circuit design

Why should stranded cables be used where machinery is subject to movement or vibration?

Stranded cables are flexible and can withstand movement and vibration better than solid conductors. This reduces the risk of conductor fatigue, breakage and loose connections. Solid conductors should never be used to supply machinery.

Correct support, vibration loops at terminations and locking washers also help prevent problems caused by vibration.

What does Appendix D of the IET On-Site Guide cover?

Methods of support for cables, conductors and wiring systems:

D1 – Cables in accessible positions

D2 – Overhead wiring between buildings

D3 – Conduit supports

D4 – Cable trunking supports

Which type of gland prevents moisture entering equipment?

A compression gland – Creates a seal around the cable entry, preventing moisture and contaminants from entering the equipment.

Which BS 7671 table specifies the maximum supportable mass for flexible cords carrying a suspended load?

Table 4F3A.

What is the maximum ambient temperature for thermoplastic and thermosetting cables?

Thermoplastic – 85°C.

Thermosetting – 90°C.

What is the construction, application and earthing requirements of SY Multi-Flex cable (BS 6500)?

SY cable – Multicore cable containing 3, 4 or 5 stranded copper conductors with 70°C PVC insulation. Used in construction, food processing, heating and air-conditioning installations where flexibility and mechanical protection are required.

Construction – The insulated cores are surrounded by PVC bedding, galvanised steel wire braid (GSWB) for mechanical protection and a transparent PVC outer sheath. The sheath protects the braid from abrasion, fraying and weathering.

Core identification – Usually brown, blue, black and green/yellow, although some manufacturers use black cores with white numbering. In cables with 3 or more cores, one core must be green/yellow to identify the CPC.

Termination and earthing – Requires a specially designed gland. A banjo washer connects the steel braid to metal-clad accessories, but the braid cannot be used as the sole means of earthing. A dedicated green/yellow CPC core must provide the earth connection.

What is the construction, uses and main precautions of MICC cable (BS EN 60702)?

MICC cable – Contains copper conductors surrounded by magnesium oxide (MgO) powder insulation, enclosed in a seamless copper sheath. Highly resistant to fire, extreme temperatures and mechanical damage. Used in fire alarms, boiler rooms and generating stations.

Duty ratings – Light duty (L) or heavy duty (H), depending on copper sheath thickness.

Moisture – MgO is hygroscopic (absorbs moisture), which reduces insulation resistance and can cause tracking between conductors. Cable ends must be sealed. Moisture can penetrate 150–200 mm; affected sections can be dried or cut back and resealed.

Identification – Example: 4H2.5 = 4 cores, heavy duty, 2.5 mm² conductors.

Termination – Brass pot with earth conductor, MICC compound, plastic pot seal, coloured sleeving and cable gland. Fit a matching PVC shroud if the cable has a PVC over sheath.

This keeps the important exam points, including the answers to the multiple-choice questions, without turning the card into a textbook.

What is the construction, purpose and key features of fire-rated cable (FP200 / Firetuf)?

Fire-rated cable – Provides a cheaper alternative to MICC for fire alarm installations. Its insulation converts into a fire-retardant, electrically insulating coating when exposed to fire, allowing circuits to continue operating without releasing harmful smoke, vapour or gases.

Construction – Contains solid copper conductors (stranded from 4 mm² upwards) with silicone insulation. A bare tinned copper CPC is included. The cores are surrounded by polyester-backed aluminium foil for additional fire protection, then covered by an outer sheath.

Identification – Available with 2 or 4 cores plus CPC. Red sheath is commonly used for fire alarms; white for emergency lighting and exit signs.

Termination – Uses a standard compression gland and requires no specialist termination tools.

What is the maximum low-temperature limit of Arctic-grade PVC flexible cords?

-20°C.

What are 2 A socket outlets used for?

Designed for low-power appliances, mainly standard and table lamps. Usually supplied by a 6 A lighting circuit, with an assumed demand of 0.5 A per outlet.

What is design current (Ib), and how is it calculated?

The expected current a circuit will draw under normal operating conditions. Calculate using \(I = P \div V\), then multiply by the number of outlets or appliances. Apply diversity where appropriate to account for loads unlikely to operate at full power simultaneously.

Example: 10 lamp holders at 100 W each:

\(I = (100 \times 10) \div 230 = 4.35\,A\)

What is diversity, and how is it applied when calculating cooker design current?

Diversity reduces the assumed current demand because it is unlikely all cooker elements will operate at full power simultaneously.

Apply the allowance:

• First 10 A at 100%.

• Add 30% of the remaining current.

• Add 5 A if a socket outlet is incorporated into the cooker control unit.

Example: A 12.8 kW cooker draws 55.65 A at full load.

\(10 + (45.65 \times 0.30) + 5 = 28.70\,A\)

This reduces the assumed design current from 55.65 A to 28.70 A, helping determine suitable cable and protective-device ratings.

There is a lot more to this, practise this Page 28

What formulas are used to calculate design current for non-purely resistive circuits?

For non-purely resistive AC loads, include power factor (cos θ) to account for the phase difference between voltage and current.

Single-phase: \(I = P/(V \times PF)\)

Three-phase: \(I_L = P/(\sqrt{3}\times V_L\times PF)\)

For three-phase, √3 accounts for the 120° phase displacement between phases.