Getting NOT Kinky - Fire Curtain Release Perimeter Line

If you have a Fire Curtain in your theatre, then part of that protective system is a fire control line that surrounds the Fire Curtain perimenter on three sides (stage left, top, & stage right) and has fusible links that will separate should there be a fire in the immediate vicinity of the Fire Curtain.  That line must be a small diameter (3/8" fiber rope or 3/32" diameter 7x19 wire rope) cable so that it bends easily and move freely through the pulley blocks at the corners of the Fire Curtain.  Ref: 2010 NFPA 80, Section 20.7.3.2.

The bend radius of the cable is known as the D:d ratio.  This represents the ratio of the pulley sheave diameter (D) to the cable diameter (d).  The for 7x19 stranded wire rope the recommended D:d ratio is 51:1, so the recommended sheave diameter for 3/32" diameter cable is 51 x 3/32, which equals 153/32, or about 5".  The minimum ratio is 34:1, which is means that a 4" pulley is typically used as they are commonly available.  Ref: American Iron and Steel Institute "Wire Rope Users Manual, 2nd Edition", Table 9.

In the picture above we have a variety of design issues:

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  The cable is 1/8" diameter plastic-coated wire rope.  It is a bit larger diameter than the required 3/32", so it would require an even larger sheave, about 6".  As you can see, the pulley is only about 1-1.5" diameter, or about 1/4th the required size.  This can cause the cable to 'take a set' and get a kink in it so that it doesn't want to move freely through the sheave.  I've seen wire rope as large as 1/4" diameter used with pulleys like this - and they definitely take a set after resting immobile for years on end.
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  The cable's plastic jacket might melt and gum-up the works should there be a fire - not good.
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   Although this rigging point isn't bearing a significant load, it's still jsut bad juju to use pot-metal dime-store clothes-line pulleys, twisted-link 'dog chain', and quick-links to string stuff up.  Man-up and install it right with load-rated equipment - after-all, this is a Fire Curtain System, not just a trick-line for a one-act play!

Test your Fire Curtain System every 90 days. Ref: 2010 NFPA 80, Section 20.7.1.3.
Have your Fire Curtain System Inspected Annually.  Ref: 2010 NFPA 80, Section 20.9.1.