ADS is the most important protective measure in BS 7671. This guide explains the principle, disconnection times, maximum Zs values for MCB types B/C/D, earthing system differences, practical loop impedance testing, and worked examples.
Free for 7 days · No charge until day 8 · Cancel anytime · Used by 1,000+ UK electricians
Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
1,000+
UK electricians
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer — DP Electrical
Automatic Disconnection of Supply (ADS) is the protective measure described in Section 411 of BS 7671:2018+A4:2026. It is the most commonly applied protection against electric shock in UK electrical installations — virtually every circuit in a domestic or commercial installation relies on ADS.
The principle is straightforward: if a fault occurs that makes an exposed-conductive-part live (for example, a live conductor contacts a metal enclosure), the protective device must disconnect the supply fast enough to prevent a lethal electric shock. ADS is not a single device — it is a combination of earthing, protective conductors, and protective devices working together.
Understanding ADS is essential for every electrician. It determines how you select protective devices, how you size cables, how you design earthing arrangements, and what you test during initial verification and periodic inspection. If ADS fails, the consequence is a sustained touch voltage on metalwork that people can contact — a direct risk to life.
Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.
Regulation 411.3.1.1 states that ADS requires the coordination of two elements:
The relationship is governed by Ohm's law. When a line-to-earth fault occurs, the fault current (If) equals the supply voltage (Uo) divided by the earth fault loop impedance (Zs):
If = Uo / Zs
Where Uo = 230V (nominal), Zs = total earth fault loop impedance in ohms
If the fault current (If) is high enough, the MCB or fuse trips within the required time. If Zs is too high, the fault current is too low, the device takes too long to trip, and the person touching the faulty equipment is exposed to a dangerous voltage for too long.
BS 7671 specifies maximum disconnection times in Regulation 411.3.2. The times differ depending on the type of circuit and the earthing system:
Regulation 411.3.2.2 requires a maximum disconnection time of 0.4 seconds for final circuits not exceeding 32A in TN systems. This covers virtually all socket-outlet circuits, lighting circuits, and fixed equipment circuits in domestic and commercial installations. The 0.4-second time is based on the physiological effects of electric shock — at 230V touch voltage, 0.4 seconds is the threshold beyond which the risk of ventricular fibrillation becomes unacceptable.
Regulation 411.3.2.3 permits a maximum disconnection time of 5 seconds for distribution circuits in TN systems. A distribution circuit supplies one or more distribution boards (sub-mains) rather than directly supplying current-using equipment. The longer time is permitted because the exposed-conductive-parts of distribution circuits are generally not accessible to the general public and because simultaneous contact with earth and a distribution conductor is less likely.
For TT systems, Table 41.1 in BS 7671 specifies a disconnection time of 0.2 seconds for final circuits not exceeding 32A, and 1 second for distribution circuits. These shorter times reflect the higher touch voltages that can occur in TT systems due to the earth electrode resistance.
| System | Final circuits up to 32A | Distribution circuits |
|---|---|---|
| TN-S / TN-C-S | 0.4s | 5s |
| TT | 0.2s | 1s |
The maximum earth fault loop impedance (Zs) for a circuit depends on the type and rating of the protective device. BS 7671 Table 41.3 provides maximum Zs values for MCBs to BS EN 60898 and RCBOs to BS EN 61009. The values differ significantly between Type B, C, and D MCBs because of their different magnetic trip thresholds:
| Rating | Type B | Type C | Type D |
|---|---|---|---|
| 6A | 7.67 ohms | 3.83 ohms | 1.92 ohms |
| 10A | 4.60 ohms | 2.30 ohms | 1.15 ohms |
| 16A | 2.87 ohms | 1.44 ohms | 0.72 ohms |
| 20A | 2.30 ohms | 1.15 ohms | 0.57 ohms |
| 32A | 1.37 ohms | 0.68 ohms | 0.34 ohms |
| 40A | 1.15 ohms | 0.57 ohms | 0.29 ohms |
Values from BS 7671 Table 41.3. These are at conductor operating temperature (70 degrees C). When testing at ambient temperature, the measured Zs must not exceed 80% of these values.
The practical impact is significant. A 32A ring final circuit protected by a Type B MCB allows a maximum Zs of 1.37 ohms. If the same circuit were protected by a Type C MCB (which would be unusual for a ring circuit, but illustrates the point), the maximum Zs drops to 0.68 ohms — almost half. This is why Type B MCBs are the standard choice for domestic circuits: they offer the most headroom on earth fault loop impedance.
The earthing system of the installation has a major impact on ADS. The earth fault loop impedance, the available fault current, and the choice of protective device all depend on whether the installation is TN-S, TN-C-S, or TT.
The supply has a separate earth conductor — typically the lead sheath or steel wire armour of the supply cable. The Ze (external earth fault loop impedance) is typically 0.35 to 0.8 ohms. ADS is straightforward in TN-S systems because the Ze is relatively low and predictable. The main concern is older TN-S supplies where the cable sheath may be deteriorating, increasing Ze over time.
The supply neutral and earth are combined in a single PEN conductor, with the neutral earthed at multiple points throughout the network. The Ze is typically very low — 0.2 to 0.35 ohms — giving excellent ADS performance. PME is the most common supply arrangement for new-build domestic properties in the UK. The low Ze means most circuits will achieve ADS with MCBs alone. However, PME carries a specific risk: if the PEN conductor is lost (broken neutral), the installation metalwork can rise to a dangerous potential.
