The LT-20 consists of a common anode  array of 20 x diodes and is used in lamp  test circuits where the lamp circuit has  a separate switched contact to avoid  feedback, i.e. not looped from relay or  contactor coil. If the lamp circuit is wired  direct from relay or contactor coil, the half wave rectifying effect of the diode will cause the relay or contactor to ‘chatter’ when the lamp test button is pressed.