Static Electricity Surge Protection
By Dan Wilson
[ Dan Wilson discusses a different type of surge: static electricity
[ charges (related to lightning) which discharge in a few microseconds.
[ The "line conditioner" device handles slow surges which occur over a
[ few milliseconds -- 1000-times the duration, but a bit less voltage!
[ The protection techniques are very different. -- Robbie
I've had a lot to do with surge protection of telecoms and signal circuits on the Festiniog Railway in Wales, which is situated in a western-facing valley where damp air blows up from the sea and drops a charge on trees and fences. The whole district is insulated from true earth ("ground" in the US) by a layer of slate, so that the charge from an area of several square miles is conducted down the railway and jumps across to anything earthy (groundy ?) en route to the sea.
Sometimes this is simply from the rails to a peat bed going over into another valley, and sometimes to our equipment. It's like minor lightning without thunder: you can be working by the line and there'll be a hellish crack and a smell of ozone -- the charge has to break down natural insula- tion at the top end to get onto the rails as well as lower down to get off them. Amazingly, no-one's been injured by this, though I've been in a phone exchange room and the fuses have all blown with a pop.
To start with we lost a lot of equipment until we understood what was happening. A surge normally comes along both wires at once, which is why a transformer (without an earthed screen) doesn't stop it: the capacitance between the windings lets it through. If the input is along mains wire one side will go back to ground at the local transformer and so there will be an element of surge across as well, maybe burning out the transformer.
We ended up with triple protection: big glass-tube gas protectors where the cable comes in, and then little ones at the input to the equipment itself, and then varistors or back-to-back power Zener diodes. (The latter fire at too low a voltage to use on mains inputs.) It's advisable to test all these every five years.
An additional ruse is to bring the supply in through tiny air-core chokes made by twirling each wire round a screwdriver five or six turns before you make the ends off. This delays the surge and enables the varistors to fire before any damage is done, but you must have gas protectors on the outer side with a central electrode, or connection, to earth, as they act the fastest. They can take an immense current but have a higher strike resistance than semiconductor protectors, which is why you need both. However at home (Kent, SE England) I have none of this and nothing has been damaged in 25 years.
Dan Wilson
[ I wonder if some of these techniques are used at the mains entrance in
[ residences and commercial buildings in lightning-prone areas, such as
[ Florida. What's inside the boxed you described, Fritz? -- Robbie
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