EMC Filter Earth Leakage Specifications
Earth 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.
It is useful to be aware of some of the techniques and issues involved in order to be able to make a more informed choice. Earth leakage for EMC filters is declared in the datasheet and enables customers to compare filters. However it is a theoretical value 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 FILTER DESIGN CONSTRAINTS
EMC filters are produced using deceptively simple combinations of inductors, capacitors and resistors. “Deceptively” because 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.
SOURCE OF EARTH LEAKAGE
However in order to reduce both common mode 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 a rule of thumb, and ignoring parasitic effects, Y capacitors attenuate common mode noise and X capacitors do the same for differential mode noise. Therefore the Y capacitors and common mode chokes work together to attenuate common mode noise. Chokes pass all of the fundamental current so tend to provide less attenuation per unit of cost and it becomes interesting to use larger and larger Y capacitors in order to achieve the required performance. Here is where some trouble can begin, which for the purposes of this article come in the guise of earth leakage current, but can also affect residual voltage, inrush and flash testing.
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 produces the same unbalanced current as would current flowing through a person to earth and it is electrocution that the RCD is present to prevent.
ISSUES ASSOCIATED WITH EARTH LEAKGE
A typical personal protection RCD will trip at 30mA earth leakage, they are not too accurate so we use the rule of thumb that the system should have a total earth leakage of less than 16mA to work reliably with all of the many RCD brands available. If the system contains a filter that has leakage already built in then there is not much left for all of the other components. The user must choose a low leakage filter which could be more expensive or lower performance or both but before any of those considerations they must navigate through the differing ways of specifying earth leakage before they can make a meaningful comparison.
COMPARING EARTH LEAKAGE VALUES
This effect is at its most complex in 3 phase only systems. The earth leakage quoted is invariably calculated using a set of assumptions about the applications in which the filter may be used. In theory when the phases are perfectly balanced and all capacitors are identical then the earth leakage will be zero. However neither of these two criteria are ever met in reality. In past years Roxburgh EMC answered this question by quoting single-fault condition leakage which is the worst possible i.e. the leakage with one phase disconnected. This gives a very high figure which is unhelpful with regard to RCD operation as it does not tell the user anything about filter leakage under normal operation. Today, we use an assumed worst case phase imbalance of 3% gained from the supply network standard EN50160 and high capacitor tolerance. This gives a more realistic but again worst case earth leakage.
However, this also is to change since UL have recently released the semi-harmonised filter standard UL60939-3 which states that since EN 50160-2000-03 states that “ac values of the negative phase sequence of the supply voltage shall be within the range 0% to 2% of the positive phase sequence component”, the leakage current is thus determined with a 2% unbalance of the network. Furthermore capacitor tolerance considerations are not taken into account. This means that our stated leakage current figures will decrease in the future for Roxburgh EMC filters with no changes having been made to the components.
If you have any questions about leakage currents please contact us:
Call: 01724 273200
E-mail: sales@dem-uk.com
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