Pancyprian Electrical Engineering III (Practical) cheat sheet

Electric shock: current through the body above roughly 30 mA for a fraction of a second can cause ventricular fibrillation; the let-go threshold is about 10 mA at 50 Hz.

Hierarchy of control for electrical work: eliminate the hazard, then isolate and prove dead, then use safe systems of work, PPE is the last line of defence.

First aid for electric shock: isolate the supply before touching the casualty, call emergency services, start CPR if breathing has stopped, treat burns.

Arc flash burns result from radiant heat; a low-voltage arc can reach several thousand degrees Celsius and ignite clothing at close range.

Site fire procedure: raise the alarm, evacuate, use the correct extinguisher (CO2 or dry powder for live electrical equipment, never water).

Risk assessment must record hazards, who may be harmed, existing controls, residual risk rating and the responsible person, and be reviewed after any change.

Manual handling of cable drums and distribution boards: assess load, use mechanical aids, keep the load close, avoid twisting; report injuries in the accident book.

Working at height for lighting and containment: use a suitable platform, never stand on the top steps of a ladder, maintain three points of contact.

Safe isolation sequence: identify the circuit, isolate at the correct device, lock off and tag, prove the tester on a known source, prove dead at the point of work, re-prove the tester.

An approved voltage indicator must comply with GS38: fused or current-limited probes, finger guards, exposed tip no more than 4 mm.

Proving dead means testing phase-to-neutral, phase-to-earth and phase-to-phase at the point of work; a single test to earth is not sufficient.

Lock-off devices and warning notices must be applied by the person carrying out the work; a padlock key is retained by that person only.

Isolation is not the same as switching off: isolation requires physical separation from all live conductors and prevention of reconnection.

Before working on a live installation, confirm the earthing and bonding are intact, use insulated tools, and only where justified as unreasonable to isolate.

Portable equipment: a voltage tester must be proved before and after use, and its proving unit or known live source recorded in the log.

Where a circuit cannot be isolated, a permit-to-work or written safe system of work is required, authorised by a competent person.

Ohm's law: V = I × R, where V is in volts, I in amperes and R in ohms; power P = V × I = I²R = V²/R.

Series circuit: same current through all components, total resistance is the sum, voltage divides in proportion to resistance.

Parallel circuit: same voltage across all branches, total current is the sum of branch currents, total resistance is less than the smallest branch resistance.

Capacitive reactance Xc = 1/(2πfC); inductive reactance XL = 2πfL; impedance Z = √(R² + (XL − Xc)²).

Power factor = true power / apparent power = cos φ; apparent power S = V × I in VA, true power P = V × I × cos φ in W.

Three-phase star connection: line voltage = √3 × phase voltage, line current = phase current; delta: line current = √3 × phase current.

A conductor carrying current produces a magnetic field; a changing magnetic field induces an EMF in a conductor (Faraday's law).

AC frequency in Cyprus is 50 Hz; RMS value = peak value / √2, so 230 V RMS corresponds to a peak of about 325 V.

Design current Ib must be ≤ protective device rating In, and In ≤ conductor current-carrying capacity It (Iz).

Correction factors: It ≥ In / (Ca × Cg × Ci), where Ca is ambient temperature, Cg is grouping and Ci is thermal insulation.

Voltage drop limit for a final circuit is 3 % of 230 V (about 6.9 V) for lighting and 5 % (about 11.5 V) for other circuits; for a three-phase 400 V circuit 5 % is 20 V.

Voltage drop for a single-phase circuit: Vd = (mV/A/m × Ib × L) / 1000, using tabulated mV/A/m values.

Earth fault loop impedance: Zs = Ze + (R1 + R2), and Zs must not exceed the maximum value in the relevant table for the protective device.

Adiabatic equation for protective conductor sizing: S = √(I²t) / k, where k depends on conductor material and insulation.

Diversity may be applied to maximum demand using the on-site guide factors; the result must not be less than the largest circuit plus an allowance for the remainder.

Cable size selection must satisfy current-carrying capacity, voltage drop, earth fault loop impedance and thermal constraints of the protective conductor.

