The machines are equipped to run at U.K. mains voltage and frequency, ie 240v 50Hz, and consume 146VA power at full load. Computers are sensitive to voltage fluctuations, and '.even more so to sudden voltage failures. It is usually prudent to install some form of protection device between the mains and the computer. Mains-born interference can take several forms, some of 'which can be avoided, for example being on the same c.:ircuit as heavy electrical machines or radio-telephone equipment. Interference from electrical storms in the form of the momentary induction of very high voltages in power transmission• cables can cause extensive damage to computers, and it is advisable to switch the machine off and disconnect it from the mains during a storm and at times when it is not in use. Simple spike eliminators offer a high degree of protection from this form of damage. If radio-frequency interference is suspected in the mains supply a voltage stabiliser of a suitable design is needed. Cables connected to the RS232 interface may also pick up induced currents due to lightening. The use of properly grounded shielded cable should minimise this, but nevertheless it is adviseable to disconnect all interface cables during storms and when the equipment is not in use.

If the mains voltage is known to fall below 90% of its nominal value or if power failures occur regularly the computer should be connected to an uninterruptable power supply unit (UPS) to prevent .damage occuring.

The computer should be properly earthed, either through the mains earth lead, or where this is not available or when the computer is connected to a UPS, by ajseparate earth lead connected to the mains cable which is plugged into the•back of the computer. The radio-frequency shielding built in to the computer depends on this, and without it RF emitted by the visual display unit (VDU) is likely to corrupt data on the Winchester disk which lies next to it. Care should be taken to keep computers away from radio transmission equipment, as this too can cause damage.

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