Switchgear and Controlgear
Switchgear and Controlgear tests whether you can specify equipment that switches, isolates and protects safely, and whether you understand the difference between isolation, functional switching and emergency switching. These distinctions account for most of the marks in this domain.
Functions and definitions
- Isolation: off-load opening of all live poles, with a visible or reliably indicated gap, lockable in the OFF position, for safe work.
- Functional switching: everyday on-load operation, which requires a device with the correct utilisation category, such as AC-23 for motor loads rather than AC-21.
- Emergency switching: rapid disconnection to remove danger, which does not by itself provide isolation.
- Protection: overcurrent devices, RCDs and motor overload relays.
Selection and installation points
Match the device to the duty: voltage, current, AC or DC, utilisation category, breaking capacity, IP rating, ambient temperature and altitude. Provide a lock-off facility, clear labelling identifying the circuit and the source, a schedule at each board, and adequate clear working space. Enclosures must contain arcs, prevent access to live parts, and suit the environment. Switchboards must not be in bathrooms or other restricted locations, should be accessible for operation and maintenance, and need fire separation where they sit in escape routes.
Control circuits
DOL starters, star-delta starters, soft starters and VFDs each impose different duties on contactors and protection. Control circuits are commonly 24 V or 110 V for safety; emergency stops should be hard-wired and not rely on software alone; interlocks and no-volt release prevent unexpected restart.
Worked example
A 30 kW, 380 V pump has a nameplate full-load current of 56 A. The isolator must be rated at least 56 A with an AC-23 category and be lockable, mounted within sight of the motor. The contactor must be AC-3 rated at 56 A or above, with a thermal overload set to the nameplate current. The short-circuit device must allow the DOL starting current of about 6 × 56 = 336 A without tripping, so the magnetic setting is raised above that while remaining below the minimum earth fault current at the motor terminals.
Traps that cost marks
Using a functional switch as an isolator; assuming a contactor or an emergency stop provides isolation; ignoring the utilisation category for motor duty; and selecting a device without a lock-off facility.
Final Circuits
Final Circuits tests whether you can design and wire the last stage of an installation correctly: lighting, socket outlet rings, radials and dedicated appliance circuits. The exam looks for the right device and cable combination, correct spur rules, and the protection each circuit type needs.
Circuit types and typical arrangements
- Lighting: 1.5 mm² protected at 6 A or 10 A, with the switch in the line conductor; consider inrush from LED drivers and use Type C or a higher rating where nuisance tripping occurs.
- Ring final circuit: 2.5 mm² on a 32 A device, with each leg sharing the load; an unfused spur may feed one socket outlet or one twin socket, and a fused spur may feed more.
- Radial: 2.5 mm² on a 20 A device serving a limited area, or 4 mm² on a 30/32 A device.
- Dedicated circuits: water heaters, air-conditioners, cookers and motors, each with its own device and isolator.
Every circuit must satisfy Ib ≤ In ≤ It, voltage drop within 4%, and Zs within the disconnection limit, with 30 mA RCD protection where the Code requires it.
Worked example
A 3 kW water heater at 220 V draws 13.6 A and is fed 18 m from the board in 2.5 mm² with a 1.5 mm² CPC, clipped direct. Tabulated capacity is 27 A; ambient derating at 40 °C gives 27 × 0.87 = 23.5 A, so a 16 A MCB satisfies 13.6 ≤ 16 ≤ 23.5. Resistance per metre for 2.5/1.5 mm² is 19.51 mΩ, so R1 + R2 = 18 × 19.51 / 1000 = 0.35 Ω, and with Ze = 0.35 Ω, Zs = 0.70 Ω against a Type B limit of 220 / (5 × 16) = 2.75 Ω. A 30 mA RCD is required because of the location, and a double-pole isolator must be provided.
Traps that cost marks
A spur taken from a spur; two sockets on one unfused spur; a ring that is broken or cross-connected incorrectly; 2.5 mm² used on a 32 A radial; omitting the RCD for a bathroom heater; and forgetting that fixed appliances on a ring must be included in the load assessment.
