SG PPE Electrical flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Ohm's Law

The relationship V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. Used to calculate any one value when the other two are known.

Power Formula

Electrical power P = V × I, also expressed as I²R or V²/R. Measured in watts and used to size protective devices and cables.

Series Resistance

Total resistance in a series circuit equals the sum of individual resistances. Current is the same through all components, while voltage divides across them.

Parallel Resistance

Total resistance is the reciprocal of the sum of reciprocals of individual resistances. Voltage is common across branches and current divides between them.

RMS Voltage

Root-mean-square value of an AC waveform, equal to peak value divided by √2 (0.707 × peak for a sine wave). It represents the equivalent heating effect of DC.

Power Factor

The ratio of true power in watts to apparent power in volt-amperes. A value below one indicates reactive load from inductance or capacitance.

Three-Phase Power

For a balanced three-phase load, power equals √3 × line voltage × line current × power factor. Used to size three-phase equipment and protection.

Conductor Resistance

Resistance equals resistivity times length divided by cross-sectional area (R = ρL/A). Longer or thinner conductors have higher resistance and greater voltage drop.

Harmonised Cable Colours

Standard UK/EU colours: brown for line, blue for neutral, and green/yellow for protective conductor. Older installations may use red and black.

Cable Derating Factors

Factors applied to tabulated current capacity to account for grouping, ambient temperature, and thermal insulation. They are multiplied together to give effective capacity.

Voltage Drop Limit

BS 7671 recommends a maximum 3% voltage drop for lighting and 5% for other circuits from the origin of the installation. Ensures equipment operates correctly.

Bending Radius

Minimum radius to which a cable may be bent without damaging insulation or conductors. Specified by the manufacturer and typically a multiple of cable diameter.

Protective Conductor

A conductor that provides a path for fault current and bonds exposed metalwork. Identified by green/yellow colouring and essential for automatic disconnection.

Earth Fault Loop Impedance

Total impedance of the path from the origin through the live conductor, the fault, and back via the protective conductor. Must be low enough for disconnection.

Disconnection Time

The maximum time permitted for a protective device to operate under fault conditions. Typically 0.4 seconds for final circuits up to 32 A and 5 seconds for distribution circuits.

RCD Sensitivity

The rated residual operating current of an RCD. 30 mA is required for additional protection, while 300 mA is used for fire protection of premises.

Main Protective Bonding

Connection of extraneous-conductive-parts to the main earthing terminal. The bonding conductor is typically at least 10 mm² or half the earthing conductor size.

TT System

An earthing arrangement where the supply neutral is earthed at the source and the installation uses its own earth electrode. Usually requires RCD protection for fault disconnection.

MCB Trip Types

Type B trips at 3-5 times rated current, Type C at 5-10 times, and Type D at 10-20 times. Selection depends on inrush current and loop impedance.

IP Rating

Ingress protection code indicating resistance to solids and liquids. The first digit covers solids and the second covers water, e.g. IP44.

Special Locations

Areas such as bathrooms, swimming pools, and agricultural buildings with additional protective requirements. Often include 30 mA RCD protection and zone restrictions.

Isolation Device

A switch or isolator that disconnects all live conductors and can be locked off. Essential for safe maintenance and repair work.

Motor Overload Protection

A protective device that prevents sustained overcurrent from damaging motor windings. Distinct from short-circuit protection and set to the motor's full-load current.

Insulation Resistance Test

A test at 500 V DC measuring resistance between live conductors and earth. A minimum of 1 MΩ is generally acceptable for most circuits.

Ring Final Circuit Test

End-to-end resistance readings r1, rn, and r2 are taken. Cross-connected readings should equal (r1+rn)/4 and (r1+r2)/4 to confirm ring integrity.

Polarity Test

Verifies that line and neutral are correctly connected at every accessory and that switches are in the line conductor. Prevents dangerous reversed connections.

RCD Trip Time Test

Verifies operation at rated residual current and five times rated current, and confirms no trip at half rated current. Ensures the device protects as intended.

Safe Isolation

A procedure to confirm a circuit is dead before work: identify, isolate, lock off, prove tester, prove dead, and re-prove tester on a known source.

BS 7671

The IET Wiring Regulations, the primary UK standard for electrical installation design and verification. It sets requirements for safety, protection, and testing.

Electricity at Work Regulations

UK statutory regulations imposing duties on employers and employees to maintain electrical systems safely. They underpin safe working practices and competence requirements.

Proving Dead

Confirming a circuit is de-energised using an approved voltage indicator tested before and after use. A critical step in safe isolation.

Short Circuit Fault

An unintended low-resistance path between live conductors, causing high current flow. Protective devices must interrupt it rapidly to prevent damage or fire.

Nuisance RCD Tripping

Unwanted RCD operation caused by accumulated leakage currents, moisture, or shared neutral faults. Diagnosed by isolating circuits and testing leakage individually.