The installation has its own earth electrode — typically a driven rod. There is no metallic return path to the transformer; the fault current returns through the general mass of earth. The Ze is typically 20 to 200 ohms (or more), depending on soil conditions and electrode type. This high impedance means overcurrent devices (MCBs and fuses) cannot achieve ADS alone — the fault current is far too low to trip them within the required time. Regulation 411.5.2 requires an RCD as the protective device in TT systems. A 30mA RCD can achieve ADS with a Zs up to 1667 ohms, making it effective even with very high earth electrode resistance.
When carrying out an EICR, always confirm the earthing system type and measure Ze before assessing individual circuits. The earthing system determines whether ADS by overcurrent device alone is feasible or whether RCD protection is essential.
16 certificate types, 70+ calculators, RAMS, quoting, invoicing, AI agents, and 46+ training courses — from £6.99/mo.
Start free trial
Earth fault loop impedance testing is a core part of both initial verification (Regulation 643.7) and periodic inspection. The test confirms that the Zs at the furthest point of each circuit is within the limits required for ADS.
Elec-Mate's schedule of test results auto-validates your Zs readings against the maximum permitted values for the protective device.
Try it free for 7 daysThese examples demonstrate how to verify ADS compliance for common domestic circuits.
Given: Ze = 0.35 ohms (TN-C-S supply). Ring circuit in 2.5mm² T&E, 60m total ring length. R1+R2 per metre from tables = 0.0246 ohms/m (at 20 degrees C).
Calculate R1+R2 for the ring: For a ring circuit, the R1+R2 at the mid-point = (total R1+R2) / 4 = (60 x 0.0246) / 4 = 1.476 / 4 = 0.369 ohms at 20 degrees C.
Correct for operating temperature: Multiply by 1.20 (correction factor for 70 degrees C PVC): 0.369 x 1.20 = 0.443 ohms.
Calculate Zs: Zs = Ze + R1+R2 = 0.35 + 0.443 = 0.793 ohms.
Compare: Maximum Zs for 32A Type B MCB = 1.37 ohms. 0.793 ohms is well within the limit. Circuit complies.
Given: Ze = 0.72 ohms (TN-S supply, older area). Radial circuit in 2.5mm² T&E, 30m cable run. R1+R2 per metre = 0.0246 ohms/m.
Calculate R1+R2: 30 x 0.0246 = 0.738 ohms at 20 degrees C.
Correct for temperature: 0.738 x 1.20 = 0.886 ohms.
Calculate Zs: 0.72 + 0.886 = 1.606 ohms.
Compare: Maximum Zs for 20A Type B MCB = 2.30 ohms. 1.606 ohms is within the limit. Circuit complies — but with less headroom than Example 1. On a TN-S supply with higher Ze, long cable runs consume more of the available Zs budget.
Given: Ze = 0.35 ohms. Ring circuit in 2.5mm² T&E, 60m total ring length. Type C MCB (incorrectly specified for a ring circuit).
Zs at 70 degrees C: 0.35 + 0.443 = 0.793 ohms (same as Example 1).
Compare: Maximum Zs for 32A Type C MCB = 0.68 ohms. 0.793 ohms exceeds the limit. Circuit does NOT comply. The Type C MCB needs a higher fault current to trip magnetically, but the circuit impedance is too high. Solution: change to a Type B MCB (maximum Zs = 1.37 ohms), or add RCD/RCBO protection.
During inspection and testing, these are the most common reasons circuits fail ADS compliance:
On an EICR, a circuit that fails ADS is typically coded C2 (potentially dangerous) — the protective measure is impaired and may not operate in the event of a fault. If the touch voltage exceeds 50V AC and disconnection will not occur, a C1 code (danger present) may be appropriate.
Verified reviews from the UK App Store.
Elec-Mate is my go to app for business and electrical work. It's feature rich without feeling cluttered. A true all in one app for quotes, certs, calculations, RAMS, EICRs, and more. I use it every day without fail, and it makes my workflow much smoother since I'm not jumping between apps anymore. The price-to-feature ratio is excellent. Any issues I've had, the developer responds within the hour and usually fixes them the same day. 100% recommend.
I've used the app and the web based version for a while now and it's well worth the investment. If you're an apprentice or experienced Spark give it a go, you won't be disappointed.
I've been using Elec-Mate for a while now, and honestly, it's one of the best apps I've ever downloaded. Every aspect of it feels thoughtfully designed, from the clean and intuitive interface to the powerful features that make everything so easy to manage. It's clear that a lot of care and attention went into building this app, and it shows in every detail.
Real feedback from real sparks
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer
Sole Trader · DP Electrical
“I've won two contracts this month because I could turn quotes around same-day with the AI cost engineer.”
Nathan Perry
Electrician · NP Electrical Services
“The study centre got me through my AM2. Mock exams and flashcards are brilliant.”
Jake Pizey
3rd Year Apprentice · Apprentice
Join 1,000+ UK electricians using Elec-Mate for loop impedance recording, automatic Zs validation, and on-site EIC/EICR certificates. 7-day free trial, cancel anytime.
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer, DP Electrical
From £6.99/mo after trial — less than a coffee a week
We use cookies to improve the app and measure what works. Cookie Policy