Glad to see me question has got you thinking. Here are my thoughts, which are not necessarily correct.
Insulation resistance, which is not what my question was about, is 500v (dc I think) and will fry any semi conductors, computers and dimmers included when tested across phase and neutral.
Zs measured with a loop meter, measures impedance (not potential difference) of the phase and earth loop under fault conditions, a suitable low reading compared with the regs ensures quick opperation of the MCB.
A good earth path is required, so I think, the current generated by the loop tester follows the quickest route to earth, therefore, not affecting any computors or the like, still connected to the circuit?
I would be grateful if any one can confirm this.