The purpose of the M.L.E. Fault Throwing Switch is to create a deliberate fault on an overhead line, responding from a signal given by the protective relays of a malfunctioning transformer. Tripping of the source circuit breaker which senses the fault will automatically cut off all of the system being supplied. It is, therefore, customary to isolate the faulty transformer quickly, in the ‘dead period’ before the circuit breaker recloses, by means of an M.L.E. type RLS spring opening switch, initiated to open by a late closing auxiliary switch on the FTS.
The switch portion is basically a single pole M.L.E. type RDB disconnector to which has been added a closing spring toggle, which in the closed ‘straight-line’ position locks the switch closed to prevent bounce and throw open by the short-circuit forces encountered on closing. At near ground level is positioned the spring close mechanism in a weatherproof galvanised housing. This also contains two powerful closing springs which in the service position are restrained by a lever released by a trip solenoid. An adjustable length of galvanised pipe couples the mechanism to a clevis on the switch.
The M.L.E. type FTS will make without welding closed on to a fault of up to 40kA peak, withstand 16kA for 1 second, and is rated up to 72.5kV.
Type FTS Fault Throwing Switches (Cat. No. FTS/15)