Chasing "ghost" trips in GFCI-protected circuits


A ground fault circuit interrupter (GFCI) is one of the most common residential, commercial, and industrial safety devices. The most common types are single-phase electrical outlets used in households near water sources, such as in kitchens, bathrooms, and outdoor receptacles. Most states have required GFCI outlets in certain areas of residential installations for decades. However, NFPA 79, the Electrical Standard for Industrial Machinery, also dictates their use in industrial applications.

Chasing ghost trips in GFCI-protected circuits

GFCI uses and maintenance

GFCIs are designed to prevent bodily harm from electrical faults that could cause electricity to flow through you to ground. When a GFCI breaker trips, it It quickly disconnects the current flowing through an unintended ground path even if the amount of current is too small to trip a typical circuit breaker. Although fuses and circuit breakers are also protective devices, they are designed to protect equipment and facilities from short circuits and large magnitude electrical faults that could cause electrical fires and property damage. The actuating energy for standard breakers and fuses far exceeds the lethal amount, but most GFCIs trip at around 6 mA.

A GFCI uses a current transformer to detect the difference between the line current supplied to the load and the neutral current returning from the load. Ideally, this difference should be zero because both currents cancel. If there is a current differential, it becomes the input to a comparator within the GFCI, which changes states when the differential is around 6 mA. When the comparator changes states, it triggers a silicon-controlled rectifier, which disconnects the power from the output of the GFCI.

GFCIs should be checked monthly by operating the push-buttons on the front labeled "Test" and "Reset." In addition to checking for proper operation, monthly testing also makes sure the mechanical components of the receptacle are in working order. Simple GFCI testers are available for less than $15, or for less than $150, you can purchase a high-quality, industrial electrical tester can check for energized circuits, test AC and DC voltages, resistance, continuity, and phase rotation between any two phases and GFCIs.

What causes a GFCI to trip

When a GFCI outlet keeps tripping, there must be a reason. Instead of just resetting the GFCI, you should also investigate the cause of the trip.

  1. Ground Faults: Ground faults occur when electrical current finds an unintended path to ground. These are often caused by worn insulation, conductive dust, water, or other soft grounds. Ground faults account for more than 80% of equipment short circuits and in 90% of those cases, it is caused by insulation deterioration on wires and cables.
  2. Human Safety Concerns: If a human becomes the unintended path, current as low as 75 mA can trigger ventricular fibrillation, which leads to cardiac arrest. This is a significant safety hazard associated with ground faults.
  3. Leakage Current: Another term for a ground fault is leakage current. All insulators, including wiring insulation, have some conductivity which can cause leakage current. If insulation is old or damaged, its resistance is lower and leakage current could become substantial.
  4. GFCI Trips: On GFCI-protected circuits, leakage current can cause unnecessary and intermittent tripping. When many pieces of equipment are operating on a circuit, the leakage current is cumulative and could cause a GFCI to trip randomly.
  5. Troubleshooting: Looking for the leakage current culprit can be costly and difficult, especially when the GFCI trips are random. Adding more equipment to a GFCI-protected circuit could exacerbate the problem.
  6. Preventive Measures: Instead of just resetting the GFCI when it trips, it's important to investigate the cause of the trip to prevent further electrical problems or potential hazards. Regular checks for insulation deterioration and proper management of circuit loads can help in this regard.

Diagnosing GFCI trips

Start by measuring the leakage current and then identify the source. Use a leakage current clamp meter to make these measurements. Leakage current clamp meters are similar to clamp meters used for measuring load currents; however, leakage current clamp meters perform much better when measuring current below 5 mA.

Test single-phase circuits by clamping the phase and neutral conductors. Test three-phase circuits by clamping around all phase conductors. If a neutral is present, clamp it too. The measured value will be any current flowing to ground. To measure the total leakage flowing to the intended ground connection, place the clamp around the ground conductor.

Measure the leakage current on each leg of the circuit to identify which one has considerably more leakage than the others. If one leg has a suspiciously high leakage current, ensure that the equipment is operating properly. Remember that surge suppression filters and capacitors on the power input of some electronic equipment can increase the overall circuit capacitance, which can increase leakage current. Determine loaded circuit leg leakage with the equipment "on"—switching the equipment "off" allows you to determine just the circuit wiring leakage.

If equipment on all legs is operating properly and the wiring is acceptable, it could be that the cumulative leakage current due to electronic equipment input filtering is just high enough to trigger the random GFCI tripping. In this case, consider redistributing the load on each circuit leg or adding circuits to provide more capacity.

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