How to Choose a Type 2 Surge Protective Device for TT/TN Systems and Distribution Cabinets

Why Type 2 Surge Protection Matters

Selecting the right Surge Protective Device (SPD) is one of the most cost-effective ways to protect low-voltage power distribution equipment from lightning surges and switching transients. For TT and TN earthing systems, the choice of wiring configuration, voltage rating and protection level directly decides whether downstream equipment survives a surge event. This guide outlines the key criteria for choosing a Type 2 (Class II) SPD.

What Is a Type 2 / Class II SPD?

A Type 2 SPD, designated Class II under EN 61643-11 and IEC 61643-11, is installed on the load side of the main distribution board to limit transient overvoltages. Typical parameters include a nominal discharge current (In) of 20kA (8/20 µs) and a maximum discharge current (Imax) of 40kA, combined with a low voltage protection level (Up) that keeps residual voltage within a safe range for sensitive equipment.

Step 1 — Match the Wiring to Your Earthing System (TT / TN)

In TT and TN earthing systems, the SPD wiring configuration must match the system topology. For single-phase TN-S and similar systems, a 1P+NPE “1+1” configuration protects both the L-N and N-PE paths. A representative product is the Modular Surge Protective Device for TT and TN Systems (“1+1” Circuit), which delivers 20kA In / 40kA Imax, with an L-N protection level of ≤1.9kV and an N-PE level of ≤1.5kV, plus a fast ≤25ns response time — well suited to single-phase distribution boxes.

Step 2 — Choose by Continuous Operating Voltage and Application

For three-phase or higher-voltage distribution cabinets, the maximum continuous operating voltage (Uc) becomes the primary selection criterion. A 385V AC Uc SPD such as the MYS5-385/20-2P (NS) Class II Surge Protective Device (SPD) is engineered for low-voltage AC distribution systems, offering 20kA In / 40kA Imax, a low ≤1.9kV Up, 35mm DIN-rail mounting, and an optional remote-signaling (NS) contact for intelligent monitoring in unattended cabinets and control panels.

Step 3 — Compare the Key Specifications

• Nominal discharge current (In) and maximum discharge current (Imax)
• Maximum continuous operating voltage (Uc)
• Voltage protection level (Up)
• Response time (ta)
• Mounting standard (35mm DIN rail, EN 60715)
• Operating temperature range (−40°C to +70°C) and IP rating
• Status indication and remote-signaling capability

Conclusion

Whichever earthing system you operate, matching the SPD’s wiring configuration and voltage rating to your installation is the foundation of reliable surge protection. Both models above comply with EN/IEC 61643-11 Class II and are built for long-term performance in industrial and building electrical installations. Contact us for a quotation or technical selection support.