Overcurrent protection, fault current and transformers

Every conductor and piece of equipment in a commercial or industrial installation depends on overcurrent devices to disconnect it before overloads and faults cause damage or fire. Lesson 3 applied this to motor circuits. This lesson covers overcurrent protection in general: how fuses and circuit breakers work and how they are rated, why the available fault current at each point of the system matters and how to estimate it, interrupting ratings, short-circuit current ratings of equipment and series ratings, the protection of dry-type transformers and their secondary conductors, and the arc flash hazard that high fault currents create for electricians working on energized equipment.

Overloads and faults

Overcurrent comes in two forms. An overload is current above rating in a normal circuit path, for example too many loads on a circuit or a motor jammed by its load; it heats conductors slowly, so protection can be time-delayed. A fault is current in an abnormal path: a short circuit between conductors, or a ground fault from a conductor to grounded metal. Fault currents can be tens of thousands of amperes, limited only by the impedance of the source and the conductors, and they must be interrupted within cycles to prevent explosive damage, melting and fire. Overcurrent devices therefore have inverse-time characteristics: the higher the current, the faster they open.

Fuses and circuit breakers

Fuses interrupt current by melting a calibrated element. Current-limiting fuses, such as Class J, RK1 and L, melt so quickly at high fault currents that they open within the first half cycle, before the current reaches its prospective peak, greatly reducing the energy let through to downstream equipment. Time-delay fuses tolerate motor starting currents. Fuses are reliable, need no maintenance, and have very high interrupting ratings (typically 200 kA or more), but must be replaced after operating and must be the correct class to fit the fuse holder (rejection features prevent installing a lower-rated class).

Circuit breakers interrupt current with contacts that open mechanically, and can be reset. Thermal-magnetic breakers use a bimetal element for overloads and a magnetic element for instantaneous trip on faults. Larger breakers use electronic trip units with adjustable settings: long time (L, overload), short time (S, delayed fault tripping for coordination), instantaneous (I) and ground fault (G), abbreviated LSIG. Breakers can be molded case, insulated case or low-voltage power breakers, with increasing ratings, adjustability and maintainability. Unlike fuses, breakers need periodic maintenance and testing, since mechanisms can stick after years without operating.

Ratings of overcurrent devices

Overcurrent devices come in standard ampere ratings, and the device protecting a conductor must generally not exceed the conductor's ampacity. Where the ampacity does not correspond to a standard rating, the next higher standard rating is often permitted for devices up to a certain size, provided the conductors are not part of a circuit supplying several receptacles for cord-and-plug loads.

240.4 requires conductors to be protected at their ampacity, with 240.4(B) permitting the next higher standard rating up to 800 A when the ampacity does not match a standard size and other conditions are met, and 240.4(D) limiting small conductors (for example 20 A for 12 AWG copper). Standard ratings are listed in 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600 A and larger.

Available fault current

The available fault current at a point is the current that would flow in a bolted short circuit there. It is greatest at the service, where only the utility transformer's impedance limits it, and decreases through feeders and transformers. Utilities supply the available fault current at the service, or the transformer data from which to calculate it. A quick upper bound for a transformer with an infinite source is the rated current divided by the per-unit impedance:

Conductor impedance then reduces the fault current at downstream equipment. The point-to-point method gives a convenient estimate for three-phase faults:

where \(L\) is the conductor length in feet, \(C\) a constant for the conductor size, material and raceway type from published tables, \(n\) the number of conductors per phase and \(V\) the line voltage. Software performs these calculations for whole systems, including motor contribution (running motors feed current into a fault for a few cycles).

Interrupting ratings and short-circuit current ratings

Two different ratings must be checked against the available fault current:

• The interrupting rating of an overcurrent device is the highest current it can safely interrupt. A breaker or fuse asked to interrupt more than its rating can fail violently, rupturing its case and failing to clear the fault.
• The short-circuit current rating (SCCR) of equipment, such as panelboards, switchboards, motor controllers, industrial control panels, air-conditioning equipment and transfer switches, is the fault current it can withstand without unacceptable damage while the specified protective device clears the fault.