Demand Assessment and Installation Design
Demand Assessment and Installation Design tests whether you can turn a list of loads into a defensible supply size, submain and switchboard. Marks go to answers that show the diversity method, the current calculation, the voltage drop check and the protection co-ordination, not just a final number.
The design sequence
- Compile connected loads by category: lighting, socket outlets, fixed appliances, air-conditioning, motors.
- Apply diversity to obtain the after-diversity maximum demand (ADMD); never apply diversity to the largest motor, and treat loads that can run together with caution.
- Convert to current using the correct power factor and the three-phase formula where applicable.
- Size the supply and submains for the ADMD with spare capacity, then verify voltage drop, PSCC, disconnection times and protective device ratings.
- Produce schedules, a single-line diagram and a load schedule, and apply to the supply company if the approved loading must change.
Typical diversity practice: lighting at about 66% of connected load, socket outlets at 100% of the first block plus 40% of the remainder, water heaters at 100% of the first plus 50% of subsequent units, and air-conditioning assessed on the actual duty cycle.
Worked example
An office fit-out has lighting 10 kW, 50 socket outlets at 300 VA each, 30 kW of air-conditioning, and two 3 kW water heaters. ADMD = (10 × 0.66) + (3 + 0.4 × 12) + 30 + (3 + 0.5 × 3) = 6.6 + 7.8 + 30 + 4.5 = 48.9 kW. At a power factor of 0.85, that is 57.5 kVA, and I = 57 500 / (√3 × 380) = 87.4 A. A 100 A three-phase supply and a 35 mm² submain are selected, then the submain voltage drop and Zs are checked before the design is signed off.
Traps that cost marks
Summing nameplate kVA with no diversity; applying diversity to the largest motor; using kW instead of kVA when calculating current; ignoring voltage drop on long submains; and forgetting spare ways and future load.
Inspection, Testing, Certification and Fault Diagnosis
Inspection, Testing, Certification and Fault Diagnosis tests the practical sequence and the interpretation of results. The examiner wants to see that you inspect before you test, test dead before you test live, and draw a conclusion you can defend on paper.
Sequence of initial verification
- Inspection of the installation against the design and the Code.
- Continuity of protective conductors, main and supplementary bonding, using a low-resistance ohmmeter with leads nulled.
- Ring final circuit continuity by the r1, rn, r2 and cross-connected methods.
- Insulation resistance at 500 V DC, minimum 1 MΩ, testing L–N, L–E and N–E with electronics disconnected and SPDs isolated.
- Polarity confirmed before energising.
- Live tests: Ze and Zs, prospective short-circuit current, RCD operation at IΔn and 5 IΔn (within 300 ms and 40 ms respectively), phase rotation and functional tests of switches, isolators and interlocks.
Results are recorded on a schedule of test results attached to the work completion or periodic certificate, with any observations coded for urgency and a date for the next inspection.
Fault diagnosis approach
Work from the supply outwards: confirm voltage at the origin, then at each distribution point, then at the point of use. Use insulation resistance to find earth faults, Zs to find high-resistance joints, a clamp meter to trace current, and thermal imaging for hot terminations. Change one variable at a time.
Worked example
A socket circuit RCD trips at random. First, ramp-test the RCD and check its 5 IΔn tripping time. Then isolate and measure insulation resistance L–E and N–E: 0.2 MΩ indicates a real earth fault, not a device fault. Split the circuit at the midpoint, retest each half, and continue until the low reading is located; the usual finding is water ingress in an outdoor socket or a damaged cable behind a fixing. Replace, retest the full circuit, and re-record the results.
Traps that cost marks
Testing insulation resistance with electronics or SPDs connected; doing live tests before dead tests; not nulling the test leads; and recording a passing Zs without confirming the device type and rating used in the limit.
Electrical Equipment, Motors, Transformers and Control Systems
This domain covers the plant at the sharp end: motors, transformers and the control gear that runs them. The exam tests nameplate interpretation, starting methods, protection settings and the safety implications of each choice.
