SG PPE Electrical cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R.

Series circuit: R_total = R1 + R2 + ...; current is common, voltage divides in proportion to resistance.

Parallel circuit: 1/R_total = 1/R1 + 1/R2 + ...; voltage is common, current divides inversely with resistance.

AC RMS values: V_rms = V_peak / √2; for a sine wave RMS is 0.707 × peak and average is 0.637 × peak.

Impedance Z = √(R² + X²); inductive reactance X_L = 2πfL, capacitive reactance X_C = 1/(2πfC).

Power factor = true power (W) / apparent power (VA); three-phase power P = √3 × V_L × I_L × cos φ.

Star (wye) and delta relationships: V_line = √3 × V_phase in star; I_line = √3 × I_phase in delta.

Kirchhoff's laws: sum of currents at a node is zero; sum of voltages around a closed loop is zero.

Conductor resistance R = ρL/A, so resistance rises with length and falls with cross-sectional area.

Cable current-carrying capacity depends on conductor size, insulation rating, installation method, grouping and ambient temperature.

Standard harmonised colours: line brown, neutral blue, protective conductor green/yellow; old UK colours red (line) and black (neutral).

Cable selection considers voltage drop, disconnection time, thermal constraints and mechanical protection.

Steel conduit and trunking must be earthed and have continuity assured across joints; plastic systems need separate CPC.

Socket outlets require fused plugs or circuit protection sized to the cable and load; accessories must match the circuit rating.

Non-sheathed cables in walls need conduit or capping; buried cables need egress protection (e.g. earthed metallic covering or RCD).

Bending radius must not be less than the manufacturer's minimum to avoid insulation and conductor damage.

Cable derating factors are multiplied together (grouping × ambient temperature × thermal insulation) to give the effective capacity.

Protective device must satisfy I_a × Z_s ≤ U_0 for the disconnection time required by the circuit type.

Earth fault loop impedance Z_s = Z_e + (R1 + R2) of the circuit conductors.

BS 7671 disconnection times: 0.4 s for TN final circuits ≤32 A, 5 s for distribution circuits and TT systems (with conditions).

RCDs to BS EN 61008/61009: 30 mA for additional protection, 300 mA for fire protection of premises.

Main protective bonding connects extraneous-conductive-parts to the MET; main bonding conductor minimum 10 mm² (or half the earthing conductor if larger).

Supplementary bonding may be required in special locations (e.g. bathrooms) to reduce touch voltage.

TT system relies on earth electrodes and RCD protection; Z_s values are typically too high for overcurrent disconnection.

Type B, C and D MCBs differ in magnetic trip thresholds (3-5×, 5-10×, 10-20× In respectively).

Fuse and MCB discrimination ensures only the device nearest the fault operates.

Every installation must have a main switch, overcurrent protection, and means of isolation for maintenance.

Special locations (bathrooms, swimming pools, agricultural) have additional requirements such as 30 mA RCD and zone restrictions.

Equipment must be selected to suit its environment (IP rating, temperature, corrosion, mechanical impact).

Motors require overload protection, isolation and often starting controls; motor circuit protection differs from cable protection.

Distribution boards need clear labelling, a schedule of circuits, and accessible protective devices.

Isolation and switching devices must break all live conductors and be capable of being locked off.

Equipment enclosures must ensure basic protection (IPXXB) against direct contact under normal conditions.

Fire safety: cables serving life safety systems need fire-resistant construction and dedicated protection.

Sequence: safe isolation, visual inspection, continuity of protective conductors, ring final circuit continuity, insulation resistance, polarity, earth electrode resistance, earth fault loop impedance, RCD tests, functional tests.

Insulation resistance test at 500 V DC; minimum acceptable value 1 MΩ for most circuits (SELV/PELV 250 V).

Ring final circuit test: end-to-end readings r1, rn, r2; expected cross-connection readings are (r1+rn)/4 and (r1+r2)/4.

Polarity test confirms line and neutral are correctly connected at every point.

Earth fault loop impedance measured by loop tester or calculated from Ze + (R1+R2).

RCD test: verify trip time at 1× IΔn and 5× IΔn, and no trip at ½ IΔn; check test button function.

Continuity of protective conductors tested with low-resistance ohmmeter after nulling leads.

Commissioning includes verifying protective device settings, labels, certificates and schedules of test results.

BS 7671 (IET Wiring Regulations) is the primary UK standard; the Electricity at Work Regulations 1989 impose statutory duties.

Live working is permitted only when it is unreasonable to work dead, and suitable precautions are taken.

Safe isolation procedure: identify supply, isolate, lock off, prove tester, prove dead, and re-prove tester.

Proving dead requires an approved voltage indicator tested on a known source before and after use.

Permit-to-work and lockout/tagout systems control work on high-energy or complex installations.

Risk assessment must consider arc flash, stored energy, and the competence of persons.

Portable appliance testing intervals are determined by equipment type and environment, not by a single fixed rule.

Regulations require adequate earthing, protective devices, and maintenance of electrical systems in safe condition.

Fault-finding approach: gather symptoms, form hypotheses, test systematically, confirm root cause before repair.

Common faults: open circuit (no continuity), short circuit (low resistance between conductors), earth fault (insulation breakdown).

Nuisance RCD tripping can result from accumulated leakage, moisture, or shared neutral faults.

High loop impedance indicates poor earthing or loose connections; low insulation resistance indicates damaged insulation or moisture.

Voltage drop measurements across a circuit reveal loose or high-resistance joints.

Continuity tests must be performed with the circuit isolated and the tester nulled.

Thermal imaging and clamp meters assist in locating overloaded or loose connections.

After repair, re-test affected circuits and update certificates and schedules.