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Leakage Current Measurement Basics

In any electrical installation, a portion of current will naturally flow through the protective ground conductor to ground, often referred to as leakage current. This current commonly travels through the insulation around conductors and the filters protecting electronic devices in homes or offices. The issue arises when leakage current causes unnecessary and intermittent tripping in Ground Fault Current Interrupters (GFCIs). In severe cases, it can increase voltage on accessible conductive parts.

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The causes of leakage current

Insulation possesses both electrical resistance and capacitance, allowing it to conduct current. Typically, the high resistance of insulation ensures minimal current leakage. However, if the insulation is damaged or aged, its resistance decreases, potentially allowing significant current flow. Longer conductors also have higher capacitance, leading to increased leakage current. Therefore, GFCI breaker manufacturers recommend a one-way feeder length not exceeding 250 feet.

Moreover, electronic equipment contains filters to protect against voltage surges and disruptions, which typically include capacitors. These capacitors add to the overall capacitance of the wiring system, thereby increasing the level of leakage current.

Minimizing the effects of leakage current

How can one eliminate or minimize leakage current effects? The process involves quantifying the leakage current and identifying its source. A leakage current clamp meter is useful for this purpose. Unlike standard clamp meters, these meters are optimized for measuring currents below 5 mA, a range most standard meters cannot accurately register.

Using a Leakage Current Clamp Meter

When placing the jaws of a clamp meter around a conductor, the current value read depends on the strength of the alternating electromagnetic field surrounding the conductors. To measure low current levels accurately, keep the jaws' mating faces clean, undamaged, and fully closed without any air gaps. Avoid twisting the jaws to prevent erroneous measurements.

The clamp meter detects the magnetic field around conductors such as a single-core cable, wire armor cable, water pipe, or the paired phase and neutral conductors of a single-phase circuit. It can also detect the magnetic fields produced by all live conductors of a three-phase circuit (like a GFCI or residual current device).

Measurement Accuracy

When testing grouped live conductors, the magnetic fields from the load currents cancel each other out. Any imbalance current results from leakage to ground or other points. A leakage clamp meter capable of reading less than 0.1 mA is required for accurate measurement.

Example Calculation

For example, measuring a 240 V ac circuit with all loads disconnected may yield a leakage value of 0.02 A (20 mA). This represents an insulation impedance of:

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

If an insulation test were conducted on a powered-down circuit, the result might be around 50 MΩ or higher. This variation occurs because the insulation tester uses a DC voltage, which does not consider capacitive effects. The insulation impedance value reflects actual conditions under normal operation.

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Leakage Current in Loaded Circuits

If you measure the same circuit loaded with office equipment (PCs, monitors, copiers, etc.), the result will differ significantly due to the capacitors in these devices' input filters. The cumulative effect of many devices operating on the circuit increases leakage current, potentially causing the GFCI to trip. Leakage current fluctuations, dependent on equipment operation, may lead to random GFCI trips, complicating diagnosis.

A clamp meter can detect and measure various alternating or changing currents passing through a conductor. Telecommunications equipment often shows higher leakage values on the meter due to functional grounding currents, harmonics, and other factors. To measure characteristic leakage at 60 Hz, use a clamp meter with a narrow band-pass filter to eliminate other frequency currents.

Measurement of leakage currents to ground

Loaded Circuit Measurement

When the load is connected and switched on, measured leakage includes currents in the load equipment. If this leakage is low, circuit wiring leakage is even lower. To measure circuit wiring leakage alone, disconnect or switch off the load.

Single-Phase Circuits

For single-phase circuits, clamp both phase and neutral conductors to measure any current flowing to ground.

Three-Phase Circuits

For three-phase circuits, clamp all three-phase conductors. If a neutral is present, include it in the clamp. Measure any current flowing to ground.

Measuring Total Leakage

To measure total leakage flowing to ground, place the clamp around the ground conductor.

Tracing the source of leakage current

This method involves a sequence of measurements to identify overall leakage and its source. Start by measuring at the main conductor to the panel, followed by subsequent measurements on circuits carrying significant leakage.

Summary

Leakage current serves as an indicator of the insulation effectiveness on conductors. High leakage levels may arise in circuits using electronic equipment with filters, potentially causing disruptions in normal equipment operation. By using a low current leakage clamp meter for systematic measurements, you can locate the source of the leakage current. This allows for load redistribution to balance the installation effectively.

Can someone recommend a leakage tester?

I need to measure ground fault leakage in various 120V 15A plug-in appliances under operating conditions. Searching hasn't yielded the exact device I need, which should be able to plug into a receptacle and measure leakage current in an inserted appliance. We often deal with portable power and run into defective appliances that trip GFIs, leading to situations where event rental companies blame our power supply. I wish to identify and prove the cause of GFI tripping.

If a portable instrument isn't available, I'm open to a panel mount instrument for a box solution. Is there an ultra-sensitive CT and panel meter to pass the hot and neutral through for accurate leakage current measurement?

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