13/08/2019
Types of lightning strikes and transients
Lightning causes damage because it generates electrical transients (very short-lived events) like overvoltage and power surges and strikes.
Direct lightning strikes can generate millions of volts and hundreds of thousands of Amps and high-level overvoltage transients. They may seriously a buildings’ physical structure, electric distribution system, and cause fires. And the electromagnetic pulse energy of a lightning strike can affect electrics and electronics 2 kilometres away!
Indirect lightning strikes are one, though not the only, cause of low-level transients. Strikes in the vicinity of a building and on powerlines lead to overvoltage induced by the electromagnetic fields from the lightning current. Though typically less damaging than direct strikes, they are enough to melt electronic circuitry.
Switching inductive, capacitive and resistive loads also generates harmful overvoltage.
Confusion between primary lightning and surge protection devices
One misconception that needs to be urgently dispelled is the widespread confusion between lightning rods and surge arresters. People think they are the same thing. Consequently, they believe that lightning rods protect buildings and their occupants. They don’t. At least not fully and not on their own.
Primary protection, however, means partial protection. The lightning rod’s conductive grid is connected to the master busbar in the switchboard, so lightning current flows from the ground into a building’s power distribution system.
Nor do lightning rods protect against surges arising from the induction effect of lightning’s electromagnetic field or from strikes to overhead lines or into the ground nearby.
That is where surge arresters (also known as surge protection devices [SPDs]) come in.
How surge arresters work
Surge arresters protect power distribution and telecommunication systems from high voltage surges caused by lightning strikes. They arrest surges currents at their point of installation, discharging them to earth so that they bypass critical equipment.
Used together with lightning rods, surge arresters form a building’s lightning protection system.
Surge arrestors also afford protection against overvoltage resulting from constant self-inducted and capacitive load switching. And because electronics are ever more critical and ubiquitous, surge protective devices play a twin role : they protect microprocessors against wear-and-tear and lightning.
Types, or class, of SPDs and their circuit breakers
You may have heard contractors and other electricity professionals talk about “classes” or “types” of SPDs. Different types and combinations thereof are installed in different protection zones to meet different needs. Let’s briefly consider Types 1 and 2.
• Type 1 arresters are generally installed in the service entrance switchboards to protect low-voltage equipment from direct lightning strike overvoltage.
• Type 2 SPDs should be fitted to all the distribution panels that serve critical equipment. They afford protection against the residual effects of lightning strikes and switching overvoltage.
To avoid any risk of permanent overvoltage which would short-circuit and damage a switchboard, the surge protective device must itself be protected by a disconnection circuit breaker (MCB).
Ideal lightning protection system
Part 4 of EN/IEC 62305 made surge protectors compulsory in buildings where lightning rods are installed. The most comprehensive lightning protection system should therefore comprise :
o A lightning rod
o A surge arrestor
o A circuit breaker
Didier Mignardot
Schneider Electric
