TECHNOLOGY IN ACTION ADVERTORIAL EMC Filter Earth Leakage Specifications E
arth leakage specifications are an important consideration when specifying an EMC filter but, as with many technical decisions, the choice is not as simple as picking the
lowest number. Published earth leakage figures are theoretical values calculated according to a set of assumptions as real leakage current depends on many parameters. These assumptions may differ between manufacturers so the headline numbers quoted might not be directly comparable. EMC filters are produced using simple combinations of inductors, capacitors and resistors. The way that the components can be combined to achieve the necessary real world performance is a delicate balance between safety, physical size, cost and manufacturability. These considerations actively impinge on each other with the laws of physics providing fundamental limitations in all directions. One of the most useful laws of physics involved is the fact that the reactance of a capacitor decreases as the value of the capacitance and the signal frequency applied increases. This is very useful in the attenuation of high frequency interference as the unwanted signals can be diverted away from the power lines through the capacitors leaving the low frequency of the mains (almost) unaffected. However in order to reduce both common and differential mode noise capacitors need to have two modes of connection: line to line also known as X capacitors and line to ground also known as Y capacitors. As the Y capacitors are connected to ground and as mains supplies are alternating the reactance of the Y capacitors is not zero and some current flows through them into ground. The larger the Y capacitor the larger the earth leakage current. If the earth leakage becomes too high a residual current device (RCD) will trip cutting power to the installation because leakage through a filter to earth appears the same as leakage through a person – otherwise known as electrocution.
Roxburgh EMC 01724 273208
www.dem-uk.com
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