How a PLC Works

Safety comes first in every industrial job. Before you work on a site, you'll need its induction, and on a construction site a General Construction Induction card (White Card). Only licensed electricians may do electrical work on installations.

Isolate before you work. Before working on a machine or its control panel, isolate it, lock it out and tag it, then test that it's dead. Never rely on the PLC being stopped or in Program mode: its outputs and the power circuit can still be live.

Emergency stops are hardwired. An emergency stop must stop the machine directly, through the wiring, and must not depend on a standard PLC program. The PLC can monitor the E-stop, but it doesn't do the stopping.

A PLC is not an isolator. A PLC output, especially a transistor output, isn't a means of isolation. A switched-off output doesn't make a machine safe to work on.

Stop devices are wired normally closed. If a wire breaks (see chapter 8), the input goes off and the machine stops.

Good installation practice:

• Keep extra-low voltage control wiring separate from low voltage power wiring.
• Run signal cables away from power cables, motors, welders and variable speed drives, which cause electrical noise.
• Earth cable screens at one end only.
• Fit suppression across inductive loads, such as contactor coils and solenoids.
• Mount the PLC in an enclosure with room for air flow, away from heat, moisture, dust and vibration.

Test safely. Test new programs on the simulators first. Never change or test a program on running plant without permission and a safe plan.

Standards (check you're using the current edition):

• AS/NZS 3000 – Wiring Rules
• AS/NZS 4836 – Safe working on or near low-voltage and extra-low voltage electrical installations and equipment
• AS 60204.1 – Safety of machinery: Electrical equipment of machines
• AS/NZS 4024.1604 – Safety of machinery: Emergency stop