PURPOSE: To prevent a relevant circuit breaker from being made even when a making command is issued so as to prevent the occurrence of a short- circuiting fault when an earthing device is earthed to a trolley wire by setting making lock data against the relevant circuit breaker.

CONSTITUTION: The states of circuit breakers 1B and 1D are fetched from a data fetching means (DI) 7A. An earthing device state discriminating means (AD) 7B fetches the information regarding a grounded-on relay from an earthing device 4 through the DI 7A. When the information regarding the relay is fetched from the AD 7B, a circuit breaker making locking signal setting means 7C retrieves a transforming facility information table accumulated in a storage device 8 and fetched from the DI 7A and a preset earthing device status collating table and sets the making locking state data corresponding to a relevant circuit breaker. Therefore, interlocking conditions are established against the breakers 1B and 1D and, even when a making command is issued, a making signal from a circuit breaker operating signal sending means 7D is blocked.