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Lets first define a household appliance. There are two categories, 'hi-tech' and 'non hi-tech'. Hi-tech will include the usual TV, Video, Hi-Fi, PC, and at a stretch a computerised washing machine (ok, I'll even stretch to the microwave oven). Such damage has been covered in the section on "equipment failure" and is recognised as highly possible to fail during a "power wielding" transient. Low voltage lighting systems are a rather grey area as they can be coupled to a dimmer circuit which in itself is a transient creator (so let's just leave this as a grey area). Non hi-tech will be the electric cooker, the immersion (hot water) heater, fridge / freezer (and combos), non-computerised washing machines, and mains powered light bulbs. These are the items under discussion here. What becomes interesting is hearing Power Quality Investigators tell their tales of woe of customers claiming for damage to a fridge owing to a "spike on the mains". Similar stories are told of electric cookers and kettles. The classic is light bulbs blowing, although there are issues surrounding these it's generally not 'spikes'. When analysing a complaint like this the question that should be foremost in one's mind is: Does the appliance allegedly damaged by such a transient contain passive components (as opposed to 'active' semiconductor riddled appliances)? To substantiate our belief that the damage in question is a myth we need to investigate the way passive components react during a transient.
Motor Elements:
Long Term Heat Damage: Heat damage from a hot spot is easily recognised. The damage is relatively wide spread and will 'distort' heat sensitive components (e.g. plastic). Hot spots on PC boards will show up as a dark area that gradually lightens to the original colour of the board the further you move away from the hot spot. Transient Damage: The second feature is 'flash marks' (a silvery-bluey tinged area). Destructive transients usually exceed the breakdown voltages or current carrying capabilities of the damage components. These flash marks are created by the metal parts, with the high current involved, vaporising and the metal depositing on the surrounding surfaces. So how are mistakes made ? 'Dry joints' (where a soldered connection has hairline cracks which have high resistance) are renowned for 'exploding' and vaporising rather spectacularly leaving a very unmistakable flash mark. What will also be seen is the heat damage is very contained. Again, the inexperienced individual will take this to be transient damage. What is not known to this individual is that such damage, at a soldered joint, was as a result of a bad joint.... not a transient. The transient would melt the surrounding printed circuit before melting the soldered joint, purely because of the physics involved - it takes more energy to melt a larger mass.
However, this was a design fault. The inexperienced individual would overlook the fact that the component around where the fault took place was a gas discharge tube. This should have absorbed such a transient. The actual fault was the design brought the two sides of the incoming mains within an unsafe distance of each other through the placing of a track on the board and one leg of the gas discharge tube. The only insulation was the solder-mask which then finally broke down allowing the full potential of the mains to arc over and cause the destruction.
We are, however, claiming that there are individuals out there that cannot make the distinction between what was a genuine case of transient damage and what was not (especially if the electricity supplier or insurance company is footing the bill). If you are involved in power quality claims issues, it would be extremely prudent to learn the difference for yourself.
© 05.09.01 |
Myths:
And finally:- The picture alongside has all the apparent tell-tale signs of transient damage. The heat damage is localised, and flash marks appear on nearby components yet nothing is distorted.