The Electricity Act and the Cyprus Electrical Installation Regulations impose duties on persons carrying out electrical work; only competent persons may certify installations.

BS 7671 (IET Wiring Regulations) is the technical standard adopted for design and verification of installations; the current edition is the 18th Edition with Amendment 2.

Fundamental principles: automatic disconnection of supply, protection by earthing and bonding, use of class II equipment, and RCD protection as additional protection.

Part P-style notification does not apply in Cyprus; the local distribution network operator and the electricity authority set connection and inspection requirements.

The Electricity at Work principles require that all electrical systems be constructed and maintained to prevent danger, including during maintenance.

Documentation required: Electrical Installation Certificate (EIC) for new work, Minor Electrical Installation Works Certificate (MEIWC) for small alterations, and periodic inspection reports (EICR) for existing installations.

Competent person means someone with sufficient technical knowledge, experience and skill to prevent danger and injury from the work undertaken.

Records of test results, schedules of test results and schedules of inspections must be retained and handed to the person responsible for the installation.

Cable reference: e.g. 2.5 mm² twin and earth with 1.5 mm² CPC is BS 6004; PVC insulated and sheathed flat cable is suitable for domestic surface and concealed work.

Copper conductor resistance at 20 °C is about 17.5 mΩ·mm²/m; this gives approximately 7.41 mΩ/m for 2.5 mm² and 4.61 mΩ/m for 4 mm².

Cable bending radius: at least 4 times the overall diameter for armoured cable and 8 times for paper-insulated cable; follow manufacturer data for flexible cables.

Cable in a wall at a depth less than 50 mm, or in a partition, requires a 30 mA RCD and protection by earthed metallic covering, conduit or a safe zone.

Safe zones for concealed cables: within 150 mm of a ceiling or corner, or vertically/horizontally aligned with an accessory.

Containment: steel conduit must be earthed and have continuity of the protective conductor; trunking must be sized so that the cable fill does not exceed 45 % of the cross-section.

Cable supports: horizontal PVC sheathed cable must be supported at intervals not exceeding 300 mm and within 150 mm of an accessory.

Fire performance: cables in escape routes may require enhanced fire resistance (e.g. FP200 or mineral-insulated) to maintain circuit integrity.

MCB types: Type B trips at 3–5 × In, Type C at 5–10 × In, Type D at 10–20 × In; Type B suits resistive loads, Type C suits inductive loads with inrush.

An RCBO combines overcurrent and residual current protection in one device; an RCD alone does not protect against overload or short circuit.

RCD sensitivity: 30 mA for additional protection against electric shock, 100 mA or 300 mA for fire protection and time-delayed types for selectivity.

Fuse breaking capacity and discrimination: a downstream device should operate before the upstream device under fault conditions to limit the outage.

Breaking capacity (Icn) of an MCB must be at least the prospective short-circuit current at its point of installation.

A 30 mA RCD must not be used as the sole means of protection against direct contact; it is additional protection only.

Selectivity between RCDs is achieved by time delay (Type S) and by rating, with the upstream device at least three times the downstream sensitivity.

Fuse rating selection: the fuse must carry the design current continuously and operate within the disconnection time required by the earthing arrangement.

TN-S: separate neutral and protective earth from the supply; TN-C-S: combined neutral and earth (PME) split at the origin; TT: installation earth electrode independent of the supply.

Maximum earth fault loop impedance for a 32 A Type B MCB on a 230 V TN system is about 1.37 Ω for 0.4 s disconnection; verify against the current table.

TT systems require an RCD, typically 100 mA or 30 mA, because the fault loop impedance is too high for overcurrent disconnection.

Main protective bonding: connect gas, water and other metallic services to the main earthing terminal with a conductor of at least 10 mm² (or 6 mm² where the supply neutral is less than 35 mm²).

Supplementary bonding is required in locations such as bathrooms to equalise potential between exposed and extraneous conductive parts; 4 mm² minimum.

The main earthing terminal must be accessible and connected to the earthing conductor, circuit protective conductors and main bonding conductors.

Earth electrode resistance should be measured with a proprietary tester; a value below 200 Ω is generally targeted for TT installations, with RCD protection.

