and when the second switching element fet 6 is conducting the series circuit comprising the two resistors r6 r7 will be shorted if under normal operating conditions no disturbances are present in the thyristor group a then the two alarm contacts a1 a2 of the thyristor group a are closed in such case the signal x1 indicating the operational state of the thyristors th a1 and th a2 is assumed to be in its low state and is designated an l signal with x1 in its low state the output y1 of the inverter p1 is in a high state and is designated an h signal the latter signal is passed on to the first input of the nor gate b and to the first input of the nand gate d similarly if no disturbances are present in the thyristor group b the two alarm contacts b1 and b2 of the thyristor group b are closed and the signal x2 indicating the operational state of the thyristors th b1 and th b2 is also in a low state i e is an l signal as a result the output y2 of the inverter p2 is a high state signal i e an h signal the latter signal is passed on to the second input of the nor gate b and the second input of the nand gate d with no disturbances at the thyristor groups a and b their thus appears two h signals at the inputs of the nor gate b these signals result in an l signal as the output signal.