The transfer case one I assume.
I don't know of an official test but I have a P38 with a locked viscous at the moment. If I coast at a slow speed on full lock, it will soon come to a stop. If I use another car (several to choose from unfortunately as I have P38itis), it doesn't come to a stop.
The 'locked' car also scrubs it's tyres, inside rear usually, very easily on anything but perfect bitumen.
As for going bang in the middle of nowhere, I think they'll be locked up way before they go bang.
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