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RCD Selection Calculator

Select the correct RCD type and sensitivity for any circuit

Reference Info & Formulas
RCD Types

Type AC: Detects AC residual currents only

Type A: AC + pulsating DC (EV chargers, PV)

Type B: AC + pulsating + smooth DC (VFDs)

Sensitivities

10mA: Medical locations

30mA: Personal protection (standard)

100mA: Fire protection (additional)

300mA: Fire protection / discrimination

Circuit Details
What is the RCD protecting?
Discrimination
Is there an upstream RCD?

Data: BS 7671:2018+A2:2022, BS EN 62423

For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.

How This Calculator Works

RCD selection matches the device type to the expected fault current waveform and the sensitivity to the required level of protection.

The calculator evaluates the connected load types to determine which RCD type is needed. Modern electronic equipment produces non-sinusoidal fault currents that older Type AC devices cannot detect. The selection process considers the waveform of possible earth fault currents, the required sensitivity for shock or fire protection, and whether time-delay is needed for discrimination with other RCDs in the installation.

I∆n (upstream) ≥ 3 × I∆n (downstream)
I∆n
= Rated residual operating current (sensitivity) of the RCD
3×
= Minimum ratio for current discrimination between upstream and downstream devices

BS 7671 Regulation 536.4.1.3

For time discrimination, the upstream device must also be time-delayed (Type S, with a minimum non-operating time of 40ms). Both current and time discrimination must be satisfied simultaneously for reliable selectivity.

Quick Reference — RCD Types

RCD types and the fault current waveforms they detect

BS EN 62423, BS 7671 Regulation 531.3.3
RCD TypeDetectsTypical Applications
Type ACSinusoidal AC onlyBeing phased out — not permitted in new installations under BS 7671
Type AAC + pulsating DC (>6mA DC)Standard for most circuits: sockets, lighting, general equipment
Type FType A + high-frequency fault currentsCircuits supplying VFD-connected single-phase equipment (washing machines, heat pumps with inverter)
Type BAC + pulsating DC + smooth DCEV chargers (without built-in DC detection), three-phase VFDs, some PV inverters

Type A is the minimum for all new installations. Type AC is no longer acceptable except for like-for-like replacement on unmodified circuits.

Quick Reference — RCD Sensitivity

RCD sensitivity ratings and their protection purpose

BS 7671 Regulation 411.3.4, 532.1
Sensitivity (I∆n)Protection TypeTypical Use
10mAEnhanced personal protectionMedical locations, high-risk environments
30mAPersonal shock protectionSocket outlets, bathrooms, outdoor circuits, concealed cables
100mAFire protectionDistribution circuits, rising mains, upstream discrimination
300mAFire protectionMain switch / incomer, upstream discrimination, cable fire protection

Practical Notes

Type AC Is Not Permitted in New Work

Type AC RCDs must not be installed on new circuits or where circuits are substantially modified. Modern electronic loads (LED drivers, switch-mode power supplies, inverters) generate DC components in fault currents that Type AC devices cannot detect. Using a Type AC RCD in a new installation is a non-compliance with Regulation 531.3.3 and a potential shock hazard.

Check Manufacturer Data for EV and PV

Before specifying an expensive Type B RCD for EV chargers or PV inverters, check the equipment's installation manual. Many modern EV chargers include 6mA DC fault detection internally, and most PV inverters incorporate an RCMU (residual current monitoring unit). Where the equipment provides its own DC fault protection, a Type A or Type F RCD is sufficient externally, saving significant cost.

Discrimination Requires Both Current and Time

Setting the upstream RCD to a higher sensitivity alone does not guarantee discrimination. You must also use a time-delayed (Type S) upstream device. Without the intentional time delay, both devices will see the fault current simultaneously and may both trip, defeating the purpose of selective protection. A common arrangement is a 100mA Type S at the board incomer with 30mA RCBOs on individual circuits.

Frequently Asked Questions

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