Leakage current is an important specification of power line filters. There has always been an undeserved negative connotation to this term. Leakage current is not a function of the quality of the components, but is a direct function of the line-to-ground capacitance value. The larger the capacitance, the lower the impedance to Common Mode currents, and the greater the Common Mode interference rejection. Hence, leakage current is a measure of filter performance--the higher the better. Why, then, do safety agencies specify a maximum allowable leakage current? This is done in order to limit the magnitude of expected ground return currents. The line-to-ground capacitors provide a path for 50/60Hz current to flow to the chassis. As long as the equipment is grounded, these currents will flow in the ground circuit and present no hazard. However, in the unlikely but always possible circumstance where the ground circuit is faulty, the earth connection may be established by the body of a person . If this should occur, the maximum leakage current specification limits the ground return current to a safe value, typically 0.5 to 5.0mA. The limits set by safety agencies are based on end user equipment specification, such as those given below.
Capacitive Current Limits
Limits for Class 1
Country
Specification
Grounded Equipment
U.S.A.
UL 478
UL 1283
5.0mA, 120V, 60Hz
.5-3.5mA, 120V, 60Hz
Canada
C22.2 No. 1
5.0mA, 120V, 60Hz
Switzerland
SEV 1054-1
IEC 335-1
.75mA, 250V, 50Hz
Germany
VDE 0804
3.5mA, 250V, 50Hz
Source: CISPR Publication 9 Since the largest component of leakage current is usually from the power line filter, it is prudent to set a maximum leakage current limit for the filter itself. There has been a tendency in the industry to specify the minimum leakage current to comply with all agency requirements, usually 0.5mA. This specification decision should not be made arbitrarily, because often the size and cost of the filter can be reduced by allowing a greater maximum leakage current.
Leakage Current Measurement
The circuit of Figure 2 (see attachment) illustrates the measurement technique for leakage current. The leakage limits apply to each side of the line independently. The test circuit provides the correct value by shunting the line-to-ground path that is not being measure d by the millimeter impedance. This test is realistic because power to a system is provided by a hot line and a neutral line, with the neutral basically at ground potential, thus providing no addition to the leakage. Note that leakage current is directly proportional to line voltage and frequency. Hence, it is unwise to specify an operating frequency greater than 60Hz (e.g. 400Hz) when leakage current limits must also be met. |
