Monday, 4 July 2016

Leakage Currents in the Right Perspective

Electrical installations must have the property of functioning satisfactorily in their electro-magnetic environment (immunity) without affecting that environment (emission) in an intolerable manner. This is referred to as electromagnetic compatibility. 

We make a differentiation between radiated and conductive interferences. With conductive interference, there are symmetrical as well as asymmetrical interferences (also known as push-pull and push-push interference). Symmetrical interference flows between phase and neutral whereas asymmetrical interference flows between phase/ neutral and earth (ground). The causes of these kinds of interferences are network switches, frequency converters, processors, switching operations in electronic or electrical installations, motor controls, etc. Symmetrical interferences are attenuated with X-capacitors. 

For the attenuation of asymmetrical interferences, current compensated chokes are used for lower interference frequencies and Y-capacitors for the higher interference frequencies. These Y-capacitors are connected between phase/neutral and earth, and conduct the asymmetrical interferences rom phase/neutral to earth. Leakage currents ensue from this. The bigger the capacitors, the better the attenuation with correspondingly higher leakage currents.

Parasitic coupling capacities of an installation or equipment as well as long power lines also contribute to the leakage current of a filter. These lead to a summation of leakage currents that flow through the earth conductor that can give rise to a safety risk. Should a person touch an item of equipment having a defective (broken) earth conductor, the leakage current will flow through that person to earth. 

On the other hand, any residual current circuit breakers connected in a building network influence the reliable operation of equipment resulting from too high a leakage current. These residual current circuit breakers detect currents flowing in the earth conductor and disconnect the supply voltage should a certain threshold value be exceeded. For this reason, there are threshold values for leakage currents that permit reliable operation and ensure that even with defective earth connections, no person is injured.

Filter manufacturers specify leakage currents in their data sheets. The IEC filter standard does not define, however, how these specifications are to be carried out. This leads then to a situation where different manufacturers are not obliged to use the same methods to determine leakage currents. Consequently, the data given by various manufacturers is not directly comparable. Equipment standards on the other hand, e.g., IEC 60950 for office equipment, 60601- 1 for medical equipment or IEC 60335-1 for domestic equipment, specify in detail which threshold values are to be kept to and what method is to be employed in determining those values.

In order to take the various requirements regarding leakage current into account, manufacturers classify their products. With this, there are filters for standard applications, medical applications, industrial applications, etc. So as to maintain threshold values, only small, and in many cases no Y-capacitors are used. SCHURTER, for example, speaks of M5 filters having a maximum leakage current of 5μA (no Y-capacitors), or from M80 filters having a maximum leakage current of 80μA. There are no standards, however, that specify what classes there are, what these are called and what corresponding threshold values are to be applied. Nevertheless, this division helps the user to quickly find the appropriate product for his application.

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