TECHNICAL GUIDE | ELECTRICAL SAFETY

A practical guide to selecting an industrial control panel SPD by system voltage, earthing arrangement, installation position, SPD type, Uc, Up, In, Imax, backup protection and monitoring requirements.

Industrial control panels require coordinated surge protection for sensitive PLCs, drives, power supplies and communication equipment.

Industrial control panels contain sensitive and valuable components such as programmable logic controllers, variable frequency drives, communication modules, sensors, industrial power supplies and monitoring devices. These components may be exposed to transient overvoltages caused by indirect lightning, utility switching, motor operation, contactors, relays and other inductive loads.

A correctly selected industrial automation surge protection system helps divert surge current toward the protective earthing and equipotential bonding network while limiting the voltage applied to downstream equipment. The SPD must be selected as part of the overall electrical protection design rather than treated as an isolated accessory.

Selection involves more than matching the nominal system voltage. Engineers should also consider the installation location, earthing arrangement, SPD classification, maximum continuous operating voltage, voltage protection level, discharge-current capacity, backup protection, wiring length and coordination with upstream and downstream protective devices.

Why Industrial Control Panels Need Surge Protection

Automation systems depend on stable power, continuous communication and accurate control signals. Even a short transient event can interfere with PLC inputs, damage a control power supply, cause communication errors or shorten the operating life of electronic components.

Circuit breakers and fuses are primarily intended for overcurrent and short-circuit protection. They are not a substitute for a surge protective device, because transient overvoltages can rise and fall much faster than conventional overcurrent protection operates.

An SPD changes from a high-impedance state to a conducting state during a surge, helping divert impulse current and limit downstream overvoltage.

Industrial Control Panel SPD Selection Process

The general principles for SPD selection, operation, location and coordination are described in IEC 61643-12:2020. The final design should also follow applicable local electrical regulations, project specifications and the manufacturer’s technical documentation.

A practical sequence for preliminary industrial control panel SPD selection.

1. Confirm the Power-System Voltage

Start by confirming the actual electrical system connected to the control panel. Record the nominal line-to-line voltage, line-to-neutral voltage, system frequency, whether the supply is single-phase or three-phase, and whether a neutral conductor is present.

The SPD’s maximum continuous operating voltage, Uc, must be suitable for the voltage continuously applied across each protection mode. An unsuitable Uc rating may lead to premature degradation, unwanted operation or inadequate protection.

For a broader pre-selection workflow, refer to the CRESIN SPD Selection Guide.

2. Identify the Earthing Arrangement

The earthing arrangement affects the SPD configuration and the required protection modes. Common low-voltage systems include TN-S, TN-C, TN-C-S, TT and IT.

A three-phase system does not automatically use the same SPD configuration in every project. The designer must consider whether a neutral conductor is present, how neutral and protective earth are arranged, and whether protection is required between L-PE, L-N and N-PE.

Requirements for earthing arrangements, protective conductors and protective bonding conductors are addressed in IEC 60364-5-54:2011+A1:2021.

3. Select the Correct SPD Type

SPD classification is determined primarily by the installation position and the surge duty expected at that point.

Typical coordinated SPD placement from the service entrance to sensitive terminal equipment.

For many internal cabinets, a Type 2 surge protective device is the main local protection stage. However, it should normally be coordinated with the protection installed at the main and sub-distribution levels. See the CRESIN power distribution surge protection solution for a layered application example.

4. Check Uc and Up

Uc is the maximum RMS voltage that can be continuously applied to the SPD under specified conditions. It should accommodate the normal system voltage and expected voltage variation while remaining compatible with the required protection level.

Up is the voltage protection level measured under specified test conditions. The selected Up should be below the impulse-withstand capability of the protected equipment. The effective voltage at the equipment can be higher than the SPD’s stated Up because connecting conductors add inductive voltage during a surge.

5. Evaluate In and Imax

In, the nominal discharge current for a Type 2 SPD, indicates repetitive discharge-current performance under the specified 8/20 μs test waveform. Imax indicates the maximum discharge current the Type 2 SPD can withstand once under specified test conditions.

