Correct Installation of a Surge Protective Device (SPD)

A surge protective device (SPD), also called a lightning protector, is an essential protection device in electrical installations.
Below explains how to correctly install it in a distribution box or cabinet.

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Installation Position

The SPD should be installed at the power supply incoming point.
It is typically mounted next to the fuse or circuit breaker so incoming surges can be discharged to ground before entering the internal circuit.

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Basic Wiring Principle

The SPD must be connected in parallel, not in series with the load.

Typical single-phase connection

• L (live) → SPD terminal L
• N (neutral) → SPD terminal N
• PE (earth) → SPD grounding terminal
  
  

Three-phase connection

Each phase must be protected:

• L1 / L2 / L3 → SPD phase terminals
• N → neutral terminal
• PE → grounding terminal
  
  

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Important Installation Rules

• Shortest possible wiring
  • Keep the wires between the breaker and SPD as short and straight as possible
  • Long wires greatly reduce protection effectiveness
    
    
• Reliable grounding
  • The grounding wire must be thick and securely connected
  • Poor grounding = ineffective surge protection
    
    
• Install a dedicated protective breaker or fuse
  • Prevent SPD failure from causing system damage
    
    
• Place upstream of equipment
  • Install before sockets, PLCs, computers, and control circuits
    
    
• Avoid loops
  • Excessive wire bending increases inductance and weakens protection
    
    
  
  

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Purpose

The SPD quickly diverts lightning or switching overvoltage to ground, preventing damage to electrical equipment.

Key idea:
The surge must be discharged to earth before it reaches the load.