Electrical installations must have the property of functioning satisfactorily in their electromagnetic environment (immunity) without affecting that environment (emission) in an intolerable manner. This is referred to as electro-magnetic compatibility. We make a differentiation between radiated and conductive interferences. With conductive interference, there are symmetrical as well as asymmetrical interferences (also known as differential and common-mode interferences, respectively). 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 switching power supplies, frequency converters, processors, switching operations in electronic or electrical installations, motor controls, and others. 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 from phase/neutral to earth. Leakage currents result from this (Figure 1). The bigger the capacitors, the better the attenuation with correspondingly higher leakage currents.

Figure 1: A typical leakage path.

Threshold values permit safe operation

Parasitic coupling capacities of an installation or equipment, as well as long cables, also contribute to the leakage current of a filter. These lead to a summation of leakage currents that flow through the earth conductor and that can pose a safety risk. The higher the impedance of the earthing conductor, the greater the safety risk for the user: should a person touch an item of equipment having a defective (broken) earth conductor, the leakage current will flow through that person to earth (Figure 2).

Figure 2: Leakage current flowing to ground.

Furthermore, any residual current-circuit breakers connected in a building network influence the reliable operation of equipment resulting from too high leakage currents. These devices detect currents flowing in the earth conductor and disconnect the supply voltage if a certain threshold value is 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.

Demands on the product developer

Manufactures of equipment and installations must ensure that their products meet these requirements regarding leakage current and electro-magnetic compatibility. And here is a conflict of objectives. Normally both basic conditions can be adhered to without any special measures being necessary. It is important, however, to understand that we are dealing with a voltage field that, with a good filtering effect, will automatically give rise to high leakage currents.

Problems with the specification of filter leakage currents

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 household appliances, specify in detail which threshold values are to be kept to and what method is to be employed in determining those values. Equipment and installation manufactures are faced thus with a problem of keeping to the standards governing their products, while having to attempt evaluation of various filter manufacturers whose information regarding leakage currents can only be compared with reservation.

Calculation models are used for the determination of leakage currents. These models are based on idealised conditions. With this, capacitor- and network-voltage tolerances are taken into consideration, but parasitic effects neglected. However, the simplification used in the idealised models lead to errors that can be neglected when compared to the relatively high tolerances used in the calculations. Thus, capacitors are specified with a ±20% capacitance tolerance, whereas in reality, experience shows that capacitances are subject to much smaller tolerances. With the determination of leakage currents, a differentiation must be made between 1- and 3-phase filters.

Leakage current with 1-phase filters

With 1-phase filters it can be assumed that neutral and earth conductors are at the same potential. For this reason the filter circuit shown in figure 3a can be simplified and represented by the replacement circuit shown in figure 3b.

Figures 3a and 3b: Filtering circuit representations and equivalences.

The leakage current can now be determined more easily by the formula

The largest leakage current to be expected is derived from a network-voltage tolerance of +10%, a capacitor tolerance of +20% and at a network frequency of 60Hz.

Leakage current with 3-phase filters

Under the assumption of a symmetrical and linear load, an ideal 3-phase filter has no leakage currents, even with large asymmetrical interferences. Figure 4 shows the section of the Ycapacitors in a 3-phase filter.

Figure 4: Y-capacitors in a 3-phase filter.

In reality, however, a 3-phase filter is always loaded in an unbalanced manner. This is due to the tolerances of the Y-capacitors, the imbalance in the supply network, the asymmetrical load, and the asymmetry within the filter resulting from a non-ideal component configuration. With 3-phase filters, the leakage currents of the vectors of individual phases are added together to give a resulting discharge current (Figure 5).

Figure 5: Adding the leakage currents of the vectors of individual phases.

Classification of leakage currents

In order to take the various requirements regarding leakage current into account, manufacturers classify their products, producing filters for standard applications, medical applications, industrial applications, etc. Since patients can come into direct contact with equipment, it is precisely in the medical field where there are increased requirements regarding leakage currents. So as to maintain threshold values, only small -and in many cases no - Y-capacitors are used. Shurter, for example, offers the M5 filters with a maximum leakage current of 5µA (no Y-capacitors), or the M80 filters with a maximum leakage current of 80µA. There are no standards, however, that specify what classes there are, what these are called or what corresponding threshold values are to be applied. But help is at hand from Schurter for users who need to quickly find the appropriate product for their application.