Circuit breakers protect every circuit in every installation. This guide explains how MCBs work, the difference between Type B, C, and D, why they replaced fuses, and what it means when one keeps tripping.
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
A circuit breaker is a switching device that automatically disconnects a circuit when the current flowing through it exceeds a safe level. Its job is to protect the cables and equipment on that circuit from damage caused by overcurrent — whether from an overload (too many appliances drawing too much current) or a short circuit (a direct fault between live conductors).
In UK domestic and commercial installations, the most common type of circuit breaker is the MCB (Miniature Circuit Breaker). Every modern consumer unit or distribution board contains MCBs — one for each circuit. You will see them labelled with ratings like B6, B16, B32, C32, or D63, where the letter indicates the type and the number is the current rating in amps.
MCBs replaced the older rewirable fuses and cartridge fuses that were standard in UK installations for decades. While fuses still exist in some older properties, all new work uses MCBs (or RCBOs, which combine MCB and RCD protection in one device).
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.
An MCB contains two separate trip mechanisms, each designed to respond to a different type of fault:
A bimetallic strip inside the MCB carries the circuit current. When the current exceeds the rated value, the strip heats up and bends. The higher the overload, the faster it bends. At a set deflection, the strip releases a mechanical latch that opens the MCB contacts. This is an inverse time characteristic — a small overload takes longer to trip than a large one. A 32A MCB carrying 40A might take several minutes to trip. At 64A (twice rated), it trips much faster. This allows for normal short-duration current spikes (like switching on a load) without nuisance tripping.
An electromagnetic solenoid (coil) surrounds the current path inside the MCB. Under normal current, the magnetic field is too weak to move the plunger. Under short circuit conditions — where the current is many times the rated value — the magnetic field becomes strong enough to pull the plunger, which releases the latch and opens the contacts almost instantaneously (typically within 10 milliseconds). The threshold at which the magnetic trip operates depends on the MCB type (B, C, or D).
When the MCB trips, an arc forms between the opening contacts. The MCB contains an arc chute — a stack of metal plates that splits the arc into smaller segments, cools it, and extinguishes it rapidly. This is critical for safely interrupting high fault currents.
For most of the 20th century, UK homes were protected by rewirable fuses — a ceramic holder with a piece of fuse wire stretched between two terminals. When too much current flowed, the wire melted and broke the circuit. The homeowner then had to replace the fuse wire with a new piece of the correct rating before power could be restored.
If you encounter a property with rewirable fuses during an EICR, this alone is not a defect — but it is often accompanied by a C3 (Improvement Recommended) observation suggesting a consumer unit upgrade.
The letter before the current rating (B32, C32, D63) indicates the instantaneous magnetic trip threshold — the multiple of rated current at which the solenoid trips the MCB without any delay.
The standard choice for domestic circuits. A B32 MCB trips magnetically between 96A and 160A. Suitable for resistive loads with no significant inrush — lighting circuits, socket outlets, immersion heaters, electric showers, cooker circuits. Type B is the default unless there is a specific reason to use C or D.
Used where loads have a moderate inrush current on startup. A C32 MCB trips magnetically between 160A and 320A. Suitable for small motors, commercial fluorescent lighting, air conditioning units, and some IT equipment. Also commonly used for EV charger circuits where the charger electronics may have brief inrush.
Used for heavy inductive loads with very high inrush currents. A D63 MCB trips magnetically between 630A and 1,260A. Suitable for large motors, transformers, welding equipment, X-ray machines, and industrial discharge lighting. Rarely seen in domestic work. Always verify the Zs is low enough for disconnection within the required time when using Type D.
The critical point: as you move from Type B to C to D, the MCB needs progressively higher fault currents for instantaneous disconnection. This means the maximum permitted Zs values are progressively lower. A circuit that complies with a Type B MCB might not comply with a Type C, because the Zs is too high for the Type C to trip fast enough. Always check BS 7671 Tables 41.3 and 41.4 (Chapter 41, Regs 411.4.202–411.4.203) before changing MCB type.
Enter the MCB type and rating in Elec-Mate and instantly see the maximum permitted Zs value from BS 7671.
Try it free for 7 days
16 certificate types, 70+ calculators, RAMS, quoting, invoicing, AI agents, and 46+ training courses — from £6.99/mo.
Start free trialWhen an MCB trips, it is doing its job — protecting the circuit. Understanding why it tripped is the first step in fixing the problem.
The total current drawn by connected loads exceeds the MCB rating. The thermal element heats up and trips after a delay. Symptoms: the MCB trips after running for a while (minutes to hours), especially when multiple high-power appliances are used simultaneously. Fix: reduce the load on the circuit or split it into two circuits.
A direct connection between line and neutral or line and earth, causing a very high current. The magnetic element trips the MCB almost instantly. Symptoms: the MCB trips immediately when switched on or when a specific appliance is plugged in. Fix: identify and repair the fault — damaged cable, faulty appliance, water ingress, or incorrect wiring.
If the MCB is actually an RCBO (or fed via an RCD), it may trip due to an earth fault — current leaking to earth through damaged insulation, moisture, or a faulty appliance. The RCD element detects the imbalance between line and neutral current and trips. See What Is an RCD for more detail on earth fault protection.
During an EICR or initial verification, several tests relate directly to the MCBs installed:
BS 7671 Table 41.3 values assume conductors at normal operating temperature. On site, you measure Zs cold (ambient temperature), so the conductor resistance is lower than it will be under load. To account for this, GN3 requires that the measured Zs on site must not exceed 80% of the tabulated maximum — the most common compliance error when reading Zs values in the field.
Common Type B MCB limits (BS 7671 Table 41.3, 230 V, Reg 411.4.202):
| MCB | BS 7671 max Zs (Ω) | Site limit ×0.80 (Ω) |
|---|---|---|
| B6 | 7.28 | 5.82 |
| B16 | 2.73 | 2.18 |
| B20 | 2.19 | 1.75 |
| B32 | 1.37 | 1.10 |
If the MCB type, rating, or breaking capacity is incorrect for the circuit, this is recorded as a C2 (Potentially Dangerous) or C1 (Danger Present) observation, depending on the severity.
Point your phone camera at the consumer unit. Elec-Mate reads every MCB type and rating from the photo — B6, B32…
Try it free for 7 daysVerified 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
Elec-Mate includes AI board scanning, built-in Zs calculators, and instant MCB reference tables. Complete EICR and EIC certificates faster on site. 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