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Filter assemblies

Three types of mains noise suppression filter assemblies are used in practice:

Collective suppressor

The collective suppressor principle results in one filter per plant. This has to cope with the entire power input. In addition, all of the connecting cables have to be shielded. Furthermore, interference generated by an «A» device can reach other devices for instance «B» or «C» through the connecting cables. The following example promises to be a more economical solution. In many cases, the single suppressor principle is the most economical solution.

Single suppressors

The principle of individual interference will in many cases the most economical solution.

Combined single and collective suppressor

From the technical point of view, only the combined application of both suppression techniques can result in a significant improvement.


Interference propagation

In the field of interference and RF suppression, the most significant means of transmission is the direct electrical connection, i.e. the connecting wiring. The radiation coupling is also important from the electromagnetic compatibility (EMC) point of view; it cannot, however, be dealt with here. 

The capacitive and inductive coupling effects occur inside the case. These could be:

- Capacitive coupling through the coupling capacity of a mains transformer.

- Inductive coupling through control system wiring in parallel.

The introduction briefly mentioned the possibility of the main filter operating with a double function. Depending on the main area of application, these filters are designated as either RF SUPPRESSION FILTERS or INTERFERENCE SUPPRESSION FILTERS.

The one filter may, therefore, appear under two references in the documentation. A filter is also classified by its mechanical design as well as its electrical data.

RF SUPPRESSION FILTERS impede the propagation of RF interference, generated by an electronic or electrical device into the mains. They also ensure an interference-free radio reception in the immediate vicinity.

INTERFERENCE SUPPRESSION FILTERS prevent mains interference from affecting electronic equipment. They enable an interference-free operation even in the case of a power supply badly affected by mains interference.

It is common to operate the mains filter in both directions in one piece of equipment, allowing it to fulfill its double function as both interference and RF suppression filters as specified.

Common- and differential mode interference

Filter engineering differentiates between common and differential mode interference originating from supply lines.

In the case of a non-earthed interference source, interference at first only propagates along the connecting lines. Like the main AC current, the parasitic current flows to the user on one lead and returns to the interference source on the other. Both these currents are in differential mode. This type of interference is therefore referred to as differential mode interference.


Due to the mechanical configuration and its parasitic capacitance, parasitic currents are also generated in the earthing circuit. This parasitic current flows on both connecting leads to the user and over an earthed lead back to the interference source. Both currents on the connecting lead are in common mode. This type of interference is therefore referred to as common mode interference.

Filter classification

For easy reading of the catalog data, SCHURTER uses the following simplified filter classification:

Differential Mode and Common Mode Attenuation

Attenuation value





20-50 dB

40-70 dB

60-80 dB

70-95 dB

Leakage Current

Leakage Current Classification

Operating leakage current





<0.1 mA

<0.5 mA

<5 mA

>5 mA

Medical Filter

SCHURTER medical filters comply with UL 60601-1 and IEC 60601-1 standard specification and are available in two versions, which differ in terms of their leakage current values.

Standard medical filters for direct person contact supplied by SCHURTER have a leakage current value of <5 μA (M5). This can only be achieved without Cy. Here, a common mode fault current against the earth is not attenuated and the filter acts only on differential mode fault currents. In addition, an inlet in protection class II can be used here, as no earth connection exists. However, if an earth connection is desired, Type (M80) can be used for indirect person contact; this has a leakage current of <80 μA which is below the required limit value of 0.1 mA. Type (M80) is manufactured to special order.


Bleed resistor

Medical filters and filters with an X-capacitor >100 nF have a bleed resistor so that no inadmissible rest voltage occurs at the touchable pins of the inlet.



All capacitors used in the filter are tested and approved according to IEC 60384-14.

According to medical standard IEC / UL 6060-1, 8.8.3 Dielectric strength test, the filter in the power line may be removed during end-use testing.

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