Deep Dive: Importance and Selection of Surge Protection Devices (SPD) in Industrial Control Panels

The Critical Role of Surge Protection Devices (SPD) in Industrial Control Systems

In industrial environments, where complex and sensitive electronic equipment is ubiquitous, protecting against damage from transient overvoltages (Surges) is paramount for maintaining production continuity and minimizing losses. Surge Protection Devices (SPDs) are not merely an option but a necessity for installation in industrial control panels to safeguard high-value equipment and entire production processes.

What is a Surge and Why Does it Pose a Threat?

A surge is a rapid and significant increase in voltage above the normal level, lasting only a few microseconds. These surges can originate from several factors, including:

• Lightning Strikes: Both direct strikes and nearby strikes that induce overvoltages in power lines.
• Switching of Large Inductive Loads: Such as the switching on/off of large motors, transformers, or capacitor banks.
• Utility Grid Faults: Including short circuits or malfunctions of equipment within the power distribution system.

These surges can cause severe damage to electronic equipment like PLCs, VFDs, sensors, industrial computers, and communication devices, leading to production disruptions, costly repairs, and unexpected downtime.

Operating Principles of Surge Protection Devices (SPDs)

An SPD acts as a protective device by diverting transient surge currents away from the equipment it protects and channeling them to the ground. This action limits the voltage supplied to the equipment to a safe level (clamping voltage). Key components used in SPDs include:

• Metal Oxide Varistors (MOVs): Semiconductor devices with high resistance at normal voltage and rapidly decreasing resistance when overvoltage occurs.
• Gas Discharge Tubes (GDTs): Consist of electrodes separated by a gas-filled gap. When overvoltage occurs, the gas ionizes and conducts current to ground.
• Silicon Avalanche Diodes (SADs) / Transient Voltage Suppressors (TVS): Diodes designed for extremely fast response to overvoltages, ideal for sensitive electronic equipment.

Types of SPD and Classification According to IEC 61643-11

SPDs are classified based on their installation location and surge protection capabilities as follows:

• Type 1 SPD: Installed at the main service entrance of the electrical system (Main Distribution Board) or where power lines directly enter a building. They have high lightning current discharge capability (Iimp) and are often used in areas with high lightning risk.
• Type 2 SPD: Installed in sub-distribution boards or main distribution panels downstream from Type 1. They protect against induced surges and switching overvoltages, with high nominal discharge current capability (In).
• Type 3 SPD: Installed close to the end equipment (e.g., power outlets, machine control panels) to protect against residual surges passing through Type 1 and Type 2. They offer lower surge protection but are highly sensitive.

Critical Factors for Proper SPD Selection

Correct SPD selection is vital to ensure effective system protection. Consider these factors:

1. Maximum Continuous Operating Voltage (Uc): Must be higher than the system's normal operating voltage to prevent unnecessary SPD activation.
2. Maximum Discharge Current (Imax): The maximum surge current the SPD can withstand once without damage.
3. Nominal Discharge Current (In): The maximum surge current the SPD can discharge 20 times without damage.
4. Voltage Protection Level (Up): The maximum voltage that the SPD will allow to pass through to the protected equipment. A lower Up value is better.
5. System Type: Such as TT, TN-S, TN-C-S, which influences the selection of SPDs with different pole counts and connection methods.
6. Installation Location: Based on Type 1, 2, or 3, depending on where it's installed in the electrical system.

Proper Installation and Wiring Practices for SPDs

The effectiveness of an SPD depends heavily on correct installation. Improper wiring will significantly reduce its protective performance. Key principles include:

1. Shortest Lead Length: The connecting wires from the SPD to the electrical system and the grounding wire (PE) should be as short as possible, preferably not exceeding 0.5 meters, to minimize inductance.
2. Proper Grounding: Ensure the control panel's grounding system is effective and compliant with standards. Connecting the SPD to the main earthing bar is crucial.
3. Overcurrent Protection: An SPD should have an upstream circuit breaker or fuse installed to protect the SPD itself in case of an excessively large surge or internal short circuit within the SPD.
4. Parallel Connection: SPDs are generally connected in parallel with the power supply and the load they are designed to protect.

SPD Wiring Example (Conceptual):

Line (L1, L2, L3) ----> Fuses/Circuit Breaker ----> SPD Input
Neutral (N) ----------> Fuses/Circuit Breaker ----> SPD Input

SPD Output (Protected Line/Neutral) ----> Industrial Equipment Load

SPD Ground Terminal ----> Main Earthing Bar (PE)

Common Mistakes and Basic Troubleshooting

• Incorrect SPD Type/Sizing: Ensure the selected SPD matches the system's Type (1, 2, 3), operating voltage (Uc), and discharge currents (In, Imax) of the system and equipment.
• Poor Grounding: Verify the resistance of the grounding system and the tightness of all ground connections.
• Excessive Lead Length: Re-install to ensure the shortest possible connecting wires to the SPD.
• Failure to Check SPD Status: Most SPDs have status indicators (e.g., green/red light) or mechanical flags to show operational status. Regularly check these; if an SPD is damaged, replace it immediately.
• Lack of Upstream Overcurrent Protection: Install appropriate fuses or circuit breakers upstream of the SPD to protect both the SPD and the system from damage.

Maintaining SPDs for Optimal Performance

While SPDs are generally low-maintenance devices, periodic inspection of their condition is vital. Always check the status indicator lights or damage flags. If an SPD is found to be damaged or non-functional, it must be replaced immediately to ensure continuous protection for the system.

Conclusion: Investing in SPDs is Investing in Business Continuity

Installing Surge Protection Devices (SPDs) in industrial control panels is a valuable investment to protect expensive electronic equipment, reduce the risk of damage, minimize production disruptions, and avoid unexpected repair costs. Understanding their operating principles, making correct selections, and adhering to installation standards will help your industrial systems operate smoothly and with maximum stability. If you have any questions or require further guidance on selecting and installing SPDs for your system, the expert engineering team at SP Automation is ready to provide technical consultation for the most suitable solutions.