Mains generator changeover contactor system..

One for the electric's crew;)..
We have a need to implement a contractor based mains generator changeover system. Of course this will need to be mechanically and electrically interlocked so as not to send gennie power back up the line;!. The gennie of some 7 KVA output will be controlled by an automatic starter system.
Its a very simple system fed off a long "ish" 230 volt overhead and the system is a simple mains incomer, RCD, thence to a Four way dist board lights, and a small ring main, all of three sockets, and Two spurs terminating in 16 Amp commando sockets which in turn feed Two 19 inch racks.
These are on MCB's at 5, 32, 16 and 16 amps respectively. All single phase.
Question is if we decide to put the changeover contactor system say after the mains incomer i.e. on the tails from the meter what would the recommend contactor amperage rating?.
Or would it be better of if we say had a small sub main after the "dist" board, the only thing thats essential to supply are the 19 inch racks the lights and ring main aren't that essential would it perhaps be better and maybe safer to put them on a MCB of say 32 amps rating off the main dist board.
Then take that the to the contactor changeover system and then the output of that thence to Two off, separate 16 amp MCB's.
This would give the system a degree of protection in that the whole has apart from the suppliers fuse (100 amp), a 32 amp MCB in line preceding it. If this is done this way I presume that a 32 amp rated contactor would be in order?..
Anyone any opinions good or bad on these contactors certainly cheaper than most others?.
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Tony Sayer

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