How important is redundancy in rigging?

http://www.boston.com/news/local/rhode-island/2014/05/04/report-aerial-performers-fall-during-circus-act-providence/jONRVASHacmTEy106u394M/story.html

"It has been determined that a carabiner in the rigging failed"

The carabiner used in the stunt is rated to hold 10,000 pounds, the statement said, while the total weight of the rigging and performers was less than 1,500 pounds.

The carabiner was the lone piece of equipment between the apparatus and a cable tethering the performers to the rafters.

This is a good excuse to post a rant I wrote a while ago.

How important is redundancy in rigging?

That depends on what level of risk you're willing to accept. Would you be comfortable with a hypothetical piece of equipment with a 1/1,000,000 chance of failure? If so, that's fine. However, you'd better be damn sure that 1/1,000,000 is accurate and not actually 1/500,000 or even 1/1,000. You'd also better be ready to take full responsibility if you underestimate the risk and end up dropping somebody due to equipment failure.

Is 1/1,000,000 not enough for comfort? A lot of people think doubling equipment doubles the statistical safety margin. Well, I have some good news and bad news for you. The bad news is that doubling equipment doesn't double the safety margin. The good news is that it increases it exponentially.

Let's look at it mathematically.
A 50% failure rate (1/2) with a redundancy can be plotted out to determine the statistical probability that both pieces of equipment will fail at the same time:

• pass/pass
• pass/fail
• fail/pass
• fail/fail

That works out to be 1/2² or 1/4 chance of complete failure. Similarly, a 1/3 chance of failure with a redundancy can be plotted and the statistical redundancy benefit determined:

• pass1/pass1
• pass1/pass2
• pass1/fail
• pass2/pass1
• pass2/pass2
• pass2/fail
• fail/pass1
• fail/pass2
• fail/fail

That works out to be 1/3² or 1/9 chance of complete failure.

Having demonstrated how redundancy in equipment yields an exponential statistical safety benefit, we can calculate for more robust equipment as well:

• 1/4² = 1/16
• 1/6² = 1/36
• 1/8² = 1/64
• 1/10² = 1/100
• 1/100² = 1/10,000

and finally,

• 1/1,000,000² = 1/1,000,000,000,000

which, if you can hit odds like that, you can simply pay for any medical bills with your lottery winnings. Even if you're wrong about that original 1/1,000,000 chance,

• 1/500,000² = 1/250,000,000,000

and

• 1/1,000² = 1/1,000,000

so you're still not exactly tempting fate.

The reason for redundancy isn't necessarily that the risk of equipment failure is unreasonably high, but that it's so easy to reduce the risk of equipment failure so much.

So, how important is redundancy in rigging? You be the judge. Just be ready, whatever you decide, to take responsibility in the unlikely event something breaks and you find the person you're suspending on the floor, instead of in the air where they belong.