MPE Limited
Hammond Road
Knowsley Industrial Park Liverpool
L33 7UL
T: +44 (0)151 632 9100

Power Line EMC Filters Overview


RFI: Radio Frequency Interference.
Traditionally considered to cover the radio spectrum 150kHz – 300MHz.

EMI: Electro Magnetic Interference.
A more modern term to cover typically 10kHz – 10GHz.

EMC: Electro Magnetic Compatibility.
The ability of equipments and systems to function as designed in an electromagnetic environment.

EMP/NEMP/HEMP: (Nuclear) Electromagnetic Pulse.
Generated by a nuclear explosion, EMP is a high energy pulse capable of damaging most types of unprotected electronic equipment. HEMP specifically relates to high altitude electromagnetic pulse.

TEMPEST: This refers to the need to prevent equipment from generating conducted or radiated EMI which could contain intelligible information.



All filters in this catalogue have wide application in the EMC field where high performance is required.
They are ideal for filtering d.c. and a.c. mains services to screened enclosures against both incoming and outgoing noise and transients.

They are equally suitable for the protection of permanently wired equipment such as computer installations.

With the increasing importance of filtering down to low frequencies, it should be noted that many of the filter ranges within this catalogue will offer 100dB performance even down to 10 kHz.


On shielded enclosures, EMP protection systems normally comprise a high energy transient suppressor, an inductive element, and a filter on all electrical conductors entering the enclosure.

Most filters in this catalogue, by virtue of their performance, are suitable for use in such systems when used with separate transient suppressors and inductors. For complete EMP protection systems comprising transient suppressors, inductors, and filters all in a single enclosure, please see our separate HEMP filter catalogues.


Where the integrity of secure information is concerned, maximum filtering performance across the widest range of frequencies is vital. This catalogue includes several ranges of filters which offer a very wide stopband performance of 100 dB from 10 kHz to well above 10GHz which can be considered for these applications.

Electrical Design Features

Multiple Line Filters with Current Compensating Inductors

All multiple line filters in this catalogue use a modern filter design employing toroidal current compensating inductors or “coupled chokes”.

Multiple windings are used on the same inductor core to achieve flux cancellation and thus full insertion loss performance up to full rated current as the inductor does not saturate.

Empirical testing has proven that this advanced filter design gives identical insertion loss performance under no-load, half-load, and full-load current conditions over the full frequency range.

The use of current compensating inductors permits the use of high permeability cores to give large inductance values.
This results in a much higher performance filter with lower leakage current and lower heat dissipation than for the older type of single line filter design still used by many manufacturers of power line filters. (Single line filters have been widely used in the past for many applications but they have a number of significant disadvantages including the loss in performance as load current increases.)

To ensure maximum benefit is obtained from MPE’s highly efficient coupled choke filter designs, the load current must always return through the filter to achieve current cancellation i.e. these filters should not be used on single lines, and on the load side of the filter the neutral must not be earthed or joined to neutrals from other sources.

As current returns through the filter in most power line applications, this type of filter design should always be the first choice for power lines as it offers the best size and performance combination.

These filter designs are equally suitable for three phase supplies with no neutral. The filter neutral line should be left unconnected, and must not be grounded.


The latest metallised plastic film dielectric capacitors are used in all of MPE’s power line filters for maximum reliability and size efficiency. In a.c. filter designs, self-healing series metallised capacitors are used with very high overvoltage capability to give a high safety factor and excellent reliability.

High quality feedthrough capacitors are used in all filters to provide good transient handling capability and optimum high frequency performance.


For enhanced safety, all filters in this catalogue are fitted with internal discharge resistor networks. These are intended to discharge the capacitors to a safe voltage within one minute or less of removing power from the filter. As the resistance of the discharge network will always be less than 1 Megohm, the use of a ‘Megger’ type instrument to check installed filters could erroneously indicate a fault so should only be used with caution (and should never be used on transient suppressed filters).


By virtue of their low inductance feedthrough construction, MPE’s filter input capacitors are extremely effective in absorbing voltage transients even for rapid pulses with nanosecond rise times.

When the energy or the voltage of incoming transients could exceed the capability of the filter capacitors, filters can be supplied with varistor transient suppressors fitted to the filter input terminals to provide additional protection. Filters are designed to permit easy renewal of transient suppressors, if necessary.

Dedicated EMP/HEMP filters are fitted with very high energy transient suppressors appropriate to their intended application.


Unless otherwise stated, filters in this catalogue are designed to meet the following overload conditions:

Current Overload:

10 times maximum rated current for 1 second.