110.9 requires equipment intended to interrupt current at fault levels to have an interrupting rating at least equal to the available current, and 110.10 requires equipment to be selected so that the protective devices clear a fault without extensive damage. Service equipment in other than dwelling units must be field marked with the available fault current and the date of the calculation (110.24), and industrial control panels (409.110), air-conditioning equipment (440.4(B)) and other equipment must be marked with their SCCR.

Fault currents tend to rise over time as utilities upgrade transformers, so an installation that was adequate when built may become underrated; marking the calculated value and date lets inspectors and electricians check new equipment against it.

Series ratings

A series-rated combination allows a downstream breaker with a lower interrupting rating to be used where the available fault current exceeds its rating, because a tested upstream device (a current-limiting fuse or breaker) limits the let-through current. Series ratings must be tested and listed for the specific combination, marked on the equipment, and are not permitted where motors connected between the devices contribute significant fault current. They save cost but sacrifice selective coordination, because both devices open for a large fault (Lesson 9 discusses coordination).

Protecting dry-type transformers

Commercial buildings use dry-type transformers to step down from the distribution voltage to the utilization voltage for receptacles and small loads (for example 600 V to 120/208 V in Canada, or 480 V to 208Y/120 V in the US). Their overcurrent protection is governed by percentages of rated current, with larger primary settings permitted when the secondary is also protected, so that transformer inrush current (ELE-370, Lesson 3) does not trip the primary device.

Table 450.3(B) covers transformers 1000 V and less: with primary-only protection, the primary device is limited to 125% of rated primary current (with next-size-up allowances for currents of 9 A or more); with both primary and secondary protection, the primary may be up to 250% and the secondary up to 125% of rated secondary current. Table 450.3(A) covers transformers over 1000 V. Dry-type transformers must meet the ventilation and installation rules of 450.9 and 450.21.

Transformer secondary conductors

The primary overcurrent device of a delta-wye transformer cannot properly protect the secondary conductors, because the current transformation and connection mean that a secondary fault or overload may not be seen as excess current on the primary. Secondary conductors therefore need their own protection, with limited lengths permitted between the transformer and the secondary overcurrent device.

240.21(C) permits secondary conductors without protection at the transformer only under specific conditions, such as not over 10 ft long with ampacity at least that of the device they supply (240.21(C)(2)), or not over 25 ft with ampacity at least one third of the primary device rating times the ratio of primary to secondary voltage (240.21(C)(6)), terminating in a single overcurrent device.

Ground fault protection of equipment

Arcing ground faults on solidly grounded wye systems above 150 V to ground can draw currents too low to trip a large overcurrent device quickly but high enough to burn down switchgear. Lesson 1 described ground fault protection required on large service disconnects. Where it is required on the service, codes and good practice may require additional levels of ground fault protection on feeders so that a ground fault on one feeder trips only that feeder rather than the whole building, particularly in health care facilities (Lesson 7), and ground fault protection settings must be coordinated with downstream devices.

Arc flash hazards

High fault currents create an arc flash hazard: an arcing fault releases enormous energy in milliseconds, producing temperatures several times hotter than the sun's surface, blast pressure, molten metal and intense light. Arc flash injuries are among the most serious in electrical work. The incident energy at a working distance (in cal/cm²) depends on the available fault current, the clearing time of the protective device and the distance, and is calculated by engineers using methods such as IEEE 1584. Faster-clearing protection dramatically reduces incident energy, which is why current-limiting fuses, zone-selective interlocking, maintenance mode switches and arc flash relays are used.

In the US, NFPA 70E, Standard for Electrical Safety in the Workplace, sets out risk assessment, electrically safe work conditions, PPE and training, and OSHA regulations require employers to protect workers from electrical hazards. NEC 110.16 requires arc flash warning labels on equipment likely to be examined or serviced while energized, with specific information on service equipment rated 1200 A or more, and 240.87 requires an arc energy reduction method (such as zone-selective interlocking, differential relaying or an energy-reducing maintenance switch) for breakers rated or adjustable to 1200 A or higher.

The first rule of arc flash safety is to de-energize equipment and verify absence of voltage before working on it (lockout, tagout and testing). Energized work is justified only when de-energizing creates greater hazards or is infeasible, and then only by qualified workers with a written energized work permit, an arc flash risk assessment and appropriate PPE.