Hello HAIDER

It's been a while since I've played with fall arrest systems, but here goes.  

There is a joke which goes: "It's not the fall that kills you, it's that sudden stop at the end."  But this is no joke, it's pure physics.

I note that you are a mechanical, so this should be easier for you to grasp than for us structural types.  Normally, for a structural engineer, if something moves it must be broken. :)

The best example I can think of is the egg-tossing competition at the country fair.  When your partner throws the egg to you, do you hold your hands out in front of you and stop it dead?  No, of course not.  You make contact with the egg as soon as possible and then slow it down gradually over the longest and slowest path which your athleticism will allow.  This will minimize the force on the egg.

And so it is with fall arrest.  The calculation you must do is Force = mass times DEceleration.  The deceleration is a function of the amount of stretch in the rope, harness and any other components of the system, including the unfortunate worker.  If the system has little elasticity, the force will be huge.

I once witnessed a worker free fall not more than 10 feet onto hard ground.  I could not believe how badly broken he was.

I believe every harness should include a shock absorber.  Check your local safety supplier.

Check also your local safety regulations.  There you may find stated minimum values.

THANKS FOR THE REPLY.

You may want to take a look at the 'Fall Simulator' and related literature at:

http://www.petzl.com/petzl/publicActiv?id=ALPI#

OSHA limits the force to applied to the individual by the fall protection system to 1800-lb.  Check their website for more info.

Tripp

For various types of fall protection equipment, search Lab Safety catalog for 'fall harness:'
http://www.labsafety.com/search/results.asp?prodKeyword=fall+harness&x=13&y=4

For a clear explanation & illustrations of OSHA Fall Protection Equipment requirements and available types of equipment, see
http://www.labsafety.com/refinfo/ezfacts/ezf130.htm
It gives details re several comments above, e.g.,:

"Fall protection lanyards may also have a shock-absorbing feature built in, thus reducing the potential fall arrest force. Remember that maximum arrest force is 900 pounds for belts, or 1800 pounds for full-body harnesses. With a belt, the use of a shock-absorbing lanyard is recommended because it limits the arresting force from a six-foot drop to 830 pounds. If a shock-absorbing lanyard is not used, the tie-off point must be high enough to limit the arrest force to less than the 900-pound limit. The height of this tie-off point will vary, depending on the lanyard material and the weight of the person involved. A lanyard used for a fall is limited to allow a maximum six-foot free fall. For this reason, most lanyards are a maximum of six feet long. However, if a higher tie-off point is used, the lanyard can be longer if the free fall distance does not exceed 6 feet."

P.S. There is an OSHA forum for this kind of question; a man of steel doesn't need a fall harness!!!