Fundamentals of leakage current measurement

Application note

In any electrical device, there is a certain amount of current flowing to the ground through a protective ground conductor. This current is generally referred to as the leakage current. The most common way for leakage current is to flow through the insulation around conductors, as well as through filters that protect electronics around your home or office. So what's the problem? In circuits protected by leakage protectors, leakage currents can cause unnecessary, intermittent trips. In the case of poles, the voltage of the parts that form a palpable conductor increases.

Causes of leakage of power supply

Insulation is both resistive and capacitive – through which current is conducted. Due to the high resistance of the insulation, the leakage current is actually very small. However, if the insulation is aged or damaged, the resistance is low, and a large amount of current may be flowing. In addition, longer conductors have a larger capacitance and can create a larger leakage current. Therefore, leakage protector manufacturers recommend limiting the length of one-way feeders to a maximum of 76.2 m.

At the same time, electrical equipment used to protect against voltage surges and other faults can also increase leakage currents, such as filters. These filters typically have capacitance on the input, which further increases the total capacitance of the cabling system, as well as the total leakage current.

Minimize leakage current

So, what if the leakage current is eliminated or minimized? Quantify leakage currents and then identify the source of the problem. One way is to use a leakage current clamp meter. These clamp meters are very similar to those used to measure load currents, but they are much better when measuring currents below 5mA. Most clamp meters just can't detect such a small current.

After clamping the jaws of the clamp meter to the conductor, the current value read by the clamp meter depends on the strength of the magnetic field around the conductor.

In order to accurately measure small currents, it is necessary to ensure that the jaws are clean and that there are no gaps or damage to the occlusal surface. Prevent twisting the jaws of the clamp table, otherwise it will form a measurement error.

power supply

load

The clamp meter detects the magnetic field around conductors, such as single-core cables, armoured cables, water pipes, etc., or the phase and neutral wires of single-phase circuits, or the full live conductors of three-phase circuits such as leakage protectors or residual current devices.

When testing the grouped live conductors of a circuit, the magnetic fields generated by the load currents cancel each other out. Any unbalanced current comes from the current that the conductor leaks to earth or elsewhere. To measure this current, the leakage current clamp should be able to detect currents less than 0.1 mA.

For example, if measured on a communication 240 V circuit, all loads are disconnected, and a leakage current of 0.02 mA (20 μA) may be formed. This value indicates that the insulation impedance is:

240 V / （20 x 10-6） = 12 MΩ。 (Ohm's Law R=V/I)

If the insulation test is performed on a circuit that is de-energized, the results may be in the range of 50 MΩ or more. This is due to the fact that the insulation tester uses DC voltage for testing without considering the capacitive effect. The insulation impedance value is the actual value that exists under normal operating conditions.

If the same circuit is measured with a load on office equipment (PCs, monitors, copiers, etc.), the results may be significantly different due to the capacitance on the input filters of these devices. When there are multiple devices on a circuit, the effect will accumulate together. In other words, the leakage current is higher, perhaps in the milliampere range. If a new device is added to a circuit with leakage protector protection, the leakage protector may be triggered. And because the total amount of leakage current varies depending on how the device operates, the leakage protector may trip randomly. This intermittent condition is very difficult to diagnose.

Clamp meters can detect and measure a wide range of communication or change currents through the conductor under test. In the presence of telecommunications equipment, the leakage current value indicated by the clamp meter may be much higher than the result of the insulation impedance occurrence at 60 Hz. This is due to the fact that telecommunications equipment often uses filters to generate functional ground currents and harmonics. If a clamp meter is used to filter out currents at other frequencies through a narrow-band bandpass filter, only the characteristic leakage current at 60 Hz can be measured.

Measure the leakage current to the ground

When a load is connected (switch flipped), the measured leakage current includes the leakage current of the load device. If the leakage current at load is acceptably low, then the line leakage current is even lower. If a separate line leakage current is required, the load is disconnected (switched off).

summary

Leakage current is an important indication of the effectiveness of insulation on a conductor. When an electrical device with a filter is connected, a high leakage current will appear when the circuit is flashed, which can cause a voltage that disturbs the normal operation of the device. Using a low-current leakage current clamp meter to perform the above measurements in an orderly manner, it may be possible to locate the source of the leakage current. If necessary, the load around the equipment can be redistributed accordingly to make it more balanced.

Fundamentals of leakage current measurement

Fundamentals of leakage current measurement

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