MAINS POWER FAULTS
A wide range of devices is available to improve the power supply quality and ensure the best protection to professional equipment highly sensitive to electrical disturbances. As a matter of fact, any equipment needs a correct powering to ensure its performance. When this condition is not complied with, there are failures, errors and premature ageing. To protect sensitive equipment, it is possible to have recourse to several devices which, depending on their characteristics, complexity and cost, ensure different levels of efficacy. The choice should be made by assessing the degree of protection required, the entity of failures and the direct and indirect costs these may cause. It is therefore useful to know the most common electrical faults in order to adopt the most adequate protection and avoid inefficiency and failures.
MAINS POWER FAULTS
Being subject to continual load variations, distribution lines are unable to supply perfectly even voltage levels. This is why any electrical equipment is built to accept shifts of at least ±5% as to the nominal value. As a matter of fact, electricity boards contractually provide for fluctuations up to ±10%. However, this limit is often exceeded due to “slow variations” (voltage drops caused by under-dimensioned lines and overloads), “over-voltages” (considerable increases of the mains voltage value arising when industries drastically cut down their energy consumption), “fast variations” (drops caused by the connection of equipment such as: discharge lamps, punching machines, electric motors etc.).
These are very brief pulse disturbances extremely dangerous for the most sensitive equipment since the voltage values may reach thousands of volts. They are caused by several events, like e.g. switching of high voltage lines, connection of power factor correction capacitors, lightning, disconnection of loads with high reactive powers, and also by lower power loads such as photocopy machines and air conditioners connected to the same line powering the sensitive equipment. Spikes are not detectable by means of an ordinary voltmeter due to their short duration; however they are one of the main causes of failures and malfunctions.
HIGH FREQUENCY DISTURBANCES
They are very common and easily detected by anyone watching TV. They are the cause of the “snowstorm” effect and those fastidious lines that sometimes appear on the screen. They are caused by the sparks generated in the AC commutator motors, the “corona effect” on the high voltage lines, the igniters of luminous signs and burners, and by the magnetic fields irradiated by radio and TV stations. Line disturbances, also known as HF noise, do not generally create problems on electromechanical equipment, but can often damage a sensitive electronic equipment.
They are caused by the ever increasing use of electrical equipment with non-linear absorption such as: rectifiers, converters, drives, switching power supplies. This fault can cause heavy overloads on lines and transformers, explosion of power factor correction capacitors, incorrect indications on measuring equipment and, generally speaking, the malfunction of any electrical equipment.
This is the most obvious event (though less frequent) because everybody perceives it. It may happen accidentally on production plants or distribution lines, or it can be programmed to reduce energy consumption. There are also micro-interruptions, which may last between microseconds and a few tenths of milliseconds, caused by short circuits or line switching. These faults are not noticed by electromechanical equipment, but they can cause damage to an electronic equipment. The switching power supplies used in almost any electronic equipment can normally compensate interruptions lasting a few milliseconds. A longer blackout can cause loss of data, program cancellations and system failures.