Lightning protection: leave it to the experts

April 11th, 2014, Published in Articles: Vector

 

by Hans van de Groenendaal, EE Publishers

Lightning protection is a serious, yet often underestimated issue, which requires professional equipment, installed professionally. Dehn Protection South Africa MD Alexis Barwise shares some strong views on the subject.

Barwise says it is inadvisable to apply a rule of thumb and to think you and your property are protected: “The most important thing to do before rushing out and spending money on lightning and surge protection is a risk assessment. You must determine the lightning strike risk to your building and to human life, as well as the financial impact, should your property be struck. The objective of the risk assessment is to calculate the required lightning protection level.”

Fig. 1: The three elements of a building protection system.

Fig. 1: The three elements of a building protection system.

It is paramount to establish whether lightning protection should be limited to just protecting against lightning or whether a total approach is needed. Barwise says the risk assessment helps one understand the risk and risk mitigation methods are then used to decrease this risk below the tolerable Rt value. These methods include external protection, fire suppression systems and also equipotential bonding.Lightning protection is serious business. It is standardised in IEC 62305:2010-12, which deals with general principles, risk management, physical damage to structures and life-threatening hazards. Electrical and electronic systems, even within structures, remain vulnerable to lightning and the electrical fields generated by it.

“Too many companies approach lightning protection from the mains board onwards, which is fine but, in many instances, not enough if lightning strikes the structure and causes physical damage, overvoltage or a fire, which may ultimately destroy the property or the electronic equipment inside.

“It is not good enough to get a contractor to install what may be considered a standard way of protecting a building. There is no standard way but there are standard principles to follow, and each building or structure presents its own unique challenges. It is advisable to employ a consultant who specialises in lightning protection.”

A lightning protection system consists of an external and internal system. In this article, the discussion focuses mainly on the external system dealing with the interception of a direct lightning strike by means of an air determination system, whichconducts the lightning current to earth and distributing the current in the earth by means of an earth termination system.

The function of an internal lightning protection system is to prevent arcing and overvoltages in the structure by establishing equipotential bonding or by keeping a separation distance between the component of the lightning protection system and other conductive elements in the structure.

External lightning protection

An external lightning protection system consists of three basic elements: an air-termination system, a down conductor and an earth-termination system, all measures which are required for connecting electrical parts to earth.(see Fig. 1)

The IEC standard suggest that if air-termination components are installed on a structure, they shall be located at corners, exposed points and edges (especially on the upper level of any facades) in accordance with one or more of the following methods: (See Fig. 2)

  • The rolling sphere method, which is suitable in all cases. Barwise explains that it is like a sphere that you roll over the building and a point of strike is determined. An air-termination system using air-termination rods (catching rod) is installed wherever the sphere touches the building. The IEC standard recommends various heights depending on the level of protection required.
  • The protection angle method, which is suitable for simple-shaped buildings, but which is subject to limits of air-termination height, which are shown in the IEC standard.
  • A “mesh” air-termination system can be used universally regardless of height. A reticulated air-termination mesh is fitted on the roof with a mesh size according to the class of lightning protection system (LPS) that is required. A mesh sizing table is detailed in the IEC standards.
Fig. 2: Rolling sphere radius, protection angle, mesh size and typical preferred distances between down conductors.

Fig. 2: Rolling sphere radius, protection angle, mesh size and typical preferred distances between down conductors.

Attention must also be given to the down conductors and the earthing. To reduce the probability of damage due to lightning, the down-conductors shall be arranged in such a way that several parallel current paths exist from the point of strike to earth; the length of the current paths is kept to a minimum; and equipotential bonding to conducting parts of the structure is performed according to the requirements of the IEC standard.

Earth-termination system

Barwise points out that the shape and dimensions of the earth-termination system are the important criteria when dealing with the dispersion of the lightning current (high-frequency behaviour) into the ground while minimising any potentially dangerous overvoltages.

“In general, a low earthing resistance (if possible, lower than 10 Ω when measured at low frequency) is recommended and from the viewpoint of lightning protection, a single integrated structure earth-termination system is preferable and is suitable for all purposes, in other words lightning protection, power systems and telecommunication systems. Earth-termination shall be bonded.”

Internal lightning protection systems

In theory, one must be able to lift power and IT systems, pipes etc. to the same potential level under lightning conditions.One can only achieve this by using devices such as lightning current arresters and surge arresters. Furthermore, the IEC uses a concept called “lightning protection zoning” which is used to select either a class 1, 2 or 3 device.

According to the IEC standards, protection of internal systems against surges requires a systematic approach consisting of coordination between lightning current arrestors and downstream surge protection devices (SPDs), both for power and signal lines using the lightning protection zone concept. When there is more than one inner light protection zone (LPZ), SPDs shall be located at the line entrance into each LPZ. In both cases, additional SPDs may be required if the distances between the SPD and the equipment protected is ≤ 5m. Refer to Annex C of IEC62305 – 4.

Overview

There is no “one-size-fits-all” solution in lightning protection – there are, however, experts in the field who have experience and who can advise on the best possible approach to take.

To comment on this article, contact vector@ee.co.za.

 

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