across resistors r7 and r13 which therefore is even greater than the voltage drop effective for the normal test and seizing process prior to the pickup of the test relay because this voltage drop occurs only across resistor r7 in order to be able to differentiate this particular instance checking for releasing connection from a normal test and seizing process using a switching unit in the idle condition it is not sufficient for the comparator v to measure the voltage drop on the input side only in order to be able to clearly distinguish these two instances from one another the comparator v is equipped on the input side with switching means by which the time functions of the input side voltage i e the lowering of the voltage drop across resistors r7 and r13 can be recognized thus a test for releasing connections can be prevented with assurance even for selectors in which no special provisions have been made for preventing the test for releasing connections as mentioned earlier the seizing circuit of switching unit u runs from check point pp through resistors r6 r7 r8 and r9 and through transistors t1 and t2 for the connection to higher impedance the portion of the seizing circuit comprising resistors r8 and r9 and transistors t1 and t2 are turned off and instead a holding circuit running through resistors r13 and r14 and through transistor t6 is turned on great importance is attached to the portion of the seizing circuit containing resistors r8 and r9 and transistors t1 and.