Start > emf-reduction > Filters
Ö

Filters to reduce dirty electricity

There are two types of dirt, and filters are only effective against one of them. Filters inserted into wall sockets do not prevent dirt from entering the home; they simply convert it.

The two types of 'dirt' are differential and common-mode, also referred to as symmetrical and asymmetrical interference. Differential mode  interference is easy to reduce, whereas reducing common-mode interference is very difficult.

Practice instead of theory

Most people find it very complicated to understand how it works, so if you already have filters installed in your wall outlets, it's easier to try it out in practice. As well as a radio, it is also a good idea to have a magnetic field meter to check normal magnetic fields. The starting point is our page about detecting dirty electricity and a testing set up as described.

Two patch cords
Testing set up. For the test, you will need two extension cords, a filter and a suitable radio. Connect one extension cord from the wall socket to the filter and the other extension cord to the filter. Use the radio to evaluate the 'dirt' radiating from the cords before and after the filter and a meter if you have one. Test with and without the filter.

Our test results

We tested two meters, one of the radios and an electromagnetic interference (EMI) line filter, which must be installed by an electrician, all of which are described on the 'Test for dirty electricity' page. The testing setup was as shown in the figure above. The test was performed without a load, which may have affected the outcome.

Meters vs. radio

No filter
 After filter
Stetzer
 1700
 60
Alpha Lab
 200
 13
Radio
 Noise
 No change

Filters are effective against differential mode interference, but not common mode interference. The radio reveals this.

Grå kub med 4 kontakter på sidan.
This is a electromagnetic interference (EMI) line filter that should be installed after the electricity meter and before the fuse box. The four contacts indicate that three phases plus neutral are filtered. However, despite its ability to filter out common-mode interference, the outcome is disappointing. It weighs 1.2 kg due to the large iron choke. But that's not enough.

How do the filters work?

There are two fundamental principles for reducing interference.

The differential mode interference

A red and a blue wave representing an symmetric intereference

The interference pattern on the two conductors of a cable is more or less mirrored by the pattern on the other conductor of the same cable. To cancel them out they need to come in contact with each other. The result of a positive voltage and an equal but negative voltage is zero voltage.

This connection can easily be made using a capacitor. This is represented by two thick lines in circuit diagrams. The capacitor acts as a solid conductor to the interference. Voltage drives electric current which starts to flow through the capacitor. The voltage across the capacitor and the two conductors drops to almost zero. Has the problem been resolved? No.

The capacitor is short-circuiting the differential mode interference.

The same current that flows through the capacitor also flows through the conductors, creating increased magnetic fields along the cable. Voltage interference is now replaced with magnetic interference.

The dirty electricity filter, which you plug into the wall socket, works on this principle with a big capacitor creating increased magnetic fields along the cable.

The common mode interference

The interference patterns on the two conductors within a cable are similar, as are the similar positive or negative voltages on each conductor. There is no mirror pattern on the other conductor to cancel the interference out.

The method of attenuating common mode interference involves choking it by allowing its energy to build up a magnetic field in magnetic material using an electric coil. As long as the magnetic field is increasing, the interference cannot pass through. A coil with that function is called a choke. Sadly, the truth is that a choke is not an effective way of attenuating common-mode interference, and there is no other method.  A choke is shown in different ways in circuit diagrams.

Two spirals with two lines in between.
A symbol for a 'choke' in a circuit diagram. Note the two dots to the left. Positioned on the same side they indicate that the choke is wired to reduce common-mode interference. Positioned one to the left and one to the right the choke will reduce differential mode interference..

Conclusion

Dirty electricity is a problem that is difficult to solve once it has occurred. Common mode interference is difficult to mitigate because simple methods are ineffective. Installing capacitors as 'filters' in wall sockets merely shifts the problem from electric fields to magnetic fields within the home. However, this outcome can be avoided by placing a filter where the electricity enters the house or further away, if it is possible to relocate the electricity meter.

In Sweden, the cables up to the electricity meter and the meter itself are owned by the electricity network company, so approval must be obtained from them to relocate the meter. The cables after the electricity meter are owned by the property owner, who can install a line filter at the desired location. This is a job for an electrician.

Test for dirty electricity using radio and meters.

Test for dirty electricity

Explanations
Close Close
Frequency (Hz):
Events occurring during a certain time period. In the electromagnetic world, frequency is expressed in hertz (Hz). It can represent the number of waves or other changes measured per second.
MHz (megahertz)
Million hertz.
GHz (gigahertz)
Billions of hertz.
Radio waves
Electromagnetic radiation used for transmitting radio signals.
Microwaves
Radio waves with a higher frequency, and therefore a higher energy content.
Microwatts per square metre (µW/m2).
The power density or just intensity of radio waves and microwaves. One microwatt is one millionth of a watt.
Microtesla (µT)
The flux density of the magnetic field, often referred to as strength.
Volt per metre (V/m)
The strength of the electric field.
dB (decibels)
Change from a baseline. +1 dB means an increase of one quarter; -1 dB means a decrease of one quarter. +/-3 dB means twice or half as much. Therefore, each 3 dB increase or decrease means a doubling or halving, so +6 dB is quadrupling from the baseline.