Handbook of Electrical Engineering

SWITCHGEAR AND MOTOR CONTROL CENTRES 161

control centres can be increased to cater for a large plant. A typical IMCS should include at least the
following components:-

• Each outgoing motor or plain feeder unit requires a MCU or a FCU.


• Each incoming interconnector and busbar circuit breaker also requires a FCU.


• A CCU should be provided for either, a complete switchboard, a group of switchboards, or all
  switchboards located in a switchroom. A dual redundant system may be chosen so as to raise the
  system reliability and availability.


• Interfacing equipment and software to communicate with a DCS and a SCADA system, if they exist.


• A local area network (LAN) to couple all the units within the IMCS, which should not be a shared
  system such as the DCS.


• Various sources of uninterruptible power will be required, if the plant is spread over a large site.
  These will provide the essential power to the IMCS processing units when other less reliable power
  supplies interrupt or fail.


• Protection circuits and devices to render the system immune from electromagnetic interference.

In switchboards that supply motors the protection, control and measurement functions will be
the most varied for the motor circuits. Generator incomers would be the next lower in complexity.
The following functions are typical of all but the lowest ratings of motors:-

• Motor current.


• Motor overload protection, settings, status, pre-alarms and time to trip.


• Unbalanced loading, settings, status and pre-alarms.


• Earth fault protection, settings, status, alarms.


• Thermal status information including inhibition of starting.


• Stalling protection, settings and status.


• Acceleration time.


• Automatic restarting information due to voltage drop at the busbars.


• Total operating time of the motor.


• Operating time since the last start command.


• Non-operating time since the last stop command.


• Number of operations of the contactor.


• Local stop command function, i.e. a push button or key.


• Manual resetting of all trips and alarms.


• Remote resetting of certain trip and alarm functions by using a password.


• Facilities to allow external hard-wired circuits, such as interlocks, to be monitored and used by
  the IMCS.


• Underpower protection of the motor and its driven equipment.


• Undervoltage tripping facilities for mechanically latched contactors.


• Historical event logging for all alarms, trips, commands and inhibiting of starting.


• Trend records should be stored for a predetermined period such as six weeks.