Surge Protection Devices: Type 1, Type 2, Type 3
How to protect KNX panels, automation systems and sensitive equipment from lightning-induced transients and switching surges.
What surge protection devices do
A surge (transient overvoltage) is a brief spike on the electrical supply — caused by a nearby lightning strike, switching of large inductive loads, or utility grid switching. A lightning strike 1 km away can induce thousands of volts on power cables via electromagnetic coupling.
SPDs (Surge Protection Devices) clamp the voltage: when voltage exceeds their protection level (Up), the SPD conducts and redirects the surge current to earth, limiting the voltage seen by downstream equipment.
KNX systems need SPD protection
KNX IP routers, logic controllers and smart meters contain microelectronics that are vulnerable to transients. A single surge without protection can destroy multiple KNX devices — far more costly than the SPD investment.
SPD types: Class I, II, III (IEC 61643-11)
MANDATORY when building has external lightning protection system (LPS — lightning rod, Faraday cage). Also required for buildings in high-lightning-density areas (>25 lightning strikes/km²/year).
Contains spark gaps or combined MOV+spark gap. Handles direct lightning current. Large physical size.
REQUIRED in all new buildings regardless of LPS presence (most European national codes require Type 2 in main and sub-distribution boards). Protects against induced surges and switching transients.
MOV (Metal Oxide Varistor) based. Compact DIN-rail device (1–2 modules). Most common SPD in electrical panels.
RECOMMENDED for sensitive equipment: KNX IP routers, energy meters, home automation servers, audio/visual equipment. Provides final-stage protection at the load.
Small MOV + transient voltage suppressor. Built into smart socket strips, equipment racks, AV equipment.
Coordination between SPD types
When Type 1 and Type 2 SPDs are used together, they must be coordinated — if installed too close to each other, the Type 1 spark gap (fast activation) can steal the surge from the Type 2 MOV, leaving the Type 2 without work. This is fine. However, if installed on the same DIN rail without separation:
- ⚠Minimum cable length between Type 1 and Type 2: 10 metres of cable (inductance slows the surge, allowing each stage to act)
- ⚠Or: use a coordinated combined Type 1+2 device (available from ABB, Schneider, Phoenix Contact) — eliminates coordination issue
- ⚠Type 2 and Type 3: minimum 5 metres cable separation, or use combined Type 2+3
SPD for KNX and data lines
SPD protection must extend to all conductors entering the building. For KNX systems:
230V AC power supply to KNX panel
→ Type 2 SPD on main incomer + Type 3 at panel
KNX TP bus cable (if entering from outside building)
→ KNX bus SPD (e.g. ABB KNX-SPD, Phoenix Contact TRABTECH KNX)
Ethernet to KNX IP router
→ Data line SPD (Cat5e/Cat6 SPD) at panel entry point
Weather station cable (outdoor sensor)
→ Signal line SPD for 12V DC sensor power and signal
Antenna cables (DECT, GSM, 433MHz)
→ Coaxial SPD at building entry
SPD selection checklist
- ✓Does the building have external LPS (lightning rod)? → Type 1 mandatory at main board
- ✓Type of earthing system: TN-C-S / TT / IT — affects SPD connection (3+0 for TN, 1+1 or 3+1 for TT)
- ✓Up (protection level) ≤ 1.5 kV for sensitive electronics (KNX devices, computers)
- ✓Uc (max continuous operating voltage) ≥ 1.1 × Un (i.e. Uc ≥ 253V for 230V system)
- ✓Back-up protection: SPD requires MCB or fuse upstream (typically 63A gG for Type 2)
- ✓Thermal disconnector: check SPD has built-in thermal disconnector to prevent fire on SPD failure
- ✓Replacement: MOV-based SPDs degrade with surges — check indicator and replace after major events
SPD protection included in every PanelCraft panel
We select and install Type 2 SPDs as standard in all our KNX and DALI panels. Type 1 available for LPS-equipped buildings.
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