Overcurrent protection and coordination on the PE Power exam

Protection questions on the PE Power exam come in two kinds. One rates a device: pick the fuse or breaker rating the code allows, or check a breaker against its fault duty. The other reads time-current curves and asks whether devices in series operate in the right order. The rating rules are in NEC Article 240. The NCEES handbook prints the ANSI device numbers under Single-Line Diagrams and the fault-duty relation under Fault Current Analysis. It has no coordination section, so those problems supply their own curves and required interval.

What you actually need to own

• Floor or ceiling. A device that must be rated at least some multiple of a current rounds up to the next standard rating. A device that may not exceed a limit rounds down, unless the rule being applied permits the next higher rating. Standard ratings are set by 240.6(A) and Table 240.6(A).
• Conductor protection. Section 240.4 holds the device at or below the conductor ampacity. 240.4(B) allows the next higher standard rating under stated conditions, for devices up to 800 A. Above that, 240.4(C) requires the ampacity to be at least the device rating. 240.4(D) caps the protection on small conductors, and it applies after the ampacity correction and adjustment factors. Taps are in 240.21(B), transformer secondary conductors in 240.21(C).
• Duty is what flows through the device. A breaker's interrupting duty is the sum of the source contributions whose path to the fault passes through that breaker. A fault duty given in MVA converts to current with S_sc = √3 · V_nom · I_sc. Close-and-latch duty is the symmetrical current times the stated asymmetry multiplier.
• Clearing time. Relay operating time, read at the time dial in service, plus the breaker's interrupting time.
• Coordination time interval. Read both devices at the same fault, each at the current it carries, and take the time between them. Between two relays the interval runs from the downstream relay's operating time to the upstream relay's operating time, so it has to cover the downstream breaker's interrupting time, the upstream relay's overtravel and a margin. Breaker and fuse curves already include clearing, so read those curve to curve. When a problem states how its interval is measured, use that. Along a radial path, add one interval for each adjacent pair of devices.
• Relay settings. The tap sets pickup. The time dial scales operating time and leaves pickup where it was. An instantaneous element has no intended delay, so it coordinates by current. Its pickup sits above the largest fault current at the point it must not reach past, usually the next downstream device, and above transformer inrush.
• Fuse edges. A fuse curve has a minimum-melting edge and a total-clearing edge. A downstream fuse is judged on total clearing. An upstream fuse is judged on minimum melting, reduced for preloading when the problem gives a factor.
• Device numbers and zones. Know 50, 51, 67, 79, 86 and 87 from the handbook table. The current transformers feeding a differential relay bound its zone.

Where people lose points

• Reporting a computed ampere value that is not a standard rating.
• Using the next-higher allowance where it is excluded. 240.21(B) does not permit it for feeder tap conductors, and 240.21(C) does not permit it for transformer secondary conductors.
• Using total fault current for a breaker's duty. A motor on the far side of the fault feeds the fault without passing through the breaker.
• Reading the wrong segment. A breaker curve has long-time, short-time and instantaneous regions. Find which one the fault current falls in.
• Comparing devices across a transformer at the same amperes. For a three-phase fault, refer the current by the voltage ratio. For a line-to-ground fault through a delta-wye bank, trace the windings, because the ratio alone overstates the delta-side line current.
• Taking an interval between devices that are not adjacent, or across a device that carries none of the fault current, such as a parallel feeder.

A note on the codes

The exam supplies its codes, including the 2020 NEC and the 2017 NESC, and scores code answers against those editions. See the PE Power handbook and code books. Know where these sit: system coordination for orderly shutdown in 240.12, selective coordination for emergency systems in 700.32, ground-fault protection of equipment at services in 230.95, transformer overcurrent protection in 450.3, and medium-voltage feeders in 240.100 and 240.101. Motor branch circuits follow Article 430 and are covered under motor starting and motor circuits.

How to study it

Sketch three breaker curves in series on log-log axes. Pick four fault currents, read all three times at each, and write the interval for each adjacent pair. Then put a transformer between two devices and repeat with referred currents. Breaker duty depends on fault current analysis.

Part of the Protection area of the PE Power exam. → Practice it on CastorPrep