Myths vs Facts

1. When in a car, the rubber tyres insulate you from ground, therefore you don't get struck. Nonsense... a lightning strike is millions of volts, voltage is the electrical pressure required to break down an insulating medium. Millions of volts will bypass a few inches of rubber without breaking a sweat. The reason you generally don't get struck in a car, is because a car is close to the ground, and there are more often than not taller objects in the vicinity that are more attractive to lightning. A lot of people think their car has been struck, when there's a big bang, and one of the windows shatters. Again, what has happened, is a nearby strike (to a tree or tall building), and the pressure wave causes the window to shatter. Should you by some strange circumstance actually experience the strike on the car, what protects you is the Faraday cage effect... a surrounding of conductive material prevents the ingress of charge into the enclosed space. For the same reason, aircraft seldom suffer damage, and they get struck by intra-cloud lightning regularly. Aircraft systems also make use of ferrites, which I'll get to later.

(C.f. Episode of Top Gear in which Hammond is seated in a car, and the car is exposed to a 700kV sustained flash.)

2. A lightning mast conducts lightning to ground. In 90% of cases, not true. Certainly, a few strokes in the lifetime of the mast may conduct to ground via the mast itself, but the purpose of the mast is to seed the near-earth air with opposite charge, cancelling charge building up in the air, preventing the strike completely. If the mast was regularly struck by lightning, two things would happen... (1) the flow of current would cause the rod to want to move (evidence by the pole seemingly climbing out of the ground), by the same principle that makes a swimming pool motor work, see paragraph on induction shortly. (2) The flow of current would induce extremely high voltages in surrounding structures' electrical and plumbing networks... clearly we would never actually want this, so a mast would appear to be counter-productive.

3. Tying supply cables in loops. Oh boy. I wonder if someone has whispered to the surge that there's a chicane in the way, and it decides to go elsewhere? To electrical current, shape of a conductor means very little. To have any protective effect, the loop in the cable must be wound around a ferrite. A ferrite is a doughnut shaped piece of iron, whose crystal structure lends itself to desirable electro-magnetic properties. The majority of computer and electronic cables that have a curious 'bulge' close to their connectors, are equipped with ferrites. A length of wire wound round a ferrite creates what is called an inductor. An inductor stores and discharges charge, and through this action, can damp certain phenomena in the electricity supply.



Here's the kicker though... ferrites are mostly employed in surge protection against network issues, notably switching surges (i.e. the supply authority is doing some switching on the network, where every make or break of current induces a small surge), and the after-effects of dips (those noticeable ones where the lights go dim, and then back to normal a second later). These phenomena are in surge protection terms, very slow acting, in the millisecond or even second range, which matches the ferrite, which will have a comparable time constant. So even ferrites are no guarantee against lightning damage.

Lightning and Induction

How often do we hear someone say, 'my house was struck by lightning, and it blew my TV/PC/whatever'? When your house gets struck directly by lightning, there will be damage to the structure (bricks and tiles missing, wiring and plumbing pipes damaged), and even thermal effects (burning).

In the vast majority of cases, the lightning strikes a nearby tree, telephone line, power line or cellphone tower, but the resident would swear on the bible that it struck the house!

How does this cause damage to the equipment in my house then? The answer lies in some brilliant 19th century physics, the laws of induction. Current flow through a conductor induces an electromagnetic field. When this invisible field cuts across nearby conductive materials, it sets up a voltage. If the voltage is high enough, it in turn is able to break down insulation, causing uncontrolled current flow. Current flow causes thermal damage (things that burn out), whereas the rising voltage causes semi-conductor components (all electronic power supplies use these components) to (what we call) puncture, destroying them.

This induction story is around us all the time... we need it to make transformers work. And it's what keeps your pool clean. In Standard 8 science, you would have been taught about motors... the outer stator, and the spinning rotor, with the rotor having brushes to carry current to and from the windings on it. Your swimming pool motor has no wiring connections from the rotor to the outside world, because it uses what is called a squirrel cage rotor. Electric field is induced in the cage, from the stator, this field causes current to flow, and the motor turns.

