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.