Rigging Fundamentals
Rigging puts weight directly over people's heads, which is why the entire discipline is built on rated limits rather than judgement calls. The number at the centre of it all is the working load limit, or WLL: the breaking strength of a component divided by a safety factor, which for overhead entertainment rigging is commonly somewhere around 7:1 to 10:1. In other words, the rated limit you work to already has a large margin built into it, and you load to that WLL or below — never toward the breaking strength itself.
It also matters that loads are not just dead weight. A static load hangs still, but a dynamic load — a motor moving, a sudden shock as something snatches tight, a performer flying — multiplies the force well beyond the resting weight, and the rig has to be rated for that peak rather than the average. Truss carries its own ratings for both span and point loads: hang a heavy fixture in the middle of a span, or run a bridle that pulls at a shallow angle, and the real force on the hardware climbs far above the simple listed weight of the gear.
Every shackle, sling, motor, and clamp must be rated, in date, and inspected before it goes up, and installed by people who are competent to do it. The reason all of this is so uncompromising is that rigging has no minor failures — it either holds or it kills someone — and that is exactly why everything overhead is rated, secured, and then checked a second time.
If it's overhead, it must be rated, secured, and checked twice.