Two systems, one objective

External lightning protection — air termination, down conductors and earthing — intercepts a strike and carries its current safely to earth. It does not stop the voltage rise. Internal surge protection limits the transient overvoltages that reach equipment along power and signal lines. Installing one without the other leaves the installation exposed.

The external system

Air termination to intercept the strike

Down conductors to carry the current to earth

An earthing system able to dissipate it

Bonding so that no dangerous potential difference develops between services

The internal system

Type 1 devices at the main incomer

Type 2 devices at distribution boards

Type 3 devices at sensitive equipment

Signal and data line protection where those lines enter or leave the building

Equipotential bonding

The most commonly neglected element. A strike raises the potential of the earthing system by a large amount for a very short time. If a metal service — water, gas, telecoms, structural steel — is not bonded to that earthing system, the difference appears across equipment connected to both. Bonding is what removes that difference.

Design sequence

Assess the risk, size the air termination and down conductors, design the earthing to a measured target, design the surge protection in coordinated stages, and bond everything together. Each step constrains the others.