A larger Imax value alone does not prove that one SPD is better for a project. Uc, Up, In, Imax, protection mode, system configuration, installation position and backup protection should be evaluated together.

6. Confirm Pole and Protection-Mode Configuration

The table is a pre-selection reference, not a substitute for reviewing the project single-line diagram and the selected SPD’s installation instructions.

7. Verify Backup Overcurrent Protection

An SPD may require an upstream fuse, MCB, MCCB or dedicated SPD backup protection device. Selection depends on the SPD’s permitted maximum backup protection, the existing upstream protective device and the prospective short-circuit current at the installation point.

Do not select the backup device only from the control panel load current. Verify the coordination information in the SPD datasheet.

8. Check Monitoring and Maintenance Requirements

For critical production lines and remote installations, useful features include visual status indication, replaceable modules, remote signalling contacts and alarm-system integration. These functions help maintenance teams identify a degraded module without waiting for a surge-related failure.

Installation Practices That Affect Protection Performance

Keep Conductors Short and Direct

The conductors between the phase lines, neutral, SPD and protective-earth bar should be kept as short and direct as practical. Long conductors, cable loops and sharp bends increase inductive voltage during a surge and may reduce the effective protection provided to downstream equipment.

Conceptual comparison of long, looped SPD conductors and short, direct connections to the protective-earth bar.

Additional product and system drawings are available in the CRESIN surge protector wiring diagram resource section.

Provide Effective Equipotential Bonding

The panel enclosure, protective-earth conductor, mounting structure and other exposed conductive parts should be connected to the required bonding network. Poor bonding can create hazardous potential differences even where an SPD is installed.

Coordinate Upstream and Downstream SPDs

A typical coordinated arrangement may use Type 1 or Type 1+2 protection at the main incoming panel, Type 2 protection at the sub-distribution or control-panel level, and Type 3 protection close to particularly sensitive equipment. Coordination should consider cable distance, protection levels, discharge-current capability and the manufacturer’s recommendations.

CSMS-B40 Type 2 SPD for Industrial Control Panels

CRESIN CSMS-B40 Type 2 surge protective device for AC power distribution and industrial control-panel applications.

The CRESIN CSMS-B40 is a Type 2 AC power SPD intended for low-voltage power-distribution and industrial control applications. Available variants should be matched to the actual system voltage, protection mode and pole configuration.

For information about CRESIN’s internal verification capabilities, visit the SPD testing laboratory.

Verify ratings, test evidence, wiring information and compliance wording before publication or project approval.

Industrial Control Panel SPD Selection Checklist

Frequently Asked Questions

Is a Type 2 SPD suitable for a PLC control panel?

A Type 2 SPD is commonly used in PLC control panels and internal distribution panels to limit induced lightning surges and switching transients. Where the facility may be exposed to lightning current, appropriate upstream Type 1 or Type 1+2 protection may also be required.

How do I choose between 3P, 4P and 3P+N?

The choice depends on the number of conductors, earthing arrangement, required protection modes and the product architecture. Review the single-line diagram and the exact manufacturer wiring instructions rather than selecting by the phrase “three-phase” alone.

Which parameter is most important: Uc, Up, In or Imax?

None should be used alone. Uc determines continuous-voltage suitability, Up describes voltage limitation, In indicates repetitive Type 2 discharge-current performance, and Imax indicates the maximum Type 2 discharge-current capability under specified test conditions.

Does an SPD require a separate fuse or circuit breaker?

It depends on the SPD and the upstream protective device. Check the maximum permitted backup protection, available short-circuit current and the manufacturer’s coordination instructions for the exact model.

Can an SPD completely prevent equipment failure?

No device can guarantee that equipment will never fail. A properly selected and installed SPD helps reduce surge-related risk, but effective protection also depends on earthing, bonding, cable routing, overcurrent protection and coordination between protection stages.