SOLAS Electrical Apprenticeship cheat sheet

Insulation resistance test: apply 500 V DC to a de-energised circuit with all loads and sensitive electronics disconnected; a 230 V final circuit must read at least 1 MΩ, and the reading is recorded after the voltage has been applied for a stable period.

Continuity of protective conductors: use a low-resistance ohmmeter with a long test lead nulled out; the measured (R1+R2) value is compared with the expected value derived from the conductor resistance per metre and the circuit length.

Polarity checks confirm that line conductors are connected to the centre contact of lampholders, that switches are in the line conductor, and that socket outlets have line, neutral and protective conductors in the correct terminals.

Ring final circuit continuity: the three-step test measures end-to-end r1, rn and r2, then cross-connects to verify that the highest reading at each socket is approximately (r1+rn)/4 and (r1+r2)/4, confirming no open or bridged ring.

Earth fault loop impedance (Zs) is measured live and compared with the maximum permitted value for the protective device; where Zs cannot be met, a 30 mA RCD or supplementary bonding may be required.

RCD test: a 30 mA device must trip at its rated residual current within 300 ms and must not trip at half rated current (15 mA); the test button is a mechanical check only and does not verify the trip threshold.

Lock-off procedure: isolate, prove dead with a voltage indicator on a known live source before and after use, apply a lock and warning notice, and retain the key with the person carrying out the work.

Periodic inspection intervals depend on installation type and use; the interval and the extent of sampling are recorded on the schedule of test results and the electrical installation condition report.

Ohm's law: V = I × R. Power: P = V × I = I²R = V²/R. In a purely resistive circuit, energy is dissipated as heat and current is in phase with voltage.

Series circuits: current is common, resistances add (RT = R1 + R2 + …), and voltage divides in proportion to resistance. Parallel circuits: voltage is common, and 1/RT = 1/R1 + 1/R2 + …

Three-phase star (wye) connection: line voltage = √3 × phase voltage (400 V line, 230 V phase), and line current equals phase current. Delta connection: line current = √3 × phase current, and line voltage equals phase voltage.

Three-phase power: P = √3 × VL × IL × cos φ for a balanced load, where cos φ is the power factor; reactive power Q = √3 × VL × IL × sin φ, and apparent power S = √3 × VL × IL in volt-amperes.

Capacitive reactance XC = 1/(2πfC) and inductive reactance XL = 2πfL; impedance Z = √(R² + (XL − XC)²), and the phase angle φ = arctan((XL − XC)/R).

Transformer ratio: V1/V2 = N1/N2 = I2/I1. An ideal transformer conserves apparent power, so a step-down in voltage is accompanied by a step-up in current.

A conductor carrying current in a magnetic field experiences a force (F = BIL sin θ); this is the motor effect. Conversely, a conductor moving through a field has an EMF induced in it (e = BLv); this is the generator effect.

Energy and cost: energy (kWh) = power (kW) × time (h); cost = kWh × unit price. A 2 kW load run for 3 hours consumes 6 kWh.

Cable current-carrying capacity (Iz) must satisfy Iz ≥ In ≥ Ib, where Ib is design current and In is the protective device rating; Iz is then corrected by Ca (ambient temperature), Cg (grouping), Ci (thermal insulation) and Cf (BS 3036 fuse) factors.

Voltage drop must not exceed 3% of nominal voltage for lighting and 5% for other circuits in a public low-voltage supply; calculate mV/A/m × Ib × length ÷ 1000.

Cable selection considers conductor material (copper or aluminium), insulation type (thermoplastic or thermosetting), number of cores, and installation method (clipped direct, in conduit, in trunking, buried).

Single-core cables in a steel enclosure must not carry separate phases of the same circuit through separate holes unless the enclosure is non-ferrous, to avoid induced eddy currents and heating.

A ring final circuit is protected by a 32 A device with 2.5 mm² line and neutral and 1.5 mm² CPC; a radial circuit uses a 20 A device with 2.5 mm² or a 16 A device with 1.5 mm².

