Leakage Current Measurement Basics


Time:

2023-12-12

In any electrical setup, a portion of the current inevitably travels through the protective ground conductor to reach the ground, commonly known as leakage current. This flow primarily occurs within the insulation encompassing conductors and in the filters safeguarding electronic devices in homes or offices. However, this raises an issue. In circuits safeguarded by GFCIs (Ground Fault Circuit Interrupters), leakage current can lead to unnecessary and sporadic tripping. In more severe instances, it might even elevate the voltage on accessible conductive parts.

 

Leakage Current Clamp Meters

 

The Causes of Leakage Current

 

The origins of leakage current lie in the characteristics of insulation, which possesses both electrical resistance and capacitance, facilitating current flow along both paths. Normally, due to the high resistance of insulation, the leakage of current is minimal. Yet, if the insulation is aged or impaired, its resistance decreases, allowing for a significant current flow. Moreover, longer conductors exhibit higher capacitance, thereby resulting in increased leakage current. Hence, manufacturers of GFCI breakers suggest limiting one-way feeder length to a maximum of 250 feet to mitigate this effect.

 

Additionally, electronic devices incorporate filters intended to shield against voltage spikes and other disturbances. These filters commonly feature capacitors at the input, contributing to the overall capacitance of the wiring system and consequently augmenting the overall level of leakage current.

 

 

Minimizing the Effects of Leakage Current

 

So, how can you reduce or mitigate the effects of leakage current? The first step is to quantify the leakage current and identify its source. One method to achieve this is by utilizing a leakage current clamp meter. These meters resemble clamp meters used for measuring load currents but offer superior performance when measuring currents below 5 mA, a range that most standard clamp meters cannot accurately register.

 

When the jaws of a clamp meter are positioned around a conductor, the current it detects relies on the strength of the alternating electromagnetic field surrounding the conductors.

 

For precise measurement of low current levels, it's crucial to protect the mating faces of the jaws from damage, ensure they remain clean, and are firmly closed together without any air gap during testing. Avoiding the twisting of the clamp meter's jaws is important to prevent inaccurate measurements.

 

The clamp meter identifies the magnetic field encompassing conductors such as a single-core cable, wire armor cable, a water pipe, etc. It can also measure paired phase and neutral conductors of a single-phase circuit or all live conductors (3-wire or 4-wire) in a three-phase circuit, like a GFCI or residual current device.

 

In cases where grouped live conductors cancel each other's magnetic fields due to load currents, any residual current signifies leakage from the conductors to ground or elsewhere. A leakage clamp meter should be capable of measuring this current, typically less than 0.1 mA.

 

For instance, on a 240 V AC circuit with all loads disconnected, a measurement might reveal 0.02 A (20 mA) of leakage. This value indicates an insulation impedance of:

 

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

 

Testing insulation on a powered-down circuit would yield a result around 50 MW or more due to the DC voltage used in insulation testing, which doesn't factor in the capacitive effect. The insulation impedance value represents the actual condition under normal operational circumstances.

 

When testing the same circuit loaded with office equipment (e.g., PCs, monitors, copiers), the result differs significantly due to the input filters' capacitance in these devices. With multiple pieces of equipment operating on a circuit, the cumulative effect can lead to higher leakage current, possibly in the order of milliamps. This increased leakage might trip a GFCI, especially as the amount of leakage varies based on the equipment's operation, causing intermittent tripping issues that are challenging to diagnose.

 

A clamp meter can detect and measure a broad range of alternating or changing currents passing through a tested conductor. However, when telecommunications equipment is present, the indicated leakage measured by a clamp meter might surpass that resulting from insulation impedance at 60 Hz. This discrepancy occurs because telecommunications equipment often includes filters generating functional grounding currents and other devices producing harmonics. To measure characteristic leakage at 60 Hz accurately, a clamp meter with a narrow band-pass filter designed to eliminate currents at other frequencies is necessary.

 

Measuring Leakage Current to Ground

 

When the load is active (turned on), the measured leakage current encompasses both the leakage in the load equipment and potentially in the circuit wiring. If the detected leakage with the load active remains within acceptable limits, it indicates that the circuit wiring's leakage alone is likely even lower. To specifically measure circuit wiring leakage, it's necessary to disconnect (switch off) the load.

 

For testing single-phase circuits, use the clamp meter to encircle both the phase and neutral conductors. The recorded value represents any current traveling towards the ground.

 

In the case of three-phase circuits, wrap the clamp meter around all three-phase conductors. If there's a neutral conductor, ensure it's also enclosed within the clamp. The resulting measured value indicates any current flowing towards the ground.

 

Measuring Leakage Current in Various Scenarios

 

To assess the total leakage directed towards the intended ground connection, position the clamp around the ground conductor.

 

Measuring Leakage Current to Ground through Unintentional Paths

 

Clamping the phase, neutral, and ground together helps identify imbalance current, pinpointing leakage at outlets or electrical panels via unintended paths to ground (e.g., a panel sitting on a concrete base). Similar imbalances may occur if other electrical bonding connections exist, such as a link to a water pipe.

 

Tracing the Source of Leakage Current

 

Conducting a sequence of measurements aids in determining overall leakage and its origins. Initiate the first measurement on the primary conductor leading to the panel, followed by subsequent measurements (2, 3, 4, and 5) to pinpoint circuits with higher levels of leakage current.

 

In Summary

 

Leakage current serves as an indicator of conductor insulation efficiency. Circuits employing electronic equipment with filters may exhibit elevated leakage current levels, potentially causing disruptions in equipment operation due to increased voltages. Employing a low-current leakage clamp for systematic measurements, as described above, facilitates the identification of leakage sources. This information enables a redistribution of loads within the installation for better balance if required.