According to FSE rule 6.11 the tractive-system-active light must be on whenever the accumulator insulation relays are closed or the voltage outside the accumulator container exceeds 60V DC or 25V AC RMS.
Therefore it is not allowed to assume that the voltage is below 60V DC, because the insulation relays are open. Just think of capacitors in your motor controller and the like.

The circuit switching the tractive-system-active light has to be independent of the other systems, therefore its power must be supplied by the control system. The circuit has to measure the voltage in the tractive system and monitor the control lines of the accumulator insulation relays.