Basic protection is insulation or barriers; fault protection is automatic disconnection, double insulation or electrical separation.

Dead tests in order: continuity of protective conductors, continuity of ring final circuit conductors, insulation resistance, polarity, then earth electrode resistance where applicable.

Insulation resistance test at 500 V DC: minimum acceptable value is 1 MΩ for a 230 V circuit; test between live conductors and between live conductors and earth.

Ring final circuit continuity test: end-to-end resistances of line, neutral and CPC; the line and neutral readings should be similar and the CPC reading about 1.67 times higher for 2.5/1.5 mm².

Live tests: earth fault loop impedance (Zs), prospective short-circuit current, RCD operation (trip time and current), and functional testing of devices.

RCD test: at ½ × IΔn it must not trip; at IΔn it must trip within 300 ms; at 5 × IΔn within 40 ms for a 30 mA device.

Polarity check confirms that line is connected to the centre contact of lampholders and to the switch, and that sockets are wired correctly.

The EIC records the extent of the installation, the characteristics of the supply, the schedule of test results and the declaration signed by the competent person.

Periodic inspection intervals: domestic installations typically 10 years or change of occupancy; commercial and industrial typically 5 years or less depending on use.

A tripping RCD may indicate a N-E fault, moisture ingress, a faulty appliance or cumulative leakage; a clamp meter can measure leakage current on the earthing conductor.

Insulation resistance readings below 1 MΩ indicate deterioration; divide the installation into sections to locate the faulty circuit.

Continuity failure on a ring final circuit often indicates a broken conductor or a loose terminal; the end-to-end test will show a high or open reading.

High earth fault loop impedance may be caused by a poor earth connection, a long circuit run or an undersized CPC; check the main earthing terminal and electrode.

Intermittent faults: check for loose terminals, damaged cable in accessories, thermal damage and moisture; use a thermal camera or insulation tester for hidden faults.

Before replacing a protective device, match the type, rating and breaking capacity; never fit a higher rating to stop tripping.

Maintenance schedule should include visual inspection, tightening of terminals, cleaning of ventilation, testing of RCDs and recording of results.

After repair, re-test the affected circuit and update the certification; a minor works certificate is appropriate for like-for-like replacement.

A consumer unit must be of a type that provides automatic disconnection and be installed so that live parts are not accessible; metal enclosures must be earthed.

The main switch isolates line and neutral; the main protective bonding conductor connects to the main earthing terminal within the consumer unit.

A split-load board uses a 30 mA RCD for socket circuits and a separate section for lighting and other circuits to limit nuisance tripping.

Distribution board labelling must identify each circuit, the protective device rating and the RCD group; a schedule of circuits must be fixed inside or adjacent.

Isolation of a distribution board requires the main switch to be locked off and the outgoing devices proved dead before work.

Control equipment: contactors and relays must be selected for the load current, duty and coil voltage; overload relays protect motors against sustained overcurrent.

Diversity in distribution boards: the sum of the ratings of outgoing devices may exceed the rating of the main switch, provided diversity is applied.

Neutral and earth bars must be separate in a distribution board downstream of the origin in a TN-C-S system; a combined bar is only at the origin.

Cable routes should be planned to avoid safe zones violations; mark positions of accessories and containment before chasing or drilling.

Terminations must be tight, with no exposed copper beyond the terminal; use the correct crimp or screw terminal and torque to manufacturer specification.

Conduit bends should be formed with a bending spring or former to avoid kinking; the internal diameter must remain clear for cable drawing.

Socket outlets should be mounted at a consistent height, typically 300 mm above floor level for domestic, and switches at 1200 mm, unless the client specifies otherwise.

Cable entries into enclosures must be through suitable glands or grommets to maintain IP rating and prevent damage to the sheath.

Labelling of circuits and warning notices must be completed before energising; verify polarity and insulation resistance before switching on.

Workmanship standards: straight runs, level accessories, no exposed cables in safe zones, correct colour coding (brown line, blue neutral, green/yellow CPC).

Handover: provide the certificate, schedule of test results, operating instructions and a demonstration to the client; record any departures from BS 7671.