Overcurrent protection

Lesson 4 chose conductor sizes and the overcurrent device ratings that protect them. This lesson looks at the devices themselves and the rules for applying them: the kinds of fuses and circuit breakers, the voltage, current and interrupting ratings every device carries, where overcurrent protection must be placed, the tap rules that allow some conductors to start without protection, and the location and accessibility requirements that let people operate devices safely. ELEC-110, Lesson 12, explained how fuses and breakers work; here the focus is on applying them to code.

In the Canadian Electrical Code, overcurrent protection is covered in Section 14 (Protection and control), with the interrupting rating requirements in Rule 14-012 and the ratings of overcurrent devices for conductors in Rule 14-104 and Table 13.

Fuses

A fuse is the simplest and often the most reliable protective device: a calibrated metal element that melts and opens the circuit. Fuses are classified by their dimensions and performance so that the right fuse can be installed and the wrong one cannot:

• Plug fuses (Edison base and type S) for old residential panels, limited to 125 V circuits. Type S fuses use adapters that prevent a larger fuse from being installed in a socket for a smaller circuit.
• Cartridge fuses in standard classes, such as H, K, R, J, T, CC and L, each with its own size, voltage rating and interrupting rating. Classes J, T, CC, R and L are current-limiting, with interrupting ratings of 200,000 A or more.
• Rejection features: class R fuses have a notch, and fuse holders for them reject older class H and K fuses, so that a current-limiting fuse cannot be replaced by one with a lower interrupting rating.

Fuses also come in time-delay (dual-element) and non-time-delay versions; time-delay fuses ride through motor starting currents (ELEC-110, Lesson 12).

Circuit breakers

Moulded-case circuit breakers are the most common overcurrent device in modern installations. Small plug-on and bolt-on breakers in panelboards have fixed thermal-magnetic trips; larger breakers may have adjustable magnetic trips or electronic trip units with long-time, short-time, instantaneous and ground fault settings. Breakers used as switches for fluorescent lighting must be marked for switching duty (SWD or HID). Breakers can be reset after a trip, but they must not simply be reset repeatedly: a breaker that trips has done its job, and the cause must be found (ELEC-110, Lesson 12).

Three ratings that must all be right

Current rating

The current rating must protect the conductors (Lesson 4): generally not more than their ampacity, with the round-up permitted to the next standard rating for devices up to 800 A, and with the small-conductor limits for 14, 12 and 10 AWG copper.

Voltage rating

A device must be rated for the voltage of the circuit. Breakers with a slash rating, such as 120/240 V or 480Y/277 V, may be used only on solidly grounded systems where the voltage from any conductor to ground does not exceed the lower figure and the voltage between conductors does not exceed the higher figure. A breaker with a straight rating, such as 240 V or 480 V, has been tested to interrupt the full voltage across a single pole and may be used on any system up to that voltage.

Interrupting rating

A device must be able to interrupt the maximum fault current available at its line terminals. Residential breakers are commonly rated 10,000 A (10 kA), commercial breakers 14 kA to 65 kA or more, and current-limiting fuses 200 kA or 300 kA. Equipment such as panelboards, motor controllers and HVAC units also carries a short-circuit current rating (SCCR), which must equal or exceed the available fault current.

Where overcurrent protection goes

The general rule is that conductors must be protected at the point where they receive their supply: an overcurrent device at the beginning of each feeder and branch circuit, in each ungrounded conductor. The grounded (neutral) conductor is not normally fused or switched on its own, because opening the neutral while the hot conductors remain live leaves equipment energized and can put loads in series (Lesson 3).

Taps

A tap is a conductor connected to a larger feeder without its own overcurrent protection at the point of connection. The code permits short taps under strict conditions, because the tap is protected only by the much larger feeder device until it reaches its own protection. The conditions limit the tap's length, require its ampacity to be a minimum fraction of the feeder protection, require it to end in a single overcurrent device that protects it, and require physical protection, typically by enclosing it in raceway.

The CEC permits taps without overcurrent protection at the point of supply under the conditions of Rule 14-100, with limits on length (short taps of a few metres) and on the ampacity of the tap compared with the feeder protection.

Location and accessibility

Overcurrent devices must be readily accessible to the people who need to operate them, so that circuits can be switched off quickly in an emergency or for maintenance. Common requirements are:

• The operating handle at its highest position must be within a maximum height above the floor or working platform, about 2 m.
• Devices must not be located where they are exposed to physical damage, near easily ignitible material such as in clothes closets, or (for dwelling panels) in bathrooms.
• Panelboards must have the required working space in front of them, kept clear of storage (Lesson 2).
• In multi-occupancy buildings, each occupant must have ready access to the overcurrent devices that serve their unit.

In Canada, the CEC sets the maximum mounting height of panelboard and overcurrent device handles and the locations where panelboards may not be installed; provincial amendments may add requirements.

Supplementary protection and coordination

Small fuses and breakers inside equipment, such as a 5 A fuse in a control transformer or a miniature breaker in a light fixture, are supplementary protection. They protect the equipment but do not replace the branch-circuit overcurrent device, and devices marked only as supplementary protectors must not be used as branch-circuit protection.

Where several devices are in series, coordination means arranging their ratings and time-current characteristics so that only the device nearest a fault operates, leaving the rest of the system in service. Selective coordination is required by code for some systems where continuity is vital, such as emergency systems, elevators and critical operations power systems, and is studied using time-current curves (ELEC-110, Lesson 12; ELEC-220).