Film-Tech Forum ARCHIVE: Why break both hot and ground on motor feed?

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Posted by Frank Angel (Member # 248) on 04-26-2007, 12:10 AM:

Working in an old booth, I discovered someone had removed the switch that traditionally sits on the side of the motor on an RCA soundhead and moved it to a 2 gang wall box in front of the machine. The wall box had a switch on one gang and a jeweled red lens on the other. Presumably the red indicator light was needed to let the projectionist know that the machine was running....just in case he couldn't figure that out by sound of the motor and maybe like the film running through the mechanism.

Anyway, I opened the box to find the switch was a double pole; it was switching both the hot and the neutral lines to the motor. So my question....would there be any good reason why this is so configured? It's a standard Robbins 110vac motor, so why would there be a need to break both lines? In fact, wouldn't lifting the neutral off the motor be someone dangerous, and wouldn't switching it on and off be more of a path for click noise to enter the system?

Posted by Monte L Fullmer (Member # 2797) on 04-26-2007, 01:20 AM:

I call it a safety for sure since, even though neutral won't bite you if the path of current is cut from the hot, but in case the path wasn't completely cut, or any potental of any complete circuit prevails - with a short of something, and you touch neutral, you're gonna get bit, or the device will not shut down completely.

Posted by Sam D. Chavez (Member # 1841) on 04-26-2007, 09:26 AM:

I have been told it was to diminish the inductive kick that produces a spark across the switch contacts when you shut the motor off.

Whether this is realy valid I don't know.

Posted by Jack Ondracek (Member # 1466) on 04-26-2007, 09:45 AM:

Could there have been some 240v motors back then? I never saw one, but that would be a reason to switch both poles.

Posted by Randy Stankey (Member # 64) on 04-26-2007, 10:15 AM:

I'm not an expert here but, I can guess that it is done in case the neutral floats for some reason. If that happens, there can be current coming out of the neutral which could cause all sorts of funky (dangerous) things to happen.

Posted by Richard Fowler (Member # 893) on 04-26-2007, 10:16 AM:

The double pole motor switch was common on RCA and earlier Simplex soundheads. Earth/ground was also provided to conduit / greenfield hardware.

Posted by Bob Koch (Member # 1062) on 04-26-2007, 12:29 PM:

Richard is correct on this. The original RCA MI 1040 had a double pole switch supplied in the motor switch box which was part of the soundhead itself.

Posted by Louis Bornwasser (Member # 3063) on 04-26-2007, 07:16 PM:

Randy is probably right. Many of these old theatres had open faced panels with a fused neutral. I was almost killed by an "open" neutral myself. (I went back later and added copper water pipe in place of the neutral fuse. This forces the "hot" to blow instead of confusing people....like me.) Louis

Posted by Frank Angel (Member # 248) on 04-26-2007, 08:47 PM:

The owner is going to have an electrician rework the booth -- seems the prior operators hadn't paid the projectionist for months and then skipped town; he cut and tore out wiring so the place will have to be rewired. I'll see what the electrical contractor wants to do with the motor switches, maybe there is a code involved. If it can stay double pole, I will ask him to leave it. I sure don't want to be messing with anything that is not 110% execution-safe, although I do find that a mild shock every now and then is good for erectile dysfunction.

Posted by Gordon McLeod (Member # 33) on 04-26-2007, 09:12 PM:

with no offence and in jest frank said "seems the prior operators hadn't paid the projectionist for months"

I hope it wasn't a Iatse memeber [Big Grin]

Posted by Frank Angel (Member # 248) on 04-26-2007, 10:19 PM:

Gordon, IATSE projectionists are much to smart to work for months without being paid! [Big Grin]

Posted by John Hawkinson (Member # 1135) on 04-26-2007, 10:28 PM:

NEC requires you to switch the hot, and that you must not switch the ground. It's optional to switch the neutral, but as is probably obvious, most people don't.

If you're paranoid, its certainly safer. I don't know about RF noise...

--jhawk

Posted by John Walsh (Member # 168) on 04-27-2007, 07:56 AM:

I've seen several poor wiring jobs in old theaters. An old theater near me (1930 or so) had light fixtures wired with only the hot lead; the fixture's neutral was simply connected to the metal box, using the conduit for the netural return.

So, I'm guessing that some RCA engineer, realizing the poor wiring that a booth might have, decided to switch hot and neutral in case someone wired the booth poorly and reversed them.

Posted by Randy Stankey (Member # 64) on 04-27-2007, 11:00 AM:

I was talking to a guy who knows all about these things and he said that, if the neutral is bonded at the service entrance like it should be, there is no reason to use a double-pole switch. But in an old building where the neutral may or may not be bonded, the neutral MIGHT be switched for extra safety. Basically, it's a "CYA" thing.

The only exception he said was that, in Europe, there may be some funky electrical codes that require it. But, since the projector is American made and has never seen the light of day in a European country, there is no reason why it would be subject to such a code.

It doesn't NEED to be but an anal retentive person might do it anyway.

Posted by Tony Ratcliff (Member # 1196) on 05-01-2007, 03:53 PM:

I worked in a drive-in booth one summer where the outlets were all the old style where both holes were the same size (and no ground hole). And some of them were wired hot on left and some hot on right.
I got a good shock one day when I plugged in the wrong way a light with a metal stand.

So maybe this type of issue existed at this theatre at some time and they wanted to make sure the power was off.

Posted by Tim Reed (Member # 137) on 05-01-2007, 09:19 PM:

Tony, there was a bare wire touching the frame somewhere inside that lamp!

Posted by Kenneth Wuepper (Member # 1174) on 05-02-2007, 06:46 AM:

A "switch" removes power from the load side of the device.
A "disconnect" removes the (2 leads in single phase or 3 leads of 3 phase)load from the power network.

Both have the same effect, removing power from the load but the "disconnect" removes all connection to the net. (Earth or Ground, the Green wire must remain connected)

Some electrical codes may require a "disconnect" be placed in the source of power to rotating machinery and that "disconnect" must be located near the machine.

The manual power "disconnect" on older installations was the motor switch. When wired into automation systems, this manual power "disconnect" is left in the circuit with the motor to meet the code requirement. In case of an automation failure, the motor can be manually stopped and safely "disconnected" by that method.