1.5 times maximum rated current for 10 minutes.

It is not recommended that any filter is operated above its continuous rated current for any extended period because of the substantial heating effect of overload current.

Voltage Overload:

1.1times maximum rated voltage continuously.

1.5 times maximum rated voltage for 1 minute. (Excluding transient suppressed types)

D.C. filters are proof voltage tested at twice rated voltage.

All a.c. filters in this catalogue are proof voltage tested between each live line and earth at 2250V d.c. and at 1150V d.c. between live and neutral lines.

These factory tests are carried out prior to fitting of any transient suppressors and should not be repeated.

A.C. filters are designed for use on mains supplies with no more than 5% total harmonic distortion.

Overvoltage testing (including “Megger” testing) should not be carried out on transient suppressed filters as the varistors could be damaged.

Volt Drop

Volt drop figures, where quoted, relate to the resistive volt drop per line. It is measured using a d.c. current equivalent to the full load r.m.s. current. This is generally of more interest to the user than a.c. volt drop as it relates directly to power dissipation.

Low Leakage Filters

To provide good low frequency performance, all conventional a.c. power line filters have large values of capacitance between live and earth lines. This gives rise to a mains frequency leakage current which can be up to several amps in very high performance filters (and may be even greater in certain single line designs).

MPE’s proprietary low leakage filter designs have no direct capacitance from live to earth but rather have capacitance from live to neutral and from neutral to earth.

This means that the continuous earth leakage current depends only on the neutral to earth voltage, and may be as little as 15mA per N-E volt at 50Hz. In practice, the value achieved may be somewhat higher depending on the harmonic content of the neutral waveform.

Low leakage filters are intrinsically safer than conventional filters because of their lower earth leakage current, and because the leakage current is derived from a low source potential.

All of our ranges of low leakage filters incorporate our modern current compensated choke design, as previously described.

To ensure that low leakage filters operate correctly, they must be used on a supply with a neutral, and terminal polarity must be observed. The neutral should not be earthed on the load side of the filter.
N.B. Conventional three phase filters will also provide a reasonably low earth leakage current under normal operating conditions due to cancelling of the earth leakage currents from the three phases. They will not, however, share the enhanced safety benefits inherent in our low leakage designs.


In all 400Hz filters used on 400Hz, leakage current is higher than for 50Hz filters because of the increased operating frequency. Where generators are used, it should be remembered that the generator must supply not only the load current but also the filter reactive (leakage) current. Consideration should be given to any harmonics on the 400Hz supply which may cause additional heating within filters.

In addition to using low loss capacitors to minimise heat dissipation, our ranges of filters suitable for 400Hz incorporate current compensating inductors as previously described. This gives the added benefit of reduced capacitance values and consequently lower leakage currents to achieve a given performance, compared to single line filter designs. It is not normally recommended that very high performance filters offering 100dB from as low as 10kHz are used on 400Hz supplies. This is because any harmonics present on the 400Hz supply can be in the stop band of the filter and can cause overheating.


Our unique ranges of very high current filters up to
2400A offer the benefits of current compensating inductors. For filters of such a high current rating, they offer exceptional performance, and have an extremely low heat dissipation. This provides very low running costs and high reliability.

The busbar connections may either be drilled or clamped, to ensure low resistance power connections are made. Because of the very high current ratings, it is important that all conductors pass through a common cable entry point to avoid eddy current heating effects.

In view of the specialist nature of these filters, customised terminal chambers can be supplied to interface to individual installations.


All filters detailed in this catalogue are high performance types designed to offer in excess of 100dB insertion loss to well beyond 10GHz in most cases.

Insertion loss performance quoted in this catalogue has been measured in the asymmetric mode in a
50 ohm system, generally in accordance with BS6299 (CISPR17). Most filters will also give a substantial symmetric mode performance.

For single phase filters incorporating current compensating inductors, measurement is carried out on the two lines together (with current flowing through both for load measurements, as would be the case in practice).

For three phase filter measurements, all input lines are connected in parallel via capacitors. The output is connected to each line in turn in parallel with neutral. This gives comparable results to a single phase filter with the same component values.

Typical measured performance figures are quoted in this catalogue. For higher performance filters, the measured values quoted may be restricted by test equipment limitations. Special measurement techniques have shown the true performance to be between 120dB and 140dB in many cases.

All new filter designs are empirically checked for insertion loss up to 4GHz. Sample tests at 10GHz and 18GHz by independent test authorities have verified that full 100dB performance has been maintained even at these very high frequencies, and good performance is still maintained at 40GHz. Please ask for details.

