There is almost nothing you can sit between you and mains that lets you recklessly play with it,
An earth leakage breaker relies on a significant enough current flowing through you for a few cycles to trip, there are medical ones that use a much lower current, though they will trip on just about anything, for the obvious reasons 90% of the time making them impractical, in other words you still get zapped and risk death, you just do this for a shorter period of time than if it was not there,
An isolation transformer can make things slightly more dangerous depending on what you are doing, for one thing the earth leakage breaker does not care if you grab both of the secondaries, and one that has been sitting in a garage for half a decade, especially an older model should be checked that there are no obvious shorts between the primary and secondary (The amount of times i have seen someone call an auto-transformer / variac an isolation transformer is laughable)
Have a think about how to describe what you are trying to do and i will be more than happy to help., and like the above poster, the general consensus is to put your measuring points in place and then power on the device, when people go poking around powered on electronics they should either know the risks well, or in the more likely case the circuit is powered by the secondary of a step-down transformer which does not expose any lethal voltages,