The accumulator insulation relay has to be mechanical since semiconductors tend to produce a short circuit between their terminals when they break down. In that case they will not be able to fulfil the requirement of an accumulator insulation relay disconnecting the poles from the car.
However, FSE recommends to use semiconductor switches and accumulator insulation relays in series to avoid stressing the mechanical relays all the time when turning the car on and off during testing for example.
REMEMBER: The accumulator insulation relay(s) must disconnect both poles of the accumulator(s).