Integrity of production filters is guaranteed by 100% testing for capacitance, power factor, inductance, voltage proof, insulation resistance, and volt drop.


It must be recognised that insertion loss measurements made in a 50 ohm system (while giving good guidance and comparative performance figures) may differ from those achieved in practical situations. This is because although mains supplies are assumed to be 50 ohm impedance as far as RFI is concerned, in practice the terminating impedances can be somewhat different from this. In fact, the source impedance is likely to be much less than 50 ohms at the lower frequencies.

BS6299 (CISPR 17) recognises this fact and suggests a second insertion loss test with 0.1 ohm source and 100 ohm load impedances. Although closer to many real system impedances, even this test cannot represent every practical situation so test results should only be used for comparative guidance.

Some filters in this catalogue (see page 26) have capacitive input and inductive output. The normal method of connection would be to use the capacitive input side towards the incoming mains to provide excellent high frequency transient response.

However, by connecting the filter with the inductive end facing the incoming mains, a better low frequency insertion loss may be provided in some practical situations such as where the source impedance is very low. This effect is also shown in the above graph.

More guidance on insertion loss in relation to system impedance can be given where actual impedances can be related to a particular filter.

Alternatively, custom filters can be designed to give optimum performance in a known system impedance.


All ranges of filters within this catalogue are of rugged construction.

The rectangular ranges of filters are housed in electro-tinned steel cases and are finished in light grey military grade paint.
To minimise any radiation coupling, input and output terminals on all rectangular filters are physically isolated within RFI tight terminal compartments. The lids to these compartments are supplied with both RFI and resilient gaskets to provide excellent protection against radiated interference. Lids protecting the internal filter circuitry are secured with blind rivets and then completely solder sealed to provide maximum RF integrity.

The stud terminals on all filters are bright nickel plated and are supplied with nuts and washers.


For optimum EMI performance, proper mounting of any filter is essential.

It is important to ensure as low as possible earth bond impedance to the unpainted base or mounting flange of the filter. This is necessary to obtain the best insertion loss from the filter, and also to carry away high pulse currents in transient suppressed filters.

We would normally recommend that filters are mounted on a steel surface which has been electroplated with tin or zinc. This should be unpainted and must be flat and smooth. Whilst other materials and finishes may be acceptable, the user should give consideration to the shielding and earth bonding properties and possible galvanic corrosion effects of any materials used. In most cases, “conductive paint” finishes are unacceptable as they do not permit a sufficiently good earth bond to be made. In any EMI filter, poor earth bonding will result in reduced insertion loss and could compromise safety.

On rectangular filters, the user must ensure complete screening around all conduit entries and mounting screws. “Blackened” screws and washers should not be used.

Where penetration tubes are used to pass cables through the base of the filter and the mounting surface, they must also provide a complete RF seal. Commercial conduit fittings are of variable quality and are generally unsuitable to fulfil this requirement due to badly fitting threads. MPE’s dedicated fixing kits have been introduced to overcome this problem and are detailed within this catalogue (see page 36).

Rectangular filters can be supplied with different cable entry hole positions to suit alternative mounting arrangements. These are illustrated on page 37.

Filter sizes and cable entries are designed to be a minimum for the filter current rating. The user is advised to verify that the standard filter terminal compartment and cable entry sizes are suitable for his application, especially where oversize cables may be used to minimise cable volt drop.

On rectangular filters, proper fitting of terminal compartment lids and gaskets is important, as shown in detail on page 37. Gaskets must be fitted in the specified positions. All lid screws must be fitted and tightened to the specified torque.

All conductors should pass through single cable entries in accordance with the IET regulations (BS…..) to avoid eddy current heating effects.


Use of Filters with RCCD’s

All high performance power line filters incorporate comparatively large values of capacitance. This means that an RCCD cannot be used ahead of the filter as the filter leakage current will cause the RCCD to trip.

This applies even to low leakage filters which may still trip RCCD’s due to inrush current and variable leakage current resulting from harmonics on the neutral supply. RCCD’s will normally function correctly when connected on the load side of the filter.


There are many applications where one type of filter design may be more effective than another or where special constraints may apply due to system requirements. Examples might be back-fed systems, unusual working voltage, current, or frequency, special system impedance etc.

In such cases, please consult us for advice on the selection of the most suitable filter design.


All high performance filters contain capacitors which will store charge even after the power has been removed from the filter.
All of the filters listed in this catalogue are fitted with internal discharge resistor networks.