Earthing systems: TN-S uses a separate protective earth from the supply; TN-C-S uses a combined neutral/earth (PME) with the earth and neutral combined in the supply and separated at the origin; TT uses a local earth electrode and requires RCD protection.

Segregation of band I (safety extra-low voltage), band II (low voltage) and band III (telecommunications) circuits is achieved by separate compartments, barriers or spacing to prevent contact and electromagnetic interference.

Cable derating in thermal insulation: where a cable is totally surrounded by insulation, apply the appropriate correction factor from the wiring regulations tables; the factor may be as low as 0.5.

The IET Wiring Regulations (BS 7671) is the national standard for electrical installations in the UK; it is not a statute, but compliance is used as evidence of reasonable care under the Health and Safety at Work etc. Act 1974.

Fundamental principles: protection against electric shock (basic and fault protection), protection against thermal effects, protection against overcurrent, and protection against voltage disturbances.

Automatic disconnection of supply requires that the disconnection time for a 230 V final circuit not exceeding 32 A is 0.4 s, and for distribution circuits 5 s; a 30 mA RCD may be used where the required Zs cannot be achieved.

Basic protection is provided by insulation, barriers or enclosures; fault protection by earthed protective conductors and automatic disconnection, or by double insulation (Class II) or electrical separation.

Special locations: bathrooms require supplementary bonding and 30 mA RCD protection for socket outlets; zones 0, 1 and 2 define proximity to the bath or shower and restrict the equipment permitted in each.

The Electricity at Work Regulations 1989 require that all electrical systems be constructed and maintained to prevent danger; portable appliance testing intervals depend on the equipment type and environment.

A minor works certificate is used for additions or alterations that do not include a new circuit; an electrical installation certificate is required for new circuits or consumer unit replacements.

First aid for electric shock: isolate the supply before touching the casualty, call emergency services, and begin CPR if the casualty is not breathing; do not apply direct contact until the supply is confirmed dead.

A diode conducts when forward-biased above its threshold (about 0.7 V for silicon, 0.3 V for germanium) and blocks in reverse; the reverse breakdown voltage must not be exceeded.

A half-wave rectifier produces one output pulse per input cycle; a full-wave bridge rectifier produces two, and the ripple frequency is twice the supply frequency.

A Zener diode operates in reverse breakdown to provide a stable reference voltage; a series resistor limits the current through the Zener.

A bipolar transistor is a current-controlled device: the collector current IC = β × IB, where β is the current gain; the base-emitter junction must be forward-biased for conduction.

A thyristor (SCR) latches on when the gate receives a pulse and the anode is positive; it turns off only when the anode current falls below the holding current.

An operational amplifier has very high input impedance and very high open-loop gain; negative feedback sets the closed-loop gain, for example a non-inverting amplifier gain of 1 + Rf/Rin.

A PLC scan cycle reads inputs, executes the program, and updates outputs; the scan time determines the response of the control system to input changes.

Inverters convert DC to AC using pulse-width modulation; the output frequency and voltage are set electronically, which allows speed control of induction motors.

A photovoltaic cell produces about 0.5–0.6 V per cell in open circuit; cells are connected in series to raise voltage and in parallel to raise current, and the array is rated in kWp at standard test conditions (1000 W/m², 25 °C).

A grid-tied PV inverter must synchronise its output to the grid frequency and voltage and must disconnect automatically on loss of grid supply (anti-islanding protection).

Maximum power point tracking (MPPT) adjusts the operating voltage of the array to extract maximum power as irradiance and temperature change.

Battery storage systems are rated in kWh (energy) and kW (power); depth of discharge and round-trip efficiency determine usable energy and losses.

A wind turbine's power output is proportional to the cube of wind speed (P ∝ v³) and to the swept area; a small increase in wind speed produces a large increase in power.

A closed-loop control system uses feedback to compare the measured output with the setpoint and reduce the error; a PID controller combines proportional, integral and derivative action.

A contactor is controlled by a coil and switches the load contacts; a thermal overload relay protects the motor by tripping when sustained overcurrent heats a bimetal strip.

A variable speed drive controls an induction motor by varying the supply frequency and voltage; reducing speed reduces the power consumed by a pump or fan approximately with the cube of speed.