Disconnection Time Calculator — Verify 0.4s and 5s Rules to BS 7671

Disconnection time is the maximum duration a protective device is permitted to take to disconnect the supply under earth fault conditions. When a live conductor comes into contact with exposed conductive parts (such as a metal appliance casing), fault current flows through the earth fault loop. The protective device must interrupt this current quickly enough to prevent a person receiving a lethal electric shock.

BS 7671:2018+A4:2026 sets specific disconnection time limits based on the type of circuit and the earthing system in use. The fundamental requirement comes from Regulation 411.3.2, which states that protective devices must disconnect the supply within the times specified in Table 41.1. These times are derived from the IEC body current curves — the relationship between current magnitude, duration, and the likelihood of ventricular fibrillation.

Getting disconnection time wrong has serious safety consequences. If a device takes too long to disconnect, anyone touching exposed metalwork during a fault could receive a sustained electric shock. The earth fault loop impedance determines the magnitude of fault current, and the protective device characteristics determine how quickly it disconnects at that current level. Both must be verified.

The maximum disconnection times are defined in BS 7671 Table 41.1 and depend on the nominal voltage and earthing system. For a 230V single-phase supply — the standard UK domestic supply — the requirements are:

These disconnection times apply to circuits where the protection against electric shock is provided by automatic disconnection of supply (ADS), which is the most common protective measure in UK installations. The cable sizing calculator and disconnection time calculator work together to ensure both thermal protection and shock protection are satisfied.

In TN systems — which include TN-S (separate neutral and earth) and TN-C-S (PME / combined neutral and earth) — the earth fault return path is through the metallic sheath or PEN conductor back to the transformer. This provides a low-impedance fault loop, meaning relatively high fault currents flow during an earth fault.

Because fault currents are high in TN systems, overcurrent protective devices (MCBs and fuses) can generally provide disconnection within the required times. The key is to verify that the actual earth fault loop impedance (Zs) at the furthest point of the circuit does not exceed the maximum value tabulated in BS 7671 for the specific protective device type and rating.

For example, a 32A Type B MCB in a TN system requires a maximum Zs of 1.37 ohms to disconnect within 0.4 seconds (Table 41.3 of BS 7671). If the measured Zs at the furthest socket on the ring final circuit exceeds 1.37 ohms, the MCB will not trip quickly enough and the circuit fails the disconnection time test. You would need to either reduce the circuit impedance (shorter cables, larger CPC) or add supplementary protection such as an RCD.

BS 7671 Regulation 411.3.3 (A4:2026) requires 30 mA RCD additional protection for all socket outlets rated up to 32 A, regardless of whether the overcurrent device meets the disconnection time on its own. In domestic premises this is an absolute requirement. In non-dwellings, the RCD may be omitted only where a documented risk assessment determines that RCD protection is not necessary — omission without that assessment is a non-compliance on an EICR.

A4:2026 also introduces Regulation 411.3.4, which requires additional protection by a 30 mA RCD for AC final circuits supplying luminaires in domestic premises. This is a new requirement — lighting circuits in dwellings that were previously unprotected by an RCD will now be coded C2 on an EICR if no RCD is present. When verifying disconnection times for lighting circuits in domestic properties, also confirm 30 mA RCD protection is in place.

TT earthing systems use a local earth electrode (typically an earth rod) for the installation's earth connection, with the fault return path running through the general mass of earth. The resistance of this earth path is significantly higher than in TN systems — typically between 10 and 200 ohms for the earth electrode alone.

Because the earth fault loop impedance is so much higher in TT systems, the fault current is correspondingly lower. In most cases, the fault current in a TT system is insufficient to operate an MCB or fuse within the required disconnection time. This is why RCD protection is essential in TT installations — the RCD detects the imbalance between live and neutral caused by the earth fault current, and disconnects even when that current is relatively small.

For a 30mA RCD to disconnect within 0.2 seconds in a TT system, BS 7671 Table 41.5 requires the Zs to not exceed 1667 ohms (calculated as 50V / 0.03A). In practice, even the worst earth electrodes easily meet this requirement. The challenge in TT systems is not usually the Zs value but rather ensuring the RCD is correctly installed, functioning properly, and regularly tested.

The Elec-Mate electrical testing calculators include specific TT system verification, checking the earth electrode resistance (Ra), the total Zs, and confirming the RCD will operate within the required time.

Protective device coordination ensures that the correct device operates first during a fault — the device closest to the fault should disconnect before upstream devices. This is known as discrimination or selectivity. While discrimination is a separate topic from disconnection time, the two are closely related because both depend on the fault current magnitude and the device operating characteristics.

The maximum permitted Zs values for common protective devices at 0.4 seconds disconnection time in TN systems are tabulated in BS 7671 as follows:

When testing on site, you compare your measured Zs against these maximum values. The Elec-Mate calculator has all the tabulated values from Tables 41.2 through 41.6 built in, so you do not need to carry the regulation book. Simply select the device type and rating, enter the measured Zs, and the calculator tells you whether the circuit passes or fails.

The prospective fault current calculator complements this tool by verifying that the prospective fault current does not exceed the breaking capacity of the protective device — another critical BS 7671 requirement under Regulation 434.