A failed PLC input card after a storm is rarely just a PLC problem. It can stop a conveyor, trip a pump station, interrupt a batch process or leave a remote asset unavailable until replacement parts arrive. Effective surge protection is therefore a system design requirement, not an accessory added after repeated failures.
Industrial sites across Australia face a combination of lightning activity, utility switching events, large motor loads and long cable runs. These conditions can introduce transient overvoltages into power, control and communications circuits. The result may be immediate equipment damage, but more often it is cumulative stress that shortens the life of sensitive electronics and creates intermittent faults that are costly to diagnose.
Why industrial equipment is exposed to surges
A surge is a short-duration rise in voltage or current that exceeds what connected equipment is designed to withstand. Lightning is the most recognised source, particularly at exposed sites, regional facilities and assets connected by overhead services. A direct lightning strike is an extreme event, but nearby strikes can still induce damaging transients in electrical and signal cabling.
Many industrial surges originate inside the facility. Switching inductive loads such as motors, solenoids, transformers and contactors creates transient energy. Variable speed drives, power factor correction equipment and switching power supplies can also contribute to electrical noise and switching disturbances. On a complex plant, the source and the affected device may be separated by substantial cable distances.
Long cable runs increase exposure. A field instrument on a tank farm, a remote I/O enclosure along a conveyor or a level transmitter at a water treatment site can be connected to the control room through cables that act as paths for induced transients. The same applies to Ethernet, serial communications, telemetry, 4-20 mA loops and 24 V DC distribution.
Surge protection must be coordinated
A single device at the main switchboard does not provide complete surge protection for an industrial control system. It reduces a portion of the incoming transient energy, but residual voltage can remain high enough to affect downstream equipment. Protection needs to be coordinated across the installation so each stage reduces the surge to a level suitable for the equipment it serves.
At the incoming supply, a high-energy surge protective device manages transients entering through the electrical service. Downstream distribution boards may require further protection where cable lengths, sub-distribution arrangements or sensitive loads justify it. Final protection close to control panels, PLCs, drives, instrumentation and communications equipment limits the remaining voltage at the point where equipment is most vulnerable.
This approach depends on the site architecture. A compact factory with short internal cable runs may require a different arrangement from a mine site with distributed pumps, long conveyors, radio links and separate field enclosures. The objective is not to fit the greatest number of devices. It is to control transient energy at each entry point and avoid leaving a path around the protection system.
Protect every conductive path
Electrical supply protection is only one part of the design. A surge can enter via any cable connected to the equipment. If a PLC panel has protected AC supply but an unprotected Ethernet cable or analogue signal cable connected to a remote device, the transient may bypass the power device altogether.
A practical assessment considers incoming mains, submains, 24 V DC supplies, analogue and digital I/O, data networks, serial communications, antenna feeders and external control wiring. Equipment with multiple connections requires special attention because differences in earth potential during a transient can drive damaging current between ports.
For example, a VSD may be supplied from a protected switchboard but receive commands from a PLC over a long control cable. The drive, PLC and associated interface circuits need protection selected for their respective supply and signal arrangements. Installing a device without accounting for the full signal path can create a false sense of security.
Selecting the right device for the circuit
Surge protective devices are not interchangeable. The voltage rating, system configuration, discharge capacity, protection level and connection method must match the circuit and the application. Selecting a device solely on price or a headline surge-current figure can lead to poor protection performance or nuisance failures.
For AC power circuits, the nominal system voltage, earthing arrangement and available fault conditions guide selection. The device must be suitable for the supply configuration, whether single-phase or three-phase, and be coordinated with upstream overcurrent protection. Installation location also matters. A device intended for main switchboard duty is not necessarily appropriate for a local control cabinet.
For DC control power, the device must suit the operating voltage and normal voltage tolerance of the circuit. A 24 V DC supply commonly feeds PLCs, I/O modules, sensors and relays, but the protection device must not conduct during normal operation or interfere with supply stability. The expected load, supply configuration and earthing reference all affect the specification.
Signal protection needs closer attention. A 4-20 mA loop, RTD circuit, thermocouple, pulse input and Ethernet connection each have different electrical characteristics. The protection device must support the required signal range, bandwidth and transmission method without creating unacceptable voltage drop, leakage, capacitance or measurement error. For communications circuits, pin configuration and network speed are also relevant.
Earthing and installation determine performance
A correctly selected surge protective device can perform poorly if it is installed with long leads, inadequate earthing or unsuitable cable routing. During a fast transient, conductor length adds inductance and increases the voltage seen by the protected equipment. The device should be mounted close to the protected circuit with short, direct conductors and a low-impedance earth connection.
Earthing is not simply a compliance checkbox. It provides the reference path that allows surge energy to be diverted away from equipment. Bonding between switchboards, control panels, cable trays and structural elements should be assessed as part of the broader earthing system. Poor bonding can create voltage differences between locations, particularly where field devices are widely distributed.
Cable segregation also supports protection performance. Routing unprotected external cables beside sensitive internal control wiring increases coupling risk. Where possible, protect cables at the point they enter an enclosure, then keep protected wiring separate from incoming unprotected conductors. This detail is often missed during retrofits, even though it can determine whether the installation performs as intended.
Apply protection where failure has consequences
Not every circuit carries the same operational risk. A disciplined specification starts with the consequence of failure, exposure to transients and cost of downtime. Critical control and safety systems usually warrant priority, including PLC and remote I/O panels, process instrumentation, pump and conveyor controls, SCADA hardware, communications gateways and security or access systems supporting operations.
Remote and exposed assets deserve particular scrutiny. Bore pumps, weather stations, weighbridges, rail signalling interfaces, solar installations and field-mounted instruments may be connected by long cables and operate in locations where lightning exposure is higher. A replacement device may be inexpensive, but travel time, production loss and troubleshooting can make the real incident cost substantial.
Surge protection should also be considered when upgrading controls. Replacing relay logic with PLCs, adding networked instrumentation or integrating VSDs increases the number of sensitive electronic devices in the plant. The protection strategy should be reviewed alongside the new control architecture rather than treated as a separate maintenance issue.
Maintenance and verification are part of the plan
Surge protective devices are consumable protection components. Each significant event can reduce their remaining capacity, even when there is no visible sign of damage. Many industrial devices include status indication or remote signalling contacts so maintenance teams can identify devices that require replacement before protection is lost.
Inspection should form part of routine switchboard and control-panel maintenance. Check status indicators, connection tightness, earthing conductors, upstream protective devices and any remote alarm integration. After a known lightning event or unexplained failure, inspect the full protection path rather than replacing only the damaged field device.
Documentation matters as well. Record device type, location, protected circuit, installation date and replacement history. This makes it easier to identify repeated exposure at a particular asset and supports consistent specification across future projects.
For industrial facilities, the right solution combines suitable devices, coordinated placement, sound earthing and installation discipline. Tech Source can assist with specifying Novaris surge protection for power, control and signal applications where project conditions and operational risk need to be properly considered. The most useful time to assess exposure is before the next unexplained fault turns a manageable design detail into unplanned downtime.