this 6 way was then plugged into a 15m 13A extension reel, but as he thought he didn't need 20 metres worth, he left 15metres of it coiled up in reel. What happened.....

 

burning smell,

smoke,

PA went out,

breakers went out,

very hot and melted extension reel!

[JimWebber]

Hmm Interesting...

 

Personally I cannot see a huge difference in figure eighting cables and coiling them...

 

1. The big heat problem discussed is due to the fact that the cables are grouped, thus excess heat cannot be dissipated (A-La IEE wiring regs grouping factors etc.)

 

2. As has already been mentioned, the magnetic field created by the live will be directly opposed by that in the nuetral. Therefore I cannot see any significant magnetic fields being generated

 

3. Admittedly a figure of eight cable is generally larger, therefore has a greater surface area in contact with the cooling air. I cannot see this making a huge difference myself...

 

4. I will admit that a figure eight is a lot easier to un-coil, as it tends not to tangle.

 

Of course, I could be wrong... (When my theatre goes up in smoke :P I'll let you all know...)