RCD tripping

[Saracen]

Every now and then, when I turn off the main isolation switch on the Boxford HSR, it takes out one of the RCDs in our consumer unit. It's not every time, so I don't believe it is a fault, but it's annoying, especially as it takes out half the household electrics. It never seems to do it on power up, just power down.

There is just about enough space in the main consumer unit to give the workshop its own RCD, but would a surge suppression gadget on the mains supply to the HSR be enough to cure the problem ?

The Boxford lathe is on the same ring but has never caused a trip when powered up or down.

There is a small consumer unit in the workshop, but I understand you can't daisy chain RCDs, so I can't fit a local one to limit the blackout.

Adrian.