What is the disconnection time for a final circuit in a TN system?

For a TN system at 120–230V AC, the maximum disconnection time for a final circuit is 0.4 seconds per BS 7671:2018 Table 41.1 (Regulation 411.3.2.2).

What is the disconnection time for a distribution circuit?

For TN systems, the maximum disconnection time for a distribution circuit is 5.0 seconds (Regulation 411.3.2.3). For TT systems it is 1.0 second (Regulation 411.3.2.4).

What is the difference between RCD Type AC and Type A?

Type AC RCDs detect sinusoidal AC fault currents only. Type A RCDs detect both sinusoidal AC and pulsating DC fault currents, making them suitable for circuits supplying electronic equipment such as LED drivers, EV chargers with onboard rectifiers, and IT equipment.

What is the max Zs for a 30mA RCD?

The maximum earth fault loop impedance for a 30mA RCD at 230V is 1667Ω per BS 7671:2018 Table 41.5.

When should I use a Type B RCD?

Type B RCDs detect all fault current types including smooth DC. They are required for EV charging installations, solar PV systems with DC-side earth faults, and medical equipment per BS EN 62423.

What sensitivity RCD do I need for socket outlets?

Socket outlets rated up to 32A in domestic and similar installations require 30mA RCD protection per Regulation 411.3.3. This also applies to mobile equipment outdoors and cables installed in walls at less than 50mm depth.

RCD Disconnection Times — BS 7671:2018 Table 41.1 Reference

Amendment 4 publishes 15 April 2026 — see what's changing →

Maximum disconnection times for protective devices

Maximum disconnection times from BS 7671:2018 Table 41.1. For automatic disconnection of supply to prevent electric shock. Times depend on system type (TN/TT), voltage, and circuit type (final/distribution).

Table 41.1 — Final circuit disconnection times

Maximum disconnection times (seconds)
Voltage range	TN AC	TN DC	TT AC	TT DC
50–120V	0.8	N/R	0.3	N/R
120–230V	0.4	5.0	0.2	0.4
230–400V	0.2	0.4	0.07	0.2
>400V	0.1	0.1	0.04	0.1

N/R = disconnection not required for protection against electric shock at this voltage.

Distribution circuit disconnection times

System type	Max time (s)
TN systems	5.0
TT systems	1.0

Regulation 411.3.2.3 (TN) and 411.3.2.4 (TT).

Special locations (reduced times)

Location	Max time (s)
Medical Group 2	0.1
Agricultural	0.2

Section 710 (medical) and Section 705 (agricultural). Applies to both TN and TT systems.

RCD types — which to use and when

Type AC

Sinusoidal AC fault currents only

Basic RCD protection. Detects AC residual currents only. Being phased out in favour of Type A for most applications.

Older domestic installations
Purely resistive loads (heaters, filament lighting)
Not suitable for circuits with electronic equipment

Type A

Sinusoidal AC + pulsating DC fault currents

The standard RCD type for most modern installations. Detects AC and pulsating DC fault currents generated by electronic equipment.

LED drivers and electronic lighting
IT equipment and computers
EV chargers with onboard rectifiers (Mode 2)
Washing machines, dishwashers, induction hobs
Minimum requirement for most circuits per BS 7671

Type F

Composite frequencies up to 1kHz

For circuits supplying equipment that generates high-frequency fault currents. Extends Type A protection to composite frequencies.

Variable frequency drives (VFDs)
UPS systems
Inverter-driven equipment (heat pumps, air conditioning)

Type B

All fault current types including smooth DC

The most comprehensive RCD type. Detects all residual current waveforms including smooth DC. Required where DC fault currents can occur.

EV charging (Mode 3, dedicated circuits)
Solar PV systems with DC-side earth faults
Medical equipment (per Section 710)
Battery storage systems
BS EN 62423

Type S (Selective / Time-delayed)

Intentional time delay for discrimination

Not a sensitivity type but a time-delay variant. Used upstream to allow downstream RCDs to trip first, preventing nuisance tripping of the whole installation.

Main switch RCDs in split-load consumer units
Board incomers where multiple RCDs are downstream
Available in Type A and Type B variants

Table 41.5 — Max Z_s for RCDs at 230V

Maximum earth fault loop impedance for RCDs to BS EN 61008 / BS EN 61009. Where an RCD provides fault protection, Z_s must ensure the residual current operates the RCD.

IΔn	Max Z_s (Ω)
30 mA	1667
100 mA	500
300 mA	167
500 mA	100

For full max Z_s tables for MCBs, fuses and MCCBs, see maxzs.co.uk

RCD sensitivity ratings — when to use each

10 mA

Additional protection — high-risk environments

Bathrooms and shower rooms (some specifications)
Medical locations (supplementary protection)
Construction sites (some specifications)
Rarely used in standard domestic installations

30 mA

General additional protection — the standard for most circuits

Socket outlets up to 32A (Reg 411.3.3)
Cables in walls at less than 50mm depth
Mobile equipment used outdoors
Bathrooms and swimming pools (Section 701, 702)
TT system final circuits
The default choice for domestic and commercial installations

100 mA

Fire protection

Distribution boards
Large installations where 30mA would cause nuisance tripping
Fire protection for cables (Reg 532.1)

300 mA

Fire protection — main distribution

Main distribution boards
Industrial installations
Fire protection where 100mA is impractical

500 mA

Fire protection — specialised equipment

Medical systems
Specialised industrial equipment
Large motor circuits

Table 41.1: Applies to automatic disconnection of supply for fault protection per Chapter 41 of BS 7671:2018.

TN systems: TN-S, TN-C-S (PME). TT systems: Own earth electrode.

120–230V AC: The most common voltage range for UK installations. 0.4s for TN final circuits, 0.2s for TT final circuits.

Amendment 4: Disconnection times unchanged in Amendment 4. See what is changing at bs7671amendment4.co.uk

Max Z_s: Full tables for MCBs, fuses, and MCCBs at maxzs.co.uk

RCD Standards: BS EN 61008 (general RCD), BS EN 61009 (RCBO), BS EN 62423 (Type B), BS 4293 (industrial), BS 7288 (socket outlets).

RCD protection is validated automatically in PUG — certification software that checks every circuit against BS 7671.

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