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Filter General


A mains filter (or line filter) is an electrical circuit which reduces both electrical disturbances of electronic equipment into the public mains supply (radio interference suppression) and the electromagnetic compatibility EMC of electrical equipment against disturbances from the power supply system (rise of the interference immunity).


Mains filters consist of resistors, chokes and capacitors. Current compensated chokes (suppress well asymmetrical or common-mode currents) are combined with uncompensated chokes (suppress symmetrical currents).

Special interference suppressing capacitors are used in mains filters. X capacitors suppress push-pull interfering voltages, Y capacitors suppress common mode interference. Both X and Y capacitors are self-healing, i.e. they do not cause any short circuit at inner electrical breakdowns.

Interactions with Residual Current Device (RCD)

The two Y capacitors at the lines of single-phase equipment of the safety class I (equipment with protective earth terminal connection or protection line) cause an increased leakage current: the full mains voltage against earth or protection line is connected to one of the capacitors, an idle current may flow of 0.5 mA (medical equipment) or 3.5 mA (other equipment of the safety class I) or 5 mA (industry assemblies) against ground.

If several equipment are operated with mains filter at a Residual Current Protective Device (RCD), the leakage currents possibly cumulates to a value above the turn off threshold (typ. 30 mA) and it triggers. This difference current is particularly high switching on in the tension maximum of the mains AC voltage, RCDs are therefore offered with a delayed response behavior.


Jon B's picture
Jon B
Hi I'm a little new to this and trying to gather information regarding electrical risks due to possible PE chassis fault and what the nock on effects could be.
Question 1: If say a machine containing 3x AKD drives, connected to 400v three phase supply and the chassis of the machine was isolated from Earth (apart from the PE terminal of supply). If the PE terminal to the machine was damaged and there was no route for earth leakage currents to pass and then say a person now touched the chassis of the machine would they likely receive a fatal electric shock from the earth leakage current passing through them to earth?  Question 2: What is the likely voltage potential of the earth leakage from a AKD drive running at x3 400V AC @ 50Hz?
jcoleman02's picture
My understanding (and I'm not an expert in electrocution) is there are some factors involved, such as the impedance of the body, path to ground, etc.  I'm not for a second going to recommend allowing this to happen.  The drive and machine must be grounded for safe operation.  But if this did happen as you describe, there is no guarantee that it would be fatal, although it definitely could be fatal.  It will be quite painful at best.  "Don't do that!!!" is the proper answer.
Jon B's picture
Jon B
Thank you for the answer to Question 1. I believe the general rule of thumb is any electric shock could cause heart fibrillation or even heart failure however, sorry to labor the point but this is then a really big deal for all machines as one single earth fault could in theory cause death? What solutions should be implemented to increase SIL protection for this issue (ie make it so it takes at least two simultaneous failures that could cause electrocution rather than just one causing electrocution).
Do you have answer to Question 2: What is the likely voltage potential of the earth leakage from a AKD drive running at x3 400V AC @ 50Hz?
Many Thanks

jcoleman02's picture
I don't have an answer to Question 2.  I'm not sure we can give a definitive answer for it.
As far as protection against shock, if you wire the drive in the recommended way, there will be multiple connections to ground:
1. PE with mains power
2. Chassis ground through back panel of cabinet
3. Chassis ground through ground lug