Protection

Choosing, setting and checking the devices that clear a fault. At 10–15 of 80 it is tied with Electrical Safety and Circuit Analysis as the largest area on the PE Power exam, with four subtopics: overcurrent protection, protective relaying, protective devices and coordination.

What the problems look like

Short calculations, code lookups, and reading a one-line or a log-log time-current plot. Recurring shapes — a device rating for a conductor, motor, transformer or tap, finished on the standard rating ladder; an interrupting-duty check against the fault current at the device; a relay setting carried through a CT or VT ratio (51 tap, 50 pickup, 27 pickup, 21 reach); a differential zone read from CT locations; a device number to identify; a recloser sequence to lockout; and a coordination interval read from fuse, breaker or relay curves.

By exam day you should be able to —

• rate a feeder device: rating ≥ I_noncontinuous + 1.25 × I_continuous (NEC 215.3; branch circuits 210.20(A)), then a standard rating (NEC 240.6(A));
• protect a motor circuit: branch device ≤ Table 430.52 percentage × table full-load current; overload ≤ 430.32(A)(1) percentage × nameplate current;
• check duty: the contributions that pass through the device ≤ its interrupting rating, with S_sc = √3 · V_nom · I_sc and close-and-latch = k × I_sym;
• move relay quantities through instrument transformers: I_sec = I_pri/CTR, tap = I_pickup,pri/CTR, Z_sec = Z_pri × CTR/VTR;
• work a differential: with both currents measured through the zone, I_op = |I₁ − I₂| > S × I_restraint;
• check a relaying CT: Z_burden,max = V_class/(20 × rated secondary current);
• compute coordination: t_clear = t_relay + t_breaker and CTI = t_upstream − t_downstream at the same fault.

Where people lose points

• Floor or ceiling. "At least" rounds up to a standard rating; "may not exceed" rounds down unless the rule permits the next higher one. The 240.4(B) allowance stops at 800 A and is barred for feeder taps and transformer secondary conductors (240.21(B), (C)). In 210.20(A) and 215.3 the 125% multiplies the continuous load, never an ampacity or a device rating.
• Motor currents. The branch device uses the table full-load current (430.6(A)(1)); the overload uses the nameplate. Pick the Table 430.52 column by device type and the row by motor type.
• Transformer tables. In Table 450.3(A), the row is location and impedance, the column side, voltage and device; in 450.3(B), the row is protection method, the column side and current band. The percentage applies to the rated current of its winding the device sits in.
• Duty is local. NEC 110.9 compares the interrupting rating at nominal circuit voltage with the current available at the device's line terminals; a breaker-duty study counts only the contributions that flow through it.
• CT ratio. 600:5 is a ratio of 120, not 600. No √3 enters the ratio itself; it enters with delta-connected CTs or a line-vs-phase voltage setting. Apply a pickup margin once.
• Zones and numbers. The CTs bound a differential zone, not the breakers; an outside fault balances and the relay restrains. Keep 67 (directional overcurrent) apart from 32 (directional power), and 87 (senses) apart from 86 (trips and holds).
• Fuse edges. A downstream fuse is judged on total clearing; an upstream fuse on minimum melting, reduced for preloading. On log axes, fix the decade from the labels.
• Current each device carries. Refer a three-phase fault across a transformer by the voltage ratio. For a line-to-ground fault behind a delta-wye bank, two delta-side line currents are 1/√3 of that balanced referral; the third is zero. Find the curve segment the current falls in, and count one interval per adjacent pair that carries the fault.

Have these at your fingertips

The CT and VT referrals, the differential operate condition and the two coordination relations. In the Handbook, the ANSI/IEEE C37.2 device numbers and suffix letters sit under Single-Line Diagrams; S_sc = √3 · V_nom · I_sc, the sequence-network fault connections and the transformer-bank sequence models under Fault Current Analysis; the ratio relation under Single-Phase Transformer Equivalent Circuits. Rating rules come from NFPA 70-2020 (NEC): Article 240 for conductors, taps and standard ratings, Article 430 for motors, Article 450 for transformers. The exam opens one chapter of a standard at a time, so know which article you need first. Coordination problems supply their own curves and required interval.

Going deeper

• Overcurrent protection and coordination on the PE Power exam — ratings and intervals
• Fault current analysis on the PE Power exam — the duty a device must interrupt
• Instrument transformers (CTs and VTs) on the PE Power exam — ratios, burden, accuracy class
• Motor starting and motor circuits on the PE Power exam — Article 430

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