Hi Bud,testing your bus-bar system is no different to testing any other circuit,isolate it,short it out one end ( i usually do it mains feed end then don't forget to take it off before its re-energised )
Disconnect any neutral links ,megger , short out phase(s) to earth R1+r2 , reconnect then earth loop,obviously you need to be at the furthest point of utilisation on the bars,job jobbed
The tap off boxs are tested seperate,like a mini installation if you like,so you would take a loop reading at your box (that now becomes your external loop within the system Ze) from your tap off box to the socket is the bit you now test and get the Zs at the furthest point.
Any Rcd would be installed at this point to feed the sockets.
There would be no need to protect the bus bars with an RCD.
Hope this helps Bud