Question 1

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

Accepted Answer

The letter indicates the magnetic trip characteristic — how much overcurrent causes instant tripping. Type B trips at 3-5 times rated current (best for domestic circuits — lighting, sockets). Type C trips at 5-10 times (used where moderate inrush currents occur — fluorescent lighting, small motors). Type D trips at 10-20 times (used for high inrush loads — welding equipment, transformers, X-ray machines). For domestic installations, Type B is standard for virtually all circuits.

Question 2

What is the difference between RCD and RCBO?

Accepted Answer

An RCD (Residual Current Device) provides earth leakage protection for a group of circuits but no overcurrent protection. An RCBO (Residual Current Breaker with Overcurrent) combines RCD and MCB protection in a single device — it provides both earth leakage AND overcurrent protection per circuit. RCBOs prevent nuisance tripping affecting multiple circuits, as each circuit has independent protection.

Question 3

What RCD type do I need?

Accepted Answer

Type AC detects sinusoidal AC earth faults only — it is the basic type but not suitable for all modern loads. Type A also detects pulsating DC faults — this is the minimum requirement for most domestic circuits under BS 7671 18th Edition. Type F adds protection for high-frequency faults from variable speed drives and inverters (e.g. washing machines, air conditioning units with VFDs).

Question 4

Why does my RCD keep tripping?

Accepted Answer

Common causes: a genuine earth fault on an appliance (test by unplugging devices one at a time), moisture in outdoor sockets or junction boxes, deteriorating cable insulation, faulty RCD (they should be tested regularly with the test button), or cumulative leakage from multiple appliances on one RCD circuit. RCBO boards solve the last issue by giving each circuit independent RCD protection.

Type	Magnetic Trip Range	Application	Examples
B	3–5 x In	Domestic, commercial lighting	Sockets, lighting, immersion heater
C	5–10 x In	Moderate inrush loads	Fluorescent banks, small motors, UPS
D	10–20 x In	High inrush loads	Welders, transformers, X-ray, compressors

Rating (A)	Type	Typical Circuit	Cable Size
6	B	Lighting (single circuit)	1.0 mm² or 1.5 mm²
10	B	Lighting (larger circuit)	1.0 mm² or 1.5 mm²
16	B	Immersion heater, towel rail	2.5 mm²
20	B	Radial socket circuit (small)	2.5 mm²
32	B	Ring final circuit (sockets)	2.5 mm²
40	B	Cooker circuit (small)	6.0 mm²
45	B	Cooker circuit, shower (9.5 kW)	6.0 mm² or 10 mm²
50	B	Large shower (10.8 kW+), cooker	10 mm²

Type	Detects	Requirement	Application
AC	AC sinusoidal only	Basic (not for all loads)	Simple resistive loads only
A	AC + pulsating DC	Minimum for most circuits	Standard domestic circuits
F	AC + pulsating DC + high frequency	For VFD/inverter loads	EV chargers, VFD washing machines
B	AC + pulsating DC + smooth DC	3-phase and DC applications	Some EV chargers, 3-phase supplies

Sensitivity (mA)	Purpose	Application
30	Personal protection (lethal shock)	All socket circuits, bathrooms, outdoor
100	Fire protection	Fire risk premises, sleeping accommodation
300	Fire protection	Fire protection on distribution boards

UK Circuit Breaker Types — MCB, RCD, RCBO

MCB Trip Curves (BS EN 60898)

Common MCB Ratings (Domestic)

RCD Types (BS EN 61008 / 61009)

RCD Ratings

Related Calculators

Frequently Asked Questions

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BS 7671 Domestic Circuit Guide

Part P RCD Requirements

UK Consumer Unit Sizes

BS 7671 18th Edition Changes

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