Lightning protection - Engineers field FAQs

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The introduction of the new and more complex standard BS EN 62305:2006 Protection against Lightning has led to many new questions and the resurfacing of several ‘old chestnuts'. We look at a few of the most frequently asked questions fielded by Furse engineers

My building has stood for 100 years, and has never been hit.  So there is no chance of it being struck now!

Not being hit by lightning in the past has no bearing on being struck in the future. The probability of a strike and whether protection should be fitted will be shown by carrying out a risk assessment. York Minster was around 600 years old when it was ‘eventually' struck by lightning in 1984, causing extensive irreplaceable damage. Remember a direct strike to the structure is not even necessary for lightning to cause damage through fire, electric shock or electronic systems failure.

I have an air finial on the tallest part of the building and a down conductor, so that should be adequate?

This is unlikely to give adequate protection in accordance with BS EN 62305 which calls for a full Faraday Cage, comprising a number of conductors on and around the building.

My building has reinforced concrete columns. Can I use these columns as down conductors?

Yes, provided you ensure the electrical continuity of one or more reinforcing bars in each column. Where sections of reinforcing bar overlap, they should be welded or clamped together, or overlapped by at least 20 times their diameter and securely bound for the entire length of the overlap.

How do I know if my building needs lightning protection and, if so, what level of lightning protection system (LPS) is required?

There's no intuitive way of doing this - you need to carry out a risk assessment in accordance with BS EN 62305:2006 Part 2. The risk assessment in BS EN 62305-2 is much more detailed and has many more parameters than the assessment contained in BS 6651. There are software packages available that can help. Furse's bespoke risk assessment software package is called StrikeRisk. It has just been updated to version 5, and a free trial version is available to download from its website - www.furse.com

I have looked at the number of parameters required to carry out a risk assessment, but cannot find all the information. What should I do?

The risk assessment carries default values, which can be used where accurate information is not available. However, these values are conservative, so you should try and obtain as much accurate information as possible.

Why is there now so much emphasis on protection of sensitive electronic systems? This wasn't a requirement of BS 6651.

The protection of electronic systems was covered in Appendix C of BS 6651, and although this was an informative annex, the philosophy is broadly similar to that of the new standard. Our increasing reliance on electronic systems means that damage or downtime can have serious financial and operational consequences and hence their protection is reflected in the single risk assessment of the new standard..

I have a lightning conductor system on my building, so will this protect my electronics within the building?

No, this will protect the structure itself but not the electronics within it. You therefore need specialist surge protection to prevent equipment damage from LEMP (lightning electro-magnetic impulse). BS EN 62305 focuses on coordinated SPDs (surge protection devices), where the locations and LEMP handling attributes of a series of SPDs are coordinated to nullify the conducted LEMP effects - thereby protecting equipment within their environment.

Is it adequate to put surge protection on the main electrical incomer only?

Although protection of the main incomer is certainly recommended, other services should be considered for protection against transient overvoltages (surges). For example, a lightning strike up to 1km from a building can transfer huge voltages onto overhead or underground cables - like data or telephone lines - through inductive or resistive coupling. Once transferred to the cable, transients will flow along it, seeking a path to earth and damaging any electronic components they encounter.

I am fitting an LPS to a building, which contains no sensitive electronics systems. Do I still need to fit Type 1 (equipotential bonding) SPDs?

Yes. Type 1 SPDs (for mains power supplies) and Category D SPDs (for data/telecom lines) form an integral part of the equipotential bonding requirements for an LPS. They are needed to prevent partial lightning currents from causing dangerous sparking and the possibility of a fire or electric shock hazards. Type 1 SPDs are not designed to protect equipment, but form the first part of a coordinated SPD set further consisting of Type 2 and 3 SPDs.

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