Question 1

When should I use a Type C MCB instead of Type B?

Accepted Answer

Type C MCBs trip at 5-10 times their rated current, compared to 3-5 times for Type B. Use Type C where the connected load has significant inrush current that would nuisance-trip a Type B device. Common examples include large banks of fluorescent or LED lighting with electronic drivers, small motors (up to about 5.5kW DOL), and air conditioning compressors. For general domestic socket and lighting circuits, Type B remains the correct choice.

Question 2

Do all circuits need RCD protection under the 18th Edition?

Accepted Answer

Amendment 2 to BS 7671:2018 significantly expanded RCD requirements. Regulation 411.3.4 now requires 30mA RCD protection for all socket outlet circuits rated up to 32A, all circuits in bathrooms and shower rooms, all circuits supplying mobile equipment outdoors, and cables concealed in walls at a depth less than 50mm. Lighting circuits in domestic premises also require RCD protection. However, certain circuits may be exempt if disconnection could cause a greater hazard — such as fire alarm or emergency lighting supplies.

Question 3

What are the advantages of an RCBO board over a split-load board?

Accepted Answer

An RCBO board gives each circuit its own combined overcurrent and RCD protection. If an earth fault trips one circuit, the rest remain live — improving selectivity and reducing nuisance whole-board outages. A split-load board groups multiple circuits under one or two RCDs, so a fault on any circuit in the group trips them all. RCBO boards cost more initially but are increasingly specified in domestic and commercial installations for reliability.

Question 4

What does the formula Ib ≤ In ≤ Iz mean?

Accepted Answer

This is the fundamental cable coordination rule from Regulation 433.1. Ib is the design current of the circuit (the actual load current), In is the rated current of the protective device (the MCB or fuse rating), and Iz is the current-carrying capacity of the cable after applying correction factors. The rule ensures the device rating covers the load, and the cable can handle whatever current the device allows to flow without tripping.

Question 5

How do I choose the correct breaker rating for a circuit?

Accepted Answer

Start by calculating the design current (Ib) from the connected load. Select a standard device rating (In) that is equal to or greater than Ib. Then verify the cable you intend to use has a corrected current-carrying capacity (Iz) equal to or greater than In. For example, a 7.4kW shower at 230V draws 32.2A (Ib), so you select a 40A MCB (In), then confirm your cable — say 10mm² T&E clipped direct at 64A (Iz) — satisfies 32.2 ≤ 40 ≤ 64.

Question 6

What is the difference between Type B, Type C, and Type D MCBs?

Accepted Answer

Type B trips at 3–5× rated current (domestic, resistive). Type C trips at 5–10× (small motors, LED banks). Type D trips at 10–20× (large motors, transformers, welding) per BS EN 60898-1.

Question 7

What circuits need RCD protection under BS 7671 Amendment 2?

Accepted Answer

Regulation 411.3.4 requires 30mA RCD for: all socket outlets up to 32A, bathroom circuits, outdoor mobile equipment, concealed cables at <50mm depth, and domestic lighting. Some circuits exempt per Regulation 411.3.5.

Question 8

What is the difference between an MCB and an RCBO?

Accepted Answer

MCB provides overcurrent protection only. RCBO combines overcurrent + RCD earth fault protection. RCBOs give each circuit independent fault protection so one fault doesn't trip other circuits.

Question 9

How do I select the right MCB rating for my circuit?

Accepted Answer

Calculate Ib (design current), select In ≥ Ib, verify cable Iz ≥ In. The relationship Ib ≤ In ≤ Iz must hold per Regulation 433.1.

Circuit Type	RCD Required?	Regulation
Socket outlets ≤ 32A	Yes	411.3.4
Bathroom / shower room circuits	Yes	701.411.3.3
Outdoor circuits / mobile equipment	Yes	411.3.4
Cables concealed in walls < 50mm deep	Yes	522.6.202
Domestic lighting circuits	Yes	411.3.4
Fire alarm / emergency lighting	May be exempt	See 411.3.5

Circuit Breaker / RCBO Selector

How This Calculator Works

Quick Reference — MCB Trip Characteristics

Quick Reference — RCD Requirements

Practical Notes

Frequently Asked Questions