Suction transformer - counters stray current and magnetic fields
In electrical installations with 4 conductors, unfortunately, it often happens that some current does not make its way back via the PEN conductor, but slips past the riser cable via water pipes, ventilation drums or similar metal structures. This is because the current chooses the path that offers the lowest electrical resistance. These so-called vagabond earth currents can cause some trouble because of the low-frequency magnetic fields they give rise to. The magnetic field only decays linearly with distance (in the vicinity of the vagabond cable) and therefore extends far away from the current-carrying conductor. The problems with magnetic fields from stray currents are that they can disturb sensitive equipment, especially monitors, and clearly exceed recommended values for what can be considered acceptable from a health point of view.
Function of the suction transformer
When part of the neutral current wanders, an imbalance of the currents in the 4 conductors of the electric cable occurs, the total current measured around all 4 conductors at the same time, is no longer 0, as it should be in a functioning system with conductors and return conductors in the same cable. In the return conductor of the cable, the neutral, the current that wanders is "missed". A resulting magnetic field forms around the cable. The suction transformer is magnetized by this field and induces a counter EMF (electromotive force) on the cable. This EMF tends to drive current into the cable's PEN conductor, which acts as a sort of suction effect on the vagabond currents. The suction transformer's EMF causes the PEN conductor to have an apparent lower resistance and the return current thus finds that path more active than transmission paths through water pipes and other external metal objects.
If Ivagabond =0, the transformer's EMF = 0 and the suction effect disappears. A certain leakage current outside the PEN conductor is thus required for the suction transformer to function. This leakage current corresponds to what is called magnetizing current for a normal transformer. The suction transformers are dimensioned so that leakage currents and resulting magnetic fields are so low that they do not interfere.
The suction transformer has a very robust construction.
There are three types of suction transformers
The openable one which easily encloses a cable, the solid one where the cable is threaded through the transformer and the complete one which is the most cost-effective.
With a suction transformer, the current goes the right way
Stray current is the most common cause of magnetic fields. Suction transformer is the simplest measure to counter stray current. It helps the neutral current to go out the same way the current came in. The neutral current otherwise escapes via the protective earth from the neutral screw in the power station to the copper pipe and takes the easiest route back to the transformer. In the case of 5-wire systems, the neutral must be isolated from protective ground to prevent stray current. In new builds with a 5-conductor system, incoming electrical service is usually only carried out with a 4-conductor cable and should be supplemented with a suction transformer.
When part of the current goes back the wrong way, that current is missing in the suction transformer. The sum of the currents in all phases and the neutral will normally be zero. But when part of the neutral current goes the wrong way, the net current becomes as high as the one that escapes. It is the net current that magnetizes the suction transformer, which in turn produces a voltage on the entire cable. This voltage makes it easier for the neutral current to go the right way. Each conductor produces its own magnetic field. The sum of all magnetic fields becomes zero when the current is going the right way.
Why buy suction transformers?
1. Simplest measure to remove unnecessary magnetic fields
2. Apply the authorities' precautionary principle
3. Adaptation for electrosensitive personnel and prevent leukemia
4. Usually more economical than changing to an electrical system with 5 conductors, i.e. isolated return conductor
5. Reduces hum interference in hearing aids