The next thing we need to understand about lightning, is that it is fast. Switching surges and dips are measured in milliseconds and seconds, lightning pulses are measured in microseconds (us). 1 us = 1 millionth of a second. To give it some perspective, our electricity is 50 hertz, or 50 cycles per second. Each cycle lasts 20 millisecond. If you had to place consecutive lightning pulses in a row, you would get 20 in once cycle of the power supply, and 1,000 in one second.

When we talk about surge and lightning protection, we use a concept called a time constant. Each surge has a rising edge, and a trailing edge. When we talk about surge and lightning time constants, we use two values. The first is the time in us, for the pulse to reach its peak. The second is the time in us, for the peak to die down to 50% of its peak value. This is shown in the following sketch...



I.e. Curve one shows 10/350us characteristics, meaning the pulses rise to maximum in 10 microseconds, and decay by half in 350 microseconds. Note that the full extent of the curve lasts a lot longer, as far as 1,000us or more. The area under the curve demonstrates the amount of induced energy that needs to be dissipated by the protective device.

Protection

This brings us neatly on to protection.

The first thing we need to understand, is that there is a difference between a surge arrester and a lightning current arrester.

The surge arresters that you buy from retailers, or that are included with your multiplug or ADSL modem package, are exactly that, surge arresters. Referring back to the curves above, device type 2 (the 5kA 8/20 jobbie) can only damp a tiny quantum of energy, compared with the lightning current arrester, type 1, whose area under the curve is massive in comparison. This provides a clue... an inexpensive surge arrester is inexpensive for a reason, and will provide almost zero protection against a proper nearby strike. A proper lightning current arrester must cost several hundred rands.

Integrated Protection

Much like protecting your property against criminals, your lightning protection also needs to be layered.

First and foremost, and without this you are farting against thunder... you need a 50kA 10/350us lightning current arrester (LCA). (The 100kA is far superior, but becomes prohibitively expensive.) This gets installed in your DB board, which is the closest point electrically, to the supply cable. In South Africa, the neutral does not need to be protected, since it is solidly bonded to the earth network, which is in turn connected to the supply authority transformer neutral. By all means, buy and install the L+N LCA, but a single phase LCA, connected between live and earth, is ample. If you have a three-phase supply, the corresponding LCA is available.

Second lady for a shave... at every sub-DB (e.g. in your outbuildings or swimming pool pump box), install a 10kA 8/20 surge arrester, between live and earth. This simply provides added protection, should something nasty get past the big bouncer in the main DB.

Thirdly, use multiplugs or plug tops with 5kA 8/20 surge arresters on sensitive equipment, i.e. your TV/DSTV/DVD/home theatre stuff in the lounge, and your PC.

Fourth, your DSTV dish and any terrestrial antenna need to bonded to earth. Most DSTV installers omit this, because they have to muck about looking for the braided copper in your roof, making up the earth network. In the old days, an earth spike was driven into the ground for the TV aerial, but this is strictly speaking not necessary these days. As long as they are connected to earth, any stray charge that accumulates will be dissipated to ground via a good conductor, rather than flashing through your DSTV equipment.

Lastly, the Telkom copper network is a great source of induced ugliness. If you are only using the telephone, purchase a surge protector designed for the Telkom RJ12 plug. It will most likely also have a 5kA 8/20 rating, and surge arresters for telco use are generally yellow in colour. Alternatively, if you have ADSL, the modem power supply module should have in/out ports for the telephone wires, and which have an internal surge device fitted.

Reputable electrical wholesalers should carry the properly rated LCAs and SAs you require. Be quite firm about your main LCA having a good kA and us rating. From a business point of view I recommend Paul van As from Surgetek. Surgetek are the agents for the Dehn range of surge and lightning protectors, imported from Germany if I recall correctly. (No, there isn't any relationship between Actom and Surgetek, or myself and Surgetek.)