Why the 55 kg test mass for half ropes?
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The half rope dynamic test is done on a single strand, as the strands can be used separately. However, it is not a single rope and is not designed to withstand multiple hard falls on a single strand.
On one strand with 80 kg, a half rope holds between 1 and 2 falls; it is difficult to differentiate between ropes with such a small number of falls.
And in reality, on a big fall, both ropes usually come under tension before the climber's fall is arrested. In this case each rope supports part of the user's weight.
For these reasons the standards committee decided: half ropes must hold 5 falls at 55 kg.
The 5 falls at 55 kg secure the usage of a half rope.
Attention, this does not mean that you can lead belay a climber on one strand of a half rope.