Lightning protection

December 18th, 2013, Published in Articles: EngineerIT

 

Lightning is a serious issue which is often under-estimated until faced with its disastrous consequences. Protection against this severe energy source requires professional equipment, professionally installed. Simply buying a few protector plugs or multi-plug extensions from the hardware store could well finish off whatever electrical and computer equipment is indoors, connected to the mains or not! 

Alex Barwise, MD of Dehn Protection South Africa has some strong views on the subject of lightning protection. “You can’t just use rule of thumb and believe that you and your property are protected. The most important thing to do before rushing out and spending money on lightning and surge protection, is to do a risk assessment. What are the lightning strike risks to your building; to human life and what is the financial impact should you experience strikes to your property? It is important to note that the objective of the risk assessment is to calculate the required lightning protection level.

“Should it be limited to just protecting against lightning or would a total approach be the way to go? Total approach means that there is a systematic approach to do complete solutions, from the risk assessment one understands the risk and then one uses risk mitigation methods to decrease this risk below the tolerable Rt value which includes external protection, fire suppression systems and also equipotential bonding.”

The protection from lightning is a serious business, which has been 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 instance not enough if  lightning strikes the structure and causes physical damage, overvoltage and/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,” said Barwise. “There is no standard way, there are standard principles to follow, but 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 will focus mainly on the external system dealing with the interception of a direct lightning strike by means of an air determination system -conducting 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 the prevention of dangerous arcing and overvoltage’s in the structure by establishing equipotential bonding or keeping a separation distance between the component of the lightning protection system and other conductive elements in the structure.

External lightning protection systems

An external lightning protection system consists of three basic elements: an air-termination system, a down conductor and an earth-termination system, all measures required for connecting an electrical part to earth.

The IEC standard suggest that if air-termination components are installed on a structure it 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 which include:

  • The rolling sphere method is suitable in all cases. Barwise explains that it is like a sphere and you roll it over the building and wherever the sphere touches, an air determination catching rod is installed. The IEC standard recommends various heights depending on the level of protection required.
  • The protection angle method is suitable for simple-shaped buildings but it is subject to limits of air-termination height which are shown in the IEC standard
  • The mesh method is a suitable form of protection where plane surfaces are to be protected. A grid is fitted but again depending on the level of protection required determines the size of the grid. For level one the IEC recommend a grid of 5 m x 5 m while for level 3 the grid size is 20 x 20 m.

Fig. 1: The illustration clearly show the three elements
of a building protection system.

Attention also has to be given to the down conductors and the earthing. In order to reduce the probability of damage due to lightning current flowing the down-conductors shall be arranged in such a way that from the point of strike to earth several parallel current paths exist; the length of the current paths is kept to a minimum; 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 when dealing with the dispersion of the lightning current (high frequency behaviour) into the ground, whilst minimising any potentially dangerous overvoltages, the shape and dimensions of the earth-termination system are the important criteria. “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 (ie. lightning protection, power systems and telecommunication systems). Earth-termination shall be bonded.”

The theory is that under lightning conditions one needs to be able to lift all things such as power systems, IT systems, pipes etc. to the same potential level.

Table 1: Rolling sphere radius, protection angle, mesh size and
typical preferred distances between down conductors.

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.

There is no such thing as all size fits all when it comes to protection against lightning strikes, there are however experts in the field who have experience and can advise the best possible approach to take.

Share your views with us by writing to hans.vandegroenendaal@ee.co.za. Your letter may win the letter of the month prize!

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