Fault detector system for starting plasma arc working apparatus

Claim 1

In combination: a plasma arc working apparatus including a torch having an electrode, a DC power supply connected in series between said electrode and the workpiece, means for igniting a pilot arc in close proximity to said electrode, said pilot arc ignition means including a high frequency pilot arc current source having an input, and a control circuit including means for extinguishing said pilot arc, wherein said apparatus is started by applying an input signal from said control circuit to said pilot arc current source whereupon pilot arc current flows in circuit with said pilot arc ignition means to ignite the pilot arc and whereupon said pilot arc transfers the main arc between said electrode and workpiece thereby establishing an arc voltage; and a fault detector system for monitoring key parameters in the starting operation of said apparatus and for automatically stopping the starting operation upon detection of a fault or malfunction, said system comprising, in combination: sensing means for separately sensing the input signal to said pilot arc current source, the flow of pilot arc current and said arc voltage as key starting parameters, said sensing means being adapted to produce separate output signals representing the presence of said key parameters; timer circuit means adapted to receive output signals from said sensing means and to produce a first output signal representing the presence of said input signal to said pilot arc current source, a second output signal representing said pilot arc current and a third output signal representing the pilot arc, said timer circuit means being adapted to set the duration of said first, second and third output signals to first, second and third predetermined time periods respectively; a first logic circuit adapted to receive said first output signal from said timer circuit means and a signal from said sensing means representing the presence of said pilot arc current, and to produce an output signal when said sensing means signal fails to occur within said first predetermined time period indicating that said pilot arc has failed to ignite; a second logic circuit adapted to receive said second output signal from said timer circuit means and a signal from said sensing means representing the presence of said arc voltage and to produce an output signal when said sensing means signal fails to occur within said second predetermined time period indicating that the main arc has fail to transfer: a third logic circuit adapted to receive said third output signal from timer circuit means and a signal from said sensing means representing said pilot arc current and to produce an output signal when the duration of said sensing means signal exceeds said third predetermined time period indicating that said pilot arc has failed to extinguish; and means adapted to receive said first, second and third output signals from said logic circuits and to produce output signals receivable by said control circuit for stopping the starting operation of said apparatus.