Fair call as long as the main unit is never down. So to ensure that what about using opto-isolators &/or fuses to prevent damage to the controller circuitry? I may not be suggesting the right solution as electronics is not my thing but you get the intention. I feel quite exposed as a failure of the system while I am on holiday may have very costly consequences. I have just had such a failure so I have an interest in making it fool proof. Whilst I try not to be that fool, even a spade thru the control wiring could produce such shorts for example. Want to make it when customers do get it wrong or an unexpected environmental event occurs, nothing dies.