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Posted on Stagecraft Digest
Subject: Re: LX safety cables
Date: Wed, 20 Nov 2002 11:28:00 -0500
From: "Karl G. Ruling"

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Standards don't have "thrusts" or any hidden meanings. They simply say what they say, and the reader has to figure out what they say means. In this case, UL 1573 says in 9.2:

"A safety cable, safety chain, or other field-installed back-up restraint device intended to 'catch' a falling object shall be capable of supporting a static load of not less than six times the intended load. When it is not evident that the backup-restraint device is capable of supporting a weight of six times the marked maximum load, the device shall be subjected to the Backup-Restraint Device Load Test, Section 40."

I'll spare you the text in Section 40, but there is no specification in any of the text, either in 9.2 or in 40, for how the backup-restraint device is supposed to be attached to the luminaire. So, this leaves it pretty much up to the manufacturer to say how the backup- restraint is supposed to be used -- and, of course, the UL inspector evaluating the product per the standard needs to agree.

Having said this, it's not unreasonable for a manufacturer to suggest that a safety cable only has to loop through the yoke of an instrument. The yoke (and all the rest of the instrument) is subject to a strength test in section 9.1 of UL 1573, and must be able to support six times the intended load, so the strength of the yoke is known quantity, and is set high enough that it is unlikely to fail. However, the pipe-clamp is another matter. It is usually not sold as part of the luminaire (maybe with it, but not part of it), and users put clamps on or take them off as needed, depending on how the instrument is being mounted for a show. Therefore, a luminiare manufacturer knows how strong the yoke is (it's part of the luminaire), but has no way of knowing the strength of the pipe clamp the user has installed. So, it makes sense to consider the backup-restraint as a device to hold the luminaire if the clamp fails or is installed incorrectly, since the clamp is the unknown part.

Keep in mind that this UL standard is the one used in the US. The European standards for luminaires get specific about where the backup restraint has to be attached. Luminaires made for the European market have anchoring points for the safety cables or safety chains built into a luminaire's body to conform to the standards used there. You'll see more anchoring points for safeties on instrument bodies sold in the US because nothing in the UL standard says they can't be there, and because it's a lot less expensive to make one product that can be sold world-wide rather than to make a US version and a non-US version.

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