Motors
Three-phase induction motors dominate. Full-load current comes from the nameplate, not from a guess based on kW. Direct-on-line starting draws about 6 to 8 times full-load current; star-delta reduces both starting current and starting torque to one third, so it suits loads that can start light. Soft starters and VFDs limit current and give control but introduce harmonics and require Type B RCDs or careful earthing. Protection includes a thermal overload relay set to the nameplate current, phase-failure protection, thermistors on larger machines, and correct IP and duty ratings.
Transformers
Turns ratio follows V1/V2 = N1/N2. Losses are iron loss (constant) and copper loss (proportional to current squared), so efficiency peaks near full load. Vector groups such as Dyn11 describe winding connections and phase displacement and matter for parallel operation. Oil-filled units need bunding and Buchholz protection; dry-type units suit indoor locations. Inrush current at energisation is several times rated current and must be allowed for in protection settings.
Control systems
Read control diagrams logically: the seal-in contact keeps a contactor closed after the start button is released, the stop button and overload contact are in series with the coil, and interlocks prevent conflicting operations. Emergency stops should be hard-wired, and safety functions should not depend on software alone. Label every conductor and device, and keep drawings current.
Worked example
A 1000 kVA transformer at 11 kV / 380 V has a primary current of 1 000 000 / (√3 × 11 000) = 52.5 A and a secondary current of 1 000 000 / (√3 × 380) = 1519 A. The LV breaker is rated above 1519 A with a breaking capacity above the LV fault level, and the LV earthing arrangement must be confirmed before the neutral is earthed or bonded.
A four-pole 50 Hz motor at 1440 rpm has synchronous speed 1500 rpm, so slip is 4%.
Traps that cost marks
Setting the overload to the nameplate current when the relay is in the delta leg of a star-delta starter; using star-delta where high starting torque is needed; ignoring VFD harmonics and DC residual current; and assuming a VFD removes the need to derate a motor running at low speed, where fan cooling is lost.
Special Installations and Locations
Special Installations and Locations tests whether you can apply the general rules with extra protection where the risk is higher: bathrooms, construction sites and hazardous areas. The exam expects zone boundaries, IP ratings, RCD requirements and equipment protection concepts stated accurately.
Bathrooms and showers
Zones are defined by distance from the bath or shower tray and by height. In zone 0 (inside the basin or tray) equipment must be IPX7 and of the type the manufacturer permits. Zones 1 and 2 require at least IPX4, with equipment suitable for the location. Socket outlets are not permitted in zones 0, 1 or 2; a shaver supply unit with an isolating transformer may be installed in zone 2 or beyond. All circuits serving the bathroom need 30 mA RCD protection, and fixed water heaters and showers need a double-pole isolator sited outside the zones with a cord that cannot be reached from the bath. Supplementary bonding of metallic pipework may be required where the conditions demand it.
Construction sites
Temporary installations must withstand rough handling. Hand-held tools operate at 110 V from a centre-tapped-to-earth transformer, giving 55 V to earth, with the centre tap earthed; 30 mA RCDs protect socket outlets and distribution boards; cables are routed overhead or protected, boards are IP44 or better, and isolators are lockable. Periodic inspection is frequent because the installation changes, and all equipment must be checked before use.
Hazardous areas
Gases and vapours are classified Zone 0, 1 and 2; dusts as Zone 20, 21 and 22. Equipment must match the zone with a suitable protection concept: Ex d flameproof, Ex e increased safety, Ex i intrinsically safe (Ex ia for Zone 0), Ex p pressurised or Ex t for dust. Apparatus groups IIA, IIB, IIC and temperature classes T1 to T6 must suit the gas present. Intrinsically safe circuits use blue identification, separate routing and barriers, and no live work is permitted.
Worked example
An 8 kW shower in a hotel bathroom draws 8000 / 220 = 36.4 A. It is fixed in zone 1, so it needs at least IPX4 and must be suitable for the location, on its own 40 A circuit with a 30 mA RCD. The pull-cord isolator goes outside the zones with a cord short enough that it cannot be reached from the bath, and all metal pipework is bonded or the RCD plus main bonding is confirmed as adequate.
Traps that cost marks
Thinking a 30 mA RCD permits a socket in a bathroom; forgetting the IP rating; using ordinary equipment in zone 2; misjudging zone extents; and using 220 V hand tools on a construction site.