 The signal lineman for the area was unavailable and so one was summoned from Kennington station.When he arrived, he gave instructions for the platform staff to hold the next train even if the signal turned green, and then he and the Stationmaster entered the tunnel to look for the fault.He quickly found a faulty signalling relay, which although it was energised had a voltage across a front contact (a contact that is made when the relay is energised).