4. My concern is that if the main catch fails why doesn't the safety operate. Renault accused me of not engaging ANY of the catches which is ridiculous, because the bonnet would not be flush then!
You do not operate the safety-catch.... it operates itself by dropping though a purpose-made opening and staying suspended under spring pressure until such time as you pull the bonnet release cable, whereupon it rises to hook against a restraining static surface. If, having released the lock, you have to insert your fingers in the gap to pull or push the safety-catch before the bonnet can be lifted, then the safety-catch is operating correctly at least when the car is not in motion.
Now if the hook
part has an obtuse angle rather than an acute one and it is made of resilient or slippery material, it may be that it will not restrain if the car is in motion, under aerofoil suction (anything above 30 mph is enough to lift it), or under facing-wind pressure (20 mph is enough). The fact that a modern bonnet has no weight to talk about (how light and how cheap can they make them?) makes it even more vulnerable. It might even be worthwhile looking at the curvature of the bonnet and consider whether the leading-edge-curve has a longer radius than the trailing edge (nearest the windscreen) which would give the bonnet a lifting-section
similar to a light-aircraft's wing. Need I say more?
The plastic part that you pull to release the safety-catch can also become deformed through heat or mechanical damage; if this does not slide smoothly through its full range, then it needs attention because it could get stuck in the open
position. I have not actually inspected a failed device so I can't venture an opinion on any specific case.
My generic opinion is that the lever that secures the Lock has been failing possibly due to resonances or road vibrations.... and the safety-catch has failed because the slider
has not allowed it to return to the safe
position after closure. A combination of factors that are both identifiable and avoidable if the makers puts their mind to it. It's not as if they are being asked to re-invent the wheel. As you probably remember, the centre spring raises the bonnet into the safe position and you then insert your fingers and pull the plastic toggle (in this case)... without doing so, the bonnet won't rise no matter how hard you try. But it is far too easy to undo the safety... so in my opinion, it should be necessary to push down slightly against the centre-spring before you can pull the toggle and allow the safety catch to pass through, thus enabling the bonnet to be lifted. Older cars had this I think, but the necessity to press the bonnet down slightly must have been discontinued as people became lazier over recent decades.
Personally, I do not favour this drop the bonnet from 18" malaki
because it leaves too much to chance. I prefer to gently lower the bonnet down onto the safety level at which stage you can check that the safety-catch has engaged, and then with the heel of both hands, push down smartly on the bonnet and listen for the unmistakeable click of the lock engaging. The instructions (to drop) given by manufacturers render the closing of the bonnet haphazard and uncheckable.
I hope this makes it a bit clearer than mud.