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CAGE Code: 5DME2
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AppNotes - Understanding EMC Stands and Specifications Page 9 (Mains Filters)

Mains Filters
Maximum power transfer occurs when source and load impedance are matched.
A power line filter is an inductor-capacitor network that aims to cause maximum mismatch between impedances, and therefore reduces the amount of EMI power to be transferred from the noise source onto the power line cable. Fig 12 shows a typical single-phase power line filter.

Figure 12

The inductors L1 and L2 are differential mode while L3/L4 are common mode. They are usually wound in current compensated fashion - on a toroidal core. This winding method allows flux due to differential mode currents and mains currents to cancel each other, while common mode currents will be added together. This gives a large inductance to common mode currents and insures that the inductor will not be saturated by the large magnetic flux produced by the mains current.

The capacitors placed between the phases, known as "X" class capacitors, must offer a high poise voltage rating and are used to attenuate differential mode interference.

The capacitor placed between the phase lines and the earth, known as "Y" class capacitors, must have a more stringent rating and are used to attenuate common mode interference (Table and Table 4).

Subgroup Pulse Peak Voltage in Operation Category of Installation to IEC 664


The switching of class X capacitors in accordance with their application L-L, L-N, and in exceptional cases by applying additional safety measures, also to earth (PE).
Capacitors must meet the requirements of class X or Y (to VDE 0565, Part 1-1, EN 132400) if installed in filters generally designed for operation with mains a-c voltage.
Class X capacitors are assigned according to those peak voltages of pulses which interfere the a-c voltage mains during operation.

Subgroup Type of Insulation By Passed According to IEC 536 Rate Voltage Range Peak Value of Impulse Voltage



insulation double or reinforced

base or additional insulation

base or additional insulation

base or additional insulation





The switching of Y capacitors is affected in accordance with their application to earth (PE).
Capacitors must meet the requirements of class X or Y (to VDE 0565, Part 1-1, EN 132400) if installed in filters generally designed for operation with mains a-c voltage.
Class Y capacitors are suited with which the failure of a capacitor could lead to dangerous electric shocks.
The value of the Y capacitor is restricted by the permissible leakage current allowed. The maximum leakage current is governed by standards and regulations and depends upon the type of equipment. The leakage current is given by:

     lL = 2.Pi.U.f.c

Where "lL" is the leakage current; "U" the voltage across the capacitor; "f" the frequency of the mains voltage across the capacitor; and "c" the capacitance.
Mains filters should be mounted as close as possible to power entry so that high frequency interference does not bypass the filter. IEC inlet modules are ideally suited for this task.
To achieve higher attenuation or an increase in the effective working frequency range more complex filters than the one shown in Fig.12 can be made using more common mode or differential mode inductors or capacitors.

General Technical Data
All technical data is given at 25C unless otherwise specified.

Current Ratings
The current ratings given for each type is the maximum allowable current authorized by safety agencies at an ambient temperature of 40C. Current at other temperatures is shown in the de rating curve, or can be ascertained by the formula:

Voltage Ratings
The maximum rated voltage of our filters is divided into two specifications, one for type testing and one for production testing. This is in accordance with guidelines laid down in various IEC recommendations. Type testing shall be a minimum of 2121DC for a minimum of 60 second between all terminals. However, the discharge resistor inside the filter shall be removed for testing according to IEC recommendations. All voltage tests shall not exceed 80% of the specified values.

Leakage Current
The leakage current to ground for each is given as a maximum value per phase, at 230V/50Hz.

Flammability Class
All filters in this catalogue are UL 94V-2 or UL 94V-0.

Disclaimer: Coil Winding Specialist, Inc., its affiliates, agents, and employees acting on behalf of Coil Winding Specialist, Inc. (collectively, CWS) disclaim any and all liability for any errors, inaccuracies or incompleteness contained herein or in any other disclosure relating to any product. CWS reserves the right at any time and from time to time to make changes to design, technical specification, or construction of any product listed on this website without notice. CWS also reserves the right to discontinue or limit sale of any product or service available throughout this website on temporary or permanently basis without notice. CWS highly recommends customers not to use its products offered on this website for inappropriate use and applications. Products should be tested for its suitability by prospective customers in all possible applications and conditions. Customers using or selling CWS products for use in other than recommended applications do so entirely at their own risk and agree to fully indemnify CWS from any damages arising or resulting from such use or sale.

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Copyright 2008, Coil Winding Specialist, Inc
Updated July, 2013

CAGE Code: 5DME2(Registered with