Now that you mention breakaway chain, what size chain do you think would be required to hold a load of 21 tonnes that you mention? My view is that using a chain that is rated for 21 tonne wouldn't suffice because that rating is for a static load. The breakaway chain would need to be of far greater load rating because it will need to withstand a shock loading in the unfortunate event of the towing mechanism failing. This page mentions 4 times the S.W.L. of the chain
http://www.chainandrope.ie/products/chains/safely-and-tow-chains.html
In the case of the 21 tonne trailer going by that criteria then an 84 tonne rated chain would be required, would this be physically too big in that it would be very awkward to be attaching?
Another point on breakaway chains is attachment to the tractor, again using the figure of 21 tonnes, how would a chain be attached to the tractor so that the tractor wouldn't be pulled apart in the event of the hitch failing and the shock load being transferred to the chain?