Load shedding and your control system: protecting PLCs, drives and panels

Wherever supply is unreliable, whether through load shedding, faults or a weak local network, automation equipment sees repeated outages, dips and restarts. The outage itself is rarely the problem. The danger is the moment power comes back.

What actually goes wrong

• Surges and spikes on restoration can damage power supplies, drives and electronics
• Brief dips and flicker trip drives on undervoltage and reset controllers
• Phase loss or imbalance can run a three-phase motor on two phases and overheat it
• Uncontrolled restarts start machines when nobody expects them, or all at once, which loads the supply
• Lost data if retentive memory or batteries are not set up properly

Layers of protection

1. Surge protective devices (SPDs) at the incoming supply and, in stages, closer to sensitive equipment. They divert surge energy to earth. Choose types suited to the exposure and fit them properly with short earth leads
2. Voltage and phase monitoring relays that disconnect motors and hold off restarts when the supply is outside limits, and only allow restart after it has been stable for a set time
3. UPS for the controls: a small UPS for PLCs, HMIs, networks and servers keeps them alive through short outages and gives a clean shutdown for longer ones
4. A DC UPS or buffer module on the 24 V rail rides through short dips without a full AC UPS
5. Good earthing and bonding, which make every other protection work better

Drives and motors

• Decide deliberately whether a drive may restart automatically. Automatic restart suits a pump but is dangerous on a machine that someone may be standing next to
• Use staggered restarts so many drives do not start at once
• Check the drive's ride-through or kinetic buffering settings, which can keep it running through short dips
• Consider line reactors, which add some protection against supply disturbance

PLC behaviour on power return

Decide what the program does after a power cycle. Does it resume, stay stopped or run a start-up routine? Check which data is retentive, and make sure the controller's backup battery or memory card setup is healthy so that recipes and counters survive.

Practical first steps

• Fit or check surge protection at the incoming supply
• Add a UPS for the PLC, HMI and network equipment on critical machines
• Review drive restart settings and confirm they are safe
• List the equipment that has failed after outages. Repeat victims are your weak points
• Stock spares for those parts. See our guide on critical spares

Where we can help

If you have repeat failures of power supplies, drives or I/O after outages, send us the details. We can quote the replacement parts, and an engineer can help review the protection around them.