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Surge Protection

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Should you use a surge protector, and if so, what sort?

The cheap ones contain thyristors which do not conduct until a certain excessive voltage is reached at which point they conduct after a short delay and in theory short out the spike. If you look at the specifications most of them contain similar quite cheap components. More expensive versions have indicator lights to show that they are OK as thyristors are often destroyed by a big spike. The problem is that a lot of spikes get through below the conduction threshold and that serious spikes can cause a lot of damage before the thyristor fires. And thyristors alone can clip certain waveforms to produce even more harmful transients prompting some companies such as Compaq to advise against their use.

More expensive protectors have filters as well as thyristors. The filters consist of series inductances (coils) and parallel capacitors in a pi style network to reject higher frequencies than 50 Hz - the sort of stuff you hear on the radio when the cake mixer starts. This cuts the high frequency energy from both spikes and noise, works without appreciable delay and so protects and complements the thyristor. Typically the filter comes first then the thyristor and then maybe another filter. Check that the components are rated to carry the necessary current. Worth $100 or so up.

You can also get spikes and noise down the telephone line for the same reasons and indeed a surge protector is a good idea for a modem too. Some of these are sold separately and some are included in the dearer surge protectors. In Australia they need to be Austel approved - a way to keep Australian manufacturers complacent, lazy and expensive and so ensure they will never be competitive overseas.

Best of all are UPS - uninterruptable power supplies - which use the 240V AC mains to charge a (for example) 24V DC battery that feeds an invertor that makes 240V AC again. Not only do you get protection from noise and spikes but also protection from voltage fluctuations and blackouts. The more you pay the more current they will provide and the longer they last - cheap ones for a few hundred dollars will run one PC for only a few minutes - much dearer ones for thousands will run a network for as long as you need. Some have network connections and software to close things down gracefully when power is low. Not cheap but could be a lot cheaper than losing your data.

If you live in an area with a reliable power supply and underground wiring you may never need these devices. If you live in the outback or next door to a welding shop you might not be able to compute without a UPS. But nothing will protect you form a direct lightning strike - an awesome event which will fry or even vaporise anything metal for quite a distance. Naturally a nuclear war would make the computer the least of your worries but EMP occupies military minds a good deal.

Final comment - all of these devices need a suitable earth system. Unless your building is fairly new it could be worthwhile to have an electrician check the mains earth and if necessary replace it.

Hope this helps.

Dr Robert Cox

I use and recommend the dual systems available from Dick Smiths and Harvey Norman from around $169 to $189. The actual brand name escapes me, it is COM.... something (maybe guard). They come with or without modem line protection (which is also important) and have line filtering, two stage surge protection (good for lightning storms) and a faster reaction rate (usually about a third of the cheaper models). It is a lot of money, but well worth it, and a lot less cheaper than an UPS. However an UPS may still be waranted if your database remains open when the computer is unattended. That will depend on how your operating system (Mac YUKK!!) and database copes with unexpected shutdowns.

Regards

Shane Kuhl

Ian Kamerman