Type AC RCD Symbol Explained: AC, A, F & B RCD Symbols Guide

doing a board change and stood in the wholesalers staring at RCDs wondering why ones £15 and another ones £180

heres what the difference actually is and which one you need without getting done by building control

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RCD Symbols Explained: Type AC, A, F & B

A practical guide to the RCD symbols and markings UK electricians see on consumer units, including what Type AC, A, F and B devices detect and when each type is normally used.

Use this alongside BS 7671 manufacturer data and the job specification before selecting or coding an RCD type.

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What is an RCD and Why Does the Type Matter?

the basics

RCD monitors live and neutral currents - if theres a difference (earth leakage) it trips in milliseconds and disconnects the supply

saves lives, prevents fires, you know the drill

why type matters now

back in the day everything was purely resistive - tungsten bulbs, basic heaters, done

now every house has LED drivers, switch-mode power supplies, USB sockets, smart tech everywhere

these electronic loads can leak DC onto the AC circuit and this is where it gets interesting

the blinding effect

standard RCD gets hit with smooth DC leakage and the ferrite core in the sensing coil becomes saturated - basically "blinded"

when a genuine AC fault happens the RCD wont trip

youve got protection that isnt actually protecting anything

regulation check

amendment 2 of 18th edition (BS 7671:2018+A2:2022) changed everything

regulation 531.3.3 says you must select RCD type based on equipment connected

this effectively banned Type AC RCDs for general domestic use

if youre still fitting Type AC on new installs youre not compliant simple as that

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Type AC RCD

what it detects

pure alternating sinusoidal residual current only

thats it - just AC

symbol

simple sine wave (~)

use cases

strictly legacy installations or purely resistive loads with zero electronics

old school tungsten lighting, basic immersion heater with no electronic timer

BS 7671 position

regulation 531.3.3 makes it clear - Type AC shall only be used for fixed equipment where you absolutely know the load has zero DC components

sparky tip

wholesalers still trying to flog cheap Type AC boards online because theyve got pallets of the things

dont fall for it on new domestic work - they wont comply with Amd 2 and building control will bounce it

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Type A RCD

what it detects

AC sinusoidal current plus pulsating DC residual current up to 6mA

symbol

sine wave (~) with a pulsating bump above a solid line

use cases

this is your new baseline

standard domestic circuits - TVs, computers, LED lighting, most modern appliances

when required

minimum standard for general use under BS 7671

you can also use it for single-phase EV chargers but only if the charger has built-in RDC-DD (residual direct current detecting device) that handles DC faults over 6mA

if it hasnt got that you need Type B

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Type F RCD

what it detects

everything Type A detects plus high-frequency mixed residual currents (up to 1kHz) plus smooth DC up to 10mA

symbol

sine wave, pulsating bump, and mixed high-frequency squiggles

use cases

single-phase inverter-driven appliances

becoming really relevant for modern white goods - washing machines with variable speed drives, air conditioning units, heat pumps

bonus feature

enhanced resistance to nuisance tripping from inrush currents these motors create

so if youre getting called back for random tripping on a heat pump circuit check if youve stuck a Type A on it when it needs Type F

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Type B RCD

what it detects

AC, pulsating DC, high-frequency AC (up to 1kHz), and smooth DC residual current

detects everything basically

symbol

sine wave, pulsating bump, and smooth straight line (—)

use cases

serious specialized kit

required for 3-phase variable speed drives, solar PV inverters, battery storage systems, medical equipment, EV chargers without built-in 6mA DC protection

cost consideration

eye-wateringly expensive compared to Type A because of the complex internal electronics needed to detect smooth DC

always check the EV or solar manufacturer instructions to see if Type B is explicitly required or if Type A is sufficient

dont just assume - read the manual

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RCD Symbols Quick Reference

Type AC: sine wave (~)

