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Author Topic: Why break both hot and ground on motor feed? Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 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?  |  IP: Logged Monte L Fullmer Film God Posts: 8367 From: Nampa, Idaho, USA Registered: Nov 2004 posted 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.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 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.  |  IP: Logged Jack Ondracek Film God Posts: 2348 From: Port Orchard, WA, USA Registered: Oct 2002 posted 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.  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 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.  |  IP: Logged Richard Fowler Film God Posts: 2392 From: Ft. Lauderdale, FL, USA Registered: Jun 2001 posted 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.  |  IP: Logged Bob Koch Expert Film Handler Posts: 183 From: williams ca Registered: Nov 2001 posted 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.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 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  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 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.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 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  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 04-26-2007 10:19 PM                        Gordon, IATSE projectionists are much to smart to work for months without being paid!  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 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  |  IP: Logged John Walsh Film God Posts: 2490 From: Connecticut, USA, Earth, Milky Way Registered: Oct 1999 posted 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.  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 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.  |  IP: Logged Tony Ratcliff Expert Film Handler Posts: 216 From: Madison, IN, USA Registered: Mar 2002 posted 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.  |  IP: Logged

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