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Leakage currents in power line filters

Posted: 22 Aug 2001     Print Version  Bookmark and Share

Keywords:power line filters 

EMC SOLUTIONS

- THAT WORK !

APPLICATIONS NOTES

LEAKAGE CURRENTS IN

POWER LINE FILTERS

MPE applications notes are provided for guidance only and are 8888MPE Limited

MPE Limited,

Hammond Road,

Knowsley Industrial Park,

Liverpool L33 7UL, UK

Tel: +44 (0)151 547 2600 Fax: +44 (0)151 546 4642

E-Mail: sales@mpe.co.uk Web Site: www.mpe.co.uk

Approved to ISO9001

/PDF document

-1-

EMC SOLUTIONS

- THAT WORK !

APPLICATIONS NOTES

LEAKAGE CURRENTS IN

POWER LINE FILTERS

MPE applications notes are provided for guidance only and are 8888MPE Limited

MPE Limited,

Hammond Road,

Knowsley Industrial Park,

Liverpool L33 7UL, UK

Tel: +44 (0)151 547 2600 Fax: +44 (0)151 546 4642

E-Mail: sales@mpe.co.uk Web Site: www.mpe.co.uk

Approved to ISO9001

-2-

APPLICATIONS NOTES

LEAKAGE CURRENTS IN POWER LINE FILTERS

All power line filters normally have capacitance from live to earth and from neutral to earth in

order to achieve a high level of asymmetric (common mode) attenuation performance.

Single Phase Filters

The capacitance from live to earth will conduct a leakage current to earth of 75mA per uF

when used on a 240V 50Hz supply. As most high performance power line filters will have a

capacitance to earth of between 10uF and 100uF, it can be seen that leakage currents in the

order of several amps can be expected to flow through the filter capacitors. This is why it is

essential that this type of filter is solidly and permanently earthed to safely carry away these

leakage currents.

Three Phase Filters

In the case of three phase filters, the leakage current is from each phase to earth, and because

the leakage currents will be 120 degrees out of phase, they will largely cancel. The resultant

leakage current will depend on how well the filter capacitors and the phase voltages are

matched. For MPE's 3 phase filters, the resultant leakage current is normally less than 5mA

per uF on each live phase.

400Hz Filters

Because of the higher supply frequency, leakage current in line to earth capacitors will be

290mA per uF for filters operated at 115V 400Hz. As above, there will be some cancellation

in three phase filters to give a resultant leakage current in the order of 20mA per uF for MPE

filters. However, it must be remembered that the generator still has to supply the leakage

current for each phase as well as its load.

-3-

Filters for Portable Equipment

Some specifications for portable equipment, which may be connected via a plug and socket

where the earth integrity cannot be guaranteed, limit the filter capacitance to 5nF. This will

limit the leakage current to a much lower level of about 0.4mA but, of course, the filter

performance attainable from such filters is greatly reduced because of the capacitance

limitation. Other specifications define a maximum permissible leakage current but this again

will restrict the filter performance.

Low Leakage Filters

MPE's Low Leakage Filters have no capacitance from live to earth. In this case the leakage

current only flows through the neutral to earth capacitors and so its magnitude is dependent

on the capacitance value, the neutral to earth voltage, and the neutral voltage waveform.

The leakage current is 0.314mA per uF per volt on neutral for a 50Hz waveform. As most

neutral waveforms have a significant harmonic content, the actual leakage current will be

greater than this.

Because the neutral voltage and waveform can not easily be controlled by the user, the precise

leakage current can not be defined. However, the main benefit of low leakage filters is that

the leakage current is derived from a low source potential i.e. neutral voltage. This type of

filter is therefore recommended where safety considerations are paramount.

It should be remembered that RCCB's can not be used on the supply side of even low leakage

filters because of switch-on inrush currents and variable harmonic content on the neutral

waveform.

8MPE 2000 Liverpool

pdfappnotes\leakage\iss2





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