Type A: sine wave + pulsating bump

Type F: sine wave + pulsating bump + composite high-frequency wave

Type B: sine wave + pulsating bump + straight line (smooth DC)

memorize these or stick a photo on your phone because the symbols are what youll see on the device

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Testing Different RCD Types

test button vs instrument testing

the physical test button homeowners should press every 3-6 months is just mechanical

deliberately unbalances the coil to check the mechanism hasnt seized

your MFT instrument test actually verifies the device operates within legally required timeframes

expected trip times

BS 7671 Amd 2 simplified testing

for standard 30mA RCD test at 1x (IΔn) using AC setting - should trip under 300ms

if its providing additional protection (sockets etc) do 5x test - should disconnect under 40ms

why wont it trip

during instrument test: if it fails MFT test theres probably existing DC leakage blinding the coil

unplug all appliances on the circuit and test again

during button test: if button doesnt work but MFT does you might have downstream neutral-to-earth fault providing parallel path bypassing the sense coil

or the mechanical button contacts are corroded and the units knackered

bin it

recording results

rather than scribbling trip times on the back of a ripped bit of paper use proper EICR software that records and syncs everything automatically

turns out it actually saves time when youre doing the cert later

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Which RCD Type Do I Need?

heres the decision logic:

purely resistive load with no electronics? Type AC (though Type A is safer and future-proof)

EV charger, solar PV, or produces >6mA smooth DC? Type B (unless EV has RDC-DD then Type A is fine)

single-phase frequency-controlled motor (heat pump / modern washing machine)? Type F

standard domestic circuit (LEDs, PCs, TVs)? Type A minimum

when in doubt check manufacturer data sheet - they usually specify what type you need

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Common Mistakes

fitting Type AC in new work

just because wholesalers got them on clearance doesnt mean you can use them

throwing Type AC main switch in new dual RCD board is direct violation of reg 531.3.3

dont do it

blindly using Type A for EVs

if charger doesnt have internal DC fault protection (>6mA) a Type A will get blinded

you must use Type B

ive seen sparks argue with me on this - read the regs mate its not optional

forgetting high-frequency loads

wiring new inverter-driven heat pump on standard Type A RCBO then getting callbacks for nuisance tripping

always check manufacturer data - they usually spec Type F or Type B

if youre getting nuisance tripping also check your earth loop impedance isnt through the roof

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Summary Table

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Quick Summary

Type AC - basically obsolete for new domestic work, dont use it

Type A - your new baseline, minimum for general circuits

Type F - inverter-driven single-phase stuff like heat pumps

Type B - solar, battery storage, EV chargers without built-in protection

regulation 531.3.3 from amendment 2 changed the game - you must select based on connected equipment

wholesaler trying to sell you cheap Type AC boards? walk away

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questions people ask

what are the different types of RCD?

Type AC (pure AC only), Type A (AC + pulsating DC up to 6mA), Type F (Type A + high frequency + smooth DC up to 10mA), Type B (detects everything including smooth DC over 10mA)

which RCD type do i need?

depends on equipment - Type A minimum for general domestic, Type F for inverter-driven appliances like heat pumps, Type B for solar PV and EV chargers without built-in DC protection

can i still use Type AC RCD?

only on purely resistive loads with zero electronics and zero chance of electronics being added later. amendment 2 effectively banned them for general domestic use

whats the difference between Type A and Type F RCD?

Type F detects high-frequency residual currents and smooth DC up to 10mA that Type A cant see. also more resistant to nuisance tripping from inverter motor inrush

do EV chargers need Type B RCD?

depends - if charger has built-in RDC-DD (6mA DC protection) you can use Type A. if it doesnt you need Type B. check manufacturer instructions

why is my Type A RCD nuisance tripping?

probably got inverter-driven load creating high-frequency leakage - heat pump, modern washing machine, air con. swap to Type F

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thats it - everything about RCD types without the waffle

pick the right one first time and you wont be getting callbacks