Registered Electrical Engineer (G&T) cheat sheet

Per-unit conversion: Z_pu = Z_actual / Z_base, with Z_base = V_base² / S_base; on a new base, Z_pu,new = Z_pu,old × (S_new/S_old) × (V_old/V_new)².

Three-phase power: S = √3 × V_LL × I_L; for a 3-winding transformer, each winding's MVA base must be referred to a common base before combining sequence impedances.

Synchronous generator short-circuit model: subtransient reactance X''d for first-cycle duty, transient X'd for several-cycle duty, synchronous Xd for steady state; E'' ≈ V + jX''d·I.

Symmetrical components: I0 = (Ia+Ib+Ic)/3, I1 = (Ia + aIb + a²Ic)/3, I2 = (Ia + a²Ib + aIc)/3, where a = 1∠120°; zero-sequence currents require a grounded neutral return path.

Transmission line parameters: inductance from geometric mean distance (GMD) over geometric mean radius (GMR); capacitance uses GMD over conductor radius; bundled conductors raise GMR and reduce reactance.

Surge impedance loading: SIL = V_LL² / Z_c, with Z_c = √(L/C) ≈ 400 Ω for a single conductor overhead line; loading near SIL minimizes reactive flow.

Stability: equal-area criterion applies to a single machine against an infinite bus; critical clearing angle occurs when accelerating area equals decelerating area.

Transformer differential protection uses CT ratio and vector-group compensation; percent differential slope settings ride through CT mismatch and tap changes while tripping internal faults.

Generator grounding: high-impedance grounding limits stator ground-fault current (typically ≤ 10 A) but requires insulation rated for full line-to-line voltage during a ground fault.

Load flow: Newton-Raphson converges quadratically and uses the Jacobian; Gauss-Seidel is simple but slow; slack bus fixes V and angle, PV buses fix P and |V|, PQ buses fix P and Q.

Fault levels: three-phase fault current I_f = V_LL / (√3 × Z_th); for a line-to-ground fault, I_f = 3V_ph / (Z1 + Z2 + Z0 + 3Z_f).

Per-unit sequence networks connect in series for a single line-to-ground fault, in parallel for a line-to-line fault, and in series-parallel for a double line-to-ground fault.

Voltage drop on a feeder: ΔV ≈ I(R cosφ + X sinφ) per phase for a lagging load; approximate ΔV/V ≈ (P·R + Q·X)/V² for small angles.

Motor starting: locked-rotor current is typically 5–8 times rated; voltage dip at the motor terminals ≈ (motor kVA_sc during start)/(source kVA_sc + motor kVA_sc).

Cable ampacity: derate for ambient temperature, soil thermal resistivity, grouping, and depth; IEC 60364 and NEC 310 both require correction factors to be applied multiplicatively.

Protection coordination: time-current curves must maintain a coordination time interval (typically 0.2–0.4 s) between downstream and upstream overcurrent devices.

Arc flash: incident energy is calculated per IEEE 1584 or NFPA 70E; the working distance and the available bolted fault current determine the incident energy and arc-flash boundary.

Earthing and bonding: TN-S, TN-C-S, TT, and IT systems differ in how the neutral and protective earth are connected; TT systems require an RCD because the earth-fault loop impedance is high.

RCD selection: 30 mA for socket outlets and portable equipment; 300 mA for fire protection; selective (type S) devices are used upstream to preserve discrimination.

Disconnection times: TN systems allow 0.4 s for final circuits ≤ 32 A and 5 s for distribution circuits; TT systems with RCDs use the manufacturer's disconnection times (typically ≤ 0.2 s).

Cable sizing: continuous current must satisfy I_b ≤ I_n ≤ I_z, and the protective device must satisfy I²t ≤ k²S² for the fault clearing energy.

Protection against indirect contact: disconnection time is achieved when Z_s ≤ U_0 / I_a, where U_0 is the nominal line-to-neutral voltage and I_a is the current that trips the device within the required time.

Isolation and switching: a lockout/tagout procedure requires verified isolation, proving dead with a voltage detector, and earthing before work on HV equipment.

IP ratings: the first digit is solid-particle protection (0–6), the second is water protection (0–8); IP54 is dust-protected and splash-proof, not dust-tight or submersible.

Hazardous areas: IEC 60079 zones 0, 1, 2 for gases and 20, 21, 22 for dust; equipment must be certified for the zone and the gas group (IIA, IIB, IIC).

Inspection and testing: insulation resistance is measured at 500 V DC for LV circuits; the minimum acceptable value is 1 MΩ, and the test must be recorded on the installation certificate.

Earned value: CV = EV − AC, SV = EV − PV, CPI = EV/AC, SPI = EV/PV; a CPI below 1.0 means cost overrun, an SPI below 1.0 means schedule delay.

Critical path method: the critical path has zero total float; free float is the amount an activity can slip without delaying its successor, and total float is slip without delaying the project.

Risk management: risk = probability × impact; the response hierarchy is avoid, reduce, transfer, and accept, and residual risk must be documented.

Contract types: lump-sum transfers quantity risk to the contractor; unit-price shifts quantity risk to the owner; cost-plus transfers cost risk to the owner but requires audit rights.

Change control: any scope change must be assessed for cost, schedule, and technical impact, approved in writing, and reflected in the baseline before work proceeds.

Quality assurance: the ITP (inspection and test plan) defines hold points and witness points; a hold point requires the client's release before the next activity can proceed.

Document control: revision status, transmittal records, and as-built markups must be traceable; superseded drawings must be withdrawn from site to prevent use.

Resource leveling changes start and finish dates to resolve over-allocation, whereas resource smoothing keeps the project finish date fixed and only adjusts within float.

Competence rule: an engineer must accept only work within their competence and must disclose any limitation or conflict of interest to the client in writing before accepting the engagement.

Public safety paramount: when a design poses an imminent danger to life or property, the engineer must notify the client and the authority having jurisdiction, even if it breaches confidentiality.

Sealing and signing: only the engineer who prepared or directly supervised the work may seal and sign drawings; sealing work not reviewed is a violation of professional practice.

Conflict of interest: an engineer must disclose any financial interest in a supplier, contractor, or product specified; undisclosed interest invalidates the professional opinion.

Confidentiality: client information must not be disclosed without consent, except where required by law, by a court order, or to prevent a serious safety hazard.

Bidding ethics: providing a competitor's price, colluding on bids, or accepting a contingency fee from a supplier without disclosure are prohibited practices.

Continuing professional development: most jurisdictions require documented CPD hours per year as a condition of licence renewal; falsifying CPD records is a disciplinary offence.

Whistleblowing: an engineer who discovers a violation of codes or regulations must first raise it internally; if unresolved, escalating to the regulator is an ethical obligation, not disloyalty.