Ground fault protection detects current leaking out of a circuit to ground and shuts the circuit off. There are two very different kinds. A ground fault circuit interrupter (GFCI) trips at a few milliamperes to protect people from electric shock. Ground fault protection of equipment (GFP) trips at much higher currents to protect large services and feeders from damaging arcing faults. Both work on the same principle, but they are not interchangeable.
What a ground fault is
In a healthy circuit, all the current that goes out on the line conductors comes back on the line or neutral conductors. A ground fault happens when some current finds another path to ground: through damaged insulation to a metal enclosure, through water, or through a person touching a live part while standing on a grounded surface.
A normal breaker or fuse responds to overcurrent. It will not trip on a small leakage current, and it may not trip quickly on an arcing ground fault, because the arc's own resistance limits the current to a level that looks like a normal load. That is the gap ground fault protection fills.
How the sensing works
Both GFCIs and GFP use the same basic idea. A current sensor surrounds all the current-carrying conductors of the circuit. If the currents going out and coming back are equal, their magnetic fields cancel and the sensor sees nothing. If some current leaves through another path, the sensor sees the difference and trips the device.
GFCI: protecting people
A GFCI is designed to trip on a leakage current small enough that it is unlikely to cause a fatal shock, in a time short enough to matter. Class A GFCIs, the type used for personnel protection in Canada, trip at around 5 milliamperes. That is far below the current needed to trip a normal breaker.
GFCIs come as receptacles, as breakers protecting a whole circuit, and as portable units. The Canadian Electrical Code requires them in locations where people are likely to be in contact with water or ground, such as near sinks and bathtubs, outdoors, in some garages and on construction sites, and in other locations set out in the code. The list has grown over successive editions, and provinces adopt editions at different times, so check the current rules for your project.
Common GFCI issues
- Nuisance tripping. Often caused by a real but small leakage, such as moisture in an outdoor box, a failing appliance, or several loads on one GFCI whose small leakages add up.
- Downstream protection. A GFCI receptacle can protect other receptacles downstream on the same circuit. Those receptacles should be labelled, or occupants may not realize they are protected and cannot find the reset.
- Failure without notice. Older GFCIs could fail in a way that left power on with no protection. Newer designs include self-testing, but regular manual tests are still good practice.
GFP: protecting equipment
Ground fault protection of equipment is built into large service and feeder breakers or provided by separate relays. It is designed to detect low-level arcing ground faults that could burn through switchgear without drawing enough current to trip the breaker's normal overcurrent elements. Its pickup settings are measured in amperes or hundreds of amperes, not milliamperes, so it does not protect people from shock.
The Canadian Electrical Code requires GFP on certain solidly grounded services and feeders, generally larger ones at voltages above 150 V to ground, which commonly includes 347/600 V systems. The specific thresholds and exceptions are in the code, and the engineer of record confirms which apply.
Why GFP needs careful design
- Coordination. GFP on the main breaker with no GFP on the feeders can mean a ground fault on one feeder trips the whole building. Feeder-level GFP or careful settings are needed to keep outages small. See selective coordination.
- Neutral arrangement. Where the neutral is grounded in more than one place, for example at a generator and at the service, current can return on unexpected paths and cause false trips or missed faults. Transfer switch and generator neutral switching must be designed with GFP in mind. See grounding and bonding.
- Emergency systems. Codes generally treat ground fault tripping on emergency power differently, often favouring alarms over automatic tripping so that life safety loads stay on. Your engineer will confirm what applies.
GFCI vs GFP at a glance
| GFCI | GFP | |
|---|---|---|
| Protects | People, from shock | Equipment, from arcing faults |
| Trips at | A few milliamperes | Amperes to hundreds of amperes |
| Where it is found | Receptacles and branch circuits | Services, feeders and large switchgear |
| Tested by | Occupants with the test button, plus periodic checks | Qualified testing personnel using injection test equipment |
A typical case
A warehouse has a 347/600 V service with GFP on the main breaker. Twice in a year, the whole building goes dark for no obvious reason, and each time the main resets without trouble. The facility manager suspects a faulty breaker. A review finds two things. First, there is no GFP on the feeders, so a ground fault anywhere in the building trips the main rather than the affected feeder. Second, a standby generator was added some years earlier with its neutral bonded at the generator as well as at the service, giving neutral current a second path that the main's sensor reads as a ground fault when certain loads run. The fix involves correcting the neutral arrangement at the transfer switch and generator, adding feeder-level ground fault protection on the largest feeders, and revising the settings so the feeders trip before the main. The main breaker itself was never the problem.
Testing and maintenance
- Press the test button on GFCI receptacles and breakers on a regular schedule, following the manufacturer's recommendation, and confirm they reset
- Replace any GFCI that does not trip on test, or will not reset
- Label receptacles protected by an upstream GFCI
- Have GFP systems performance-tested when first installed, as the code and manufacturer require, and record the results
- Retest GFP periodically and after any change to the service, neutral or grounding arrangement
- Keep GFP settings in the coordination study settings table and confirm the installed settings match
Do not bypass a tripping device. A GFCI or GFP that trips repeatedly is usually detecting something real. Have a qualified electrician find the cause. Disabling or replacing the device with a plain breaker removes the protection the code requires.
How ehZee can help
Ground fault protection settings are part of our short-circuit and coordination studies, so they work with the rest of the protective devices. We can also review where GFCI protection is needed in a renovation or new design, and how the neutral and grounding arrangement affects GFP.
Common questions
Is a GFCI the same as an AFCI?
No. An arc fault circuit interrupter detects the signature of arcing in wiring, such as a damaged cord or loose connection, to reduce fire risk. A GFCI detects leakage to ground to reduce shock risk. Some devices combine both functions.
Why does our main breaker trip when the generator runs?
This can be a sign that neutral current is returning on an unintended path because of how the generator and transfer switch neutral are arranged, which GFP sees as a ground fault. It needs an engineering review, not a change of settings.
Can a GFCI protect equipment?
It will trip on equipment leakage, but it is designed for personnel protection on branch circuits. Large services and feeders need GFP, which is set for that purpose.