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Although we earlier defined inrush as "the current drawn by a device when switched on", and the fact there are no exceptions, doesn't help when having to cope with them. During normal operating this is usually not an issue or, if it is, it is fairly localised. When the inrush of a complete circuit happens collectively then the story takes on a whole new characteristic. The problem usually presents itself during the re-energising of a vast extent of a network, or a large building. During this time every piece of equipment will be wanting to take what is required to get them going, the worst being television monitors and sets, and computer monitors. These can take up to 20 times their usual running current. The typical method of saving the fuses during a re-energization process is to split the area to be energised and to then energise in smaller 'chunks'. Unfortunately, this can not always be controlled, especially if the input supply suffers a relatively long power outage. When the supply is restored the inrush then becomes so large it pops the main fuses. What is required is to take the principle behind re-energising in chunks and to make this automatic, regardless of whether humanly controlled or when recovering from a power failure. The principle is referred to as "sequential power energising systems.
An example of such a sequential power strip is shown above. Sadly, this supplier refuses to supply me with a sample of their product making me very suspicious as to what is wrong with it! I printed the photo to give others the opportunity to construct something simliar and possibly let me have one to promote. The application of such distribution systems is often done incorrectly. Such systems tend to be installed in an 'habitual' manner meaning the monitors are all in socket 1, PCs in socket 2, etc. When the power is applied to the building, the situation of all the monitors being switched on in batches is therefore negated. The proper method is to ensure the monitors reside at different sequences such that they are switched on at different times after power is restored. This will minimise the inrush and so protect the fuses. Having the monitor turn on a second or two after the PC will in no way compromise the integrity of any 'plug and play' system. Obviously, one needs to take into consideration devices such as SCSI interfaces, ensuring the required bits are powered up ahead of the PC, but these are few and far between. Costs are also not that high as an array of desks could be powered up through a single 10 or 16 way adapter. All that is needed is something that will delay and stagger the 'bo-ing' of modern colour monitors.
© 17.09.02 |
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