Overcurrent Devices and Protection

serving the Ottawa area for over 30 years!


Any device that will automatically open an electrical circuit during a short circuit are called overcurrent devices. Typically, these are breakers or fuses.



A device which will automatically open a circuit during an overload but not necessarily during a short circuit condition is called an overload device. Exercise caution in applying these terms overcurrent and overload. Even though fuses and circuit breakers will open the circuit during periods of overload, they are overcurrent devices because they are capable of interrupting short circuits. The critical difference is the term “short circuit.”


Fuses are devices that come in various forms. The small screw in type (Edison base) is the most familiar. Larger cartridge and knife blade types perform the same function but have a more extensive type range. Fuses interrupt from 10 000 to 250 000 A. That last figure is correct: 250 000 A. Latest developments have pushed these levels to around the 300 000 A range.

Don’t confuse their interrupting capacity with their rated capacity, however. The rated capacity is the maximum current that the fuse will allow the circuit to carry without opening. A 15A fuse may also have an interrupting capacity of 200 000 A. These fuses are called HRC fuses. HRC stands for “high rupture capacity.” Fuses with this type of capability could get found in residence; however, it would be rare. Fault currents in homes rarely exceed 10 000 A.


Circuit breakers are the most modern overcurrent protection found in homes. These devices routinely have operating characteristics in the 10 000 A range. Although not near as high an interrupting capacity as the fuse, it is entirely satisfactory for residential circuit protection. The popularity of the breaker over the fuse is that there is less chance of tampering by the homeowner and rarely will you find oversized breakers. A word of caution: exercise great care when inspecting breaker panels. Although seldom done by the homeowner, it is easy to change a breaker. If the new breaker is oversized, it is as dangerous as an oversized fuse.

Did you know that the breaker is designed as a “one-time” device? No one knows the condition of the breaker after a fault. It, like the fuse, should be replaced if this happens.


The ground fault circuit interrupters detect failures in insulation and open the circuit at low current values very quickly. When these devices detect a difference in current flow between the circuit conductors of 0.005 A, they respond by shutting off the circuit within 0.0025 s. Two common GFCI’s are available: receptacle or breaker. Both of these devices must be tested frequently, at least once a month. There is a test button to accomplish this, and a reset button on the device. Test all GFCI’s during your inspection. They must trip, and they must reset. Failure to trip and reset constitutes a hazardous situation and should be reported immediately to the present homeowner for their protection as well as in the report to your client.


Class B GFCI’s are designed to trip when a ground fault of 20 mA or more occurs. These can be adjusted to suit specific criteria, unlike a Class A GFCI. According to the National Electrical Manufacturers’ Standard No. 280, the Class B GFCI would only get used for swimming pool underwater lighting that got installed before May 1965.


A GFCI does not limit the magnitude of the ground-fault current. It limits the time that a ground-fault current will flow. A GFCI does not protect against shock hazard should a person initiate contact with two of the line conductors on the load side of the GFCI.


Arc-fault circuit interrupters get used in branch circuits that supply receptacles installed in the sleeping facilities of a dwelling. An arc-fault device protects from the effects of arc-faults by recognizing characteristics unique to arcing and functions to de-energize the circuit where an arc-fault is detected.

Always seek help from a professional electrician before attempting any work yourself. Contact Mike Fuller Electric in Ottawa today for your overcurrent protection and device inquires.

Tel: (613) 225-3249

Email: [email protected]

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Mike Fuller Electric Ltd.

1692 Ortona Avenue
Ottawa ON

Working hours

Monday-Friday: 7AM-4PM
By appointment on Saturday and Sundays


Tel: (613) 225-3249

Email: [email protected]




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