jbarchuk:
One little oopsie and the machine is toast. I'd go along with three condoms but be much more comfortable with four.
It is not recommended to "double up" on condoms. The explanation is that the different layers "cinch" against each other and the persistent abrasion is likely to wear through them. 
On the other hand it is a common practice to "double glove" in the operating theatre. The extra layers do tend to become uncomfortable, especially over three or more hours. The under-gloves are black or green to make tears visible. It is however a spurious argument for the practice, to suggest that this could reduce the risk of needle-stick injury (as should be fairly obvious).
Current practice would be to use opto-couplers - alone - for this sort of monitoring. Capacitors plus a protective resistor can be used instead of just resistors to limit the current to the opto-couplers though the circuit cited above utilises high-value resistors with little heat dissipation to make use of the fact that you require only a tiny current to control CMOS logic inputs.