Fault current and equipment ratings

A short circuit can draw tens of thousands of amperes from a building's distribution system, far more than any load. Every circuit breaker, fuse, panelboard and switch that might carry or interrupt that current must be rated for it, or it can explode, start a fire or fail to clear the fault. Fault current also sets the severity of arc flash hazards to workers. Electricians need to estimate available fault current, read and apply equipment ratings, and recognize when a change, such as a larger transformer or a new utility supply, makes existing equipment inadequate. This lesson covers the sources of fault current, calculating it at the transformer and along feeders, the effect of motors, interrupting and short-circuit current ratings, current-limiting devices and series ratings, labelling requirements, and the link to arc flash.

Where fault current comes from

When a bolted short circuit occurs, the current is limited only by the impedance of everything between the sources and the fault: the utility system, transformers, conductors and busway, and the fault itself. The sources include:

• The utility, through the service transformer. The transformer's impedance usually dominates in buildings.
• Motors running at the time of the fault: an induction motor acts briefly as a generator, driven by its own inertia and residual flux, contributing roughly four to six times its full-load current for the first few cycles.
• On-site generators, UPSs and inverter-based sources, each with its own characteristic contribution (inverters typically contribute only 1.1 to 1.5 times rated current, see ELE-380, Lesson 3, for the theory).

Calculations start from the worst case: the maximum utility contribution (often given by the utility as available fault current or MVA at the service, or assumed infinite on the primary for conservatism), all motors running, and a bolted three-phase fault, which usually gives the highest current on solidly grounded low-voltage systems.

Fault current at a transformer secondary

Lesson 8 showed that with an infinite source, the fault current at a transformer's secondary terminals is its full-load current divided by its per-unit impedance:

Transformer impedances on nameplates are subject to manufacturing tolerance (often ±7.5% or ±10%), so conservative calculations reduce the nameplate impedance by the tolerance, raising the fault current. With a finite utility source, the source impedance adds to the transformer impedance and reduces the current.

Fault current along a feeder

Conductors add impedance between the source and a downstream fault, so fault current falls with distance. A convenient method converts everything to ohms per phase and adds the impedances:

• Source and transformer impedance referred to the secondary: \(Z_T = V_{LL}/(\sqrt3\,I_{sc})\), where \(I_{sc}\) is the fault current at the transformer secondary.
• Feeder impedance: the conductor resistance and reactance per phase for its length (Lesson 10), combined as \(\sqrt{(R_T + R_f)^2 + (X_T + X_f)^2}\), or, as a simpler conservative approximation, by adding magnitudes.
• Fault current at the downstream point: \(I_{sc,2} = V_{LL}/(\sqrt3\,Z_{\text{total}})\).

Manufacturers publish the equivalent point-to-point method, which uses a conductor constant \(C\) for each size and raceway type and computes a multiplier \(M = 1/(1 + f)\) with \(f = \sqrt3\,L\,I_{sc}/(C\,V_{LL})\) for three-phase faults; the two approaches give similar results.

Motor contribution

Running induction motors add their own fault current, which flows from the motor toward the fault. A common estimate is four times the full-load current of all motors that may be running, added to the utility contribution at the point of interest. The contribution decays within a few cycles, so it matters most for the first-cycle (momentary) duty of breakers and for equipment close to large motor groups.

Interrupting ratings and short-circuit current ratings

Two kinds of rating apply:

• Interrupting rating (also called interrupting capacity, AIC or kAIC): the highest current at rated voltage that a fuse or circuit breaker can safely interrupt. A breaker rated 10 kA attempting to interrupt 25 kA may fail violently.
• Short-circuit current rating (SCCR) or withstand rating: the fault current that equipment such as panelboards, switchboards, motor controllers, transfer switches and industrial control panels can withstand until the protective device clears the fault, without exceeding damage limits. An assembly's SCCR is limited by its weakest component and the protection ahead of it.

The CEC (Rule 14-012) requires overcurrent devices to have an interrupting rating sufficient for the voltage and the available fault current at the point of installation, and equipment to be suitable for the fault current it may be exposed to. Equipment certification marks list these ratings, and electrical inspection authorities increasingly ask for the available fault current to be documented on drawings or labels.

Typical ratings: residential breakers 10 kA; commercial panelboard breakers 10, 14, 18, 22, 25, 35, 42 or 65 kA, and up to 100 kA or more for high-interrupting types; current-limiting fuses (such as Class J, L, RK1 and T) 200 kA or 300 kA.

Current-limiting devices and series ratings

A current-limiting fuse or breaker opens so fast, within the first half-cycle, that the fault current never reaches its prospective peak. The let-through current and energy (\(I^2t\)) are a small fraction of what the circuit could deliver. Current-limiting fuses can protect downstream equipment with lower withstand ratings, when the combination has been tested.

Series ratings allow a downstream breaker with a lower interrupting rating to be used where available fault current exceeds that rating, provided an upstream fuse or breaker of a tested combination limits the current. Series-rated combinations must be tested and listed together, marked on the equipment, and not used where motors connected between the two devices contribute significant fault current. Changing either device to a different type invalidates the rating.

When fault current changes

Available fault current is not fixed. It rises when:

• The utility replaces the service transformer with a larger or lower-impedance unit, or strengthens its system.
• A building's transformer is replaced or paralleled with another (Lesson 9).
• Feeders are shortened, or conductors enlarged (for voltage drop, Lesson 10).
• On-site generation or large motors are added.

Each such change may leave existing panels and breakers underrated. Good practice, and in many jurisdictions a code or labelling requirement, is to recalculate and update fault current labels when the supply or distribution changes, and to ask the utility for its maximum available fault current when planning new services.

Fault current and arc flash

An arcing fault releases energy as heat, light, pressure and molten metal. The incident energy at a worker's position depends on the arcing current (typically 40% to 90% of the bolted fault current at low voltage), the duration until the protective device clears it, the distance and the equipment configuration. Arc flash studies following IEEE 1584 calculate incident energy and arc flash boundaries for each piece of equipment, used to select PPE and to label equipment under CSA Z462 in Canada or NFPA 70E in the US.

A counter-intuitive point: lower fault current does not always mean lower arc flash energy. If the current falls below the instantaneous trip setting of the upstream breaker, the breaker clears the fault on its slower time-delay characteristic, and the much longer duration can more than offset the lower current. Reducing clearing time, with maintenance switches, zone-selective interlocking or current-limiting fuses, is usually the most effective way to reduce arc flash energy.