When you’re working on domestic electrical installations — from upgrading an old consumer unit to fault-finding on a socket circuit — the circuit breaker you choose is more than just a switch. It’s a safety device. One that can make all the difference in protecting the customer, the property, and the electrical system itself from overloads, short circuits, and earth faults.
If you’re an independent electrician, electrical contractor, or maintenance engineer, knowing which device to select (and why) is essential to staying compliant with BS 7671 Wiring Regulations — and giving clients confidence in your work.
Why It Matters
In domestic environments, most faults stem from overloaded circuits, faulty appliances, or ageing equipment. Choosing the correct MCB, RCBO, or RCD helps prevent fires, protect lives, and reduce nuisance tripping. It also means fewer call-backs, fewer failed EICRs, and a better reputation for reliable, safe work.
Let’s walk through the most common types of circuit protection you’ll come across on residential jobs — and when to use each one.
MCBs (Miniature Circuit Breakers)
Best for: Standard lighting and socket circuits in most homes
Purpose: Protects against overloads and short circuits
MCBs are still widely used in domestic installations. You’ll typically see them on lighting and power circuits, with current ratings including:
- 6A (lighting)
- 16A or 20A (radials)
- 32A (ring finals)
- 40A (cookers or showers)
Top tip: Don’t just match the amp rating — check the tripping curve. That leads us to…
Type B vs Type C MCBs: What’s the Difference?
Understanding curve types is key to selecting the right device:
- Type B: Trips at 3–5x rated current — ideal for general domestic use (lighting, sockets, basic appliances).
- Type C: Trips at 5–10x rated current — better for circuits with higher inrush currents like garage tools or small domestic workshops.
Caution: Installing a Type C where a Type B is required could delay fault disconnection — a potential safety issue.
RCBOs (Residual Current Breaker with Overcurrent)
Best for: Individual circuits where you need both earth leakage and overload protection
Why use them: They combine the function of an MCB and RCD into one unit
RCBOs are now best practice in many new builds and consumer unit upgrades. They reduce nuisance tripping by isolating faults to a single circuit — especially useful for:
- Kitchen socket rings
- Outdoor circuits
- Bathroom fans
- Garage supplies
- Compliant with 18th Edition Regs (Reg 411.3.3)
RCDs (Residual Current Devices)
Best for: Split-load boards or older installs where one RCD protects multiple MCBs
Purpose: Trips when it detects earth leakage — critical for life protection
While RCBOs are more commonly specified today, many domestic setups still use 30mA RCDs to protect groups of circuits. This is fine in certain situations, but be mindful: if one circuit trips, everything else on that RCD goes off too.
SPDs & AFDDs: Do You Need Them?
As electrical safety standards evolve, devices like SPDs and AFDDs are becoming more relevant — even in domestic work.
Surge Protection Devices (SPDs)
Best for: New consumer units or homes with sensitive or high-value electronics
Why they matter: Protect against overvoltages from lightning or power surges — now required in most new installations under the 18th Edition.
Consider installing SPDs when:
- Upgrading boards
- Working in areas prone to lightning
- Clients have home offices, smart systems, or expensive AV equipment
Arc Fault Detection Devices (AFDDs)
Best for: High-risk areas like HMO bedrooms or socket outlets in sleeping areas
Note: Still relatively rare in average domestic jobs, but increasingly specified in premium installs or rented properties where safety scrutiny is higher.
Final Checks Before You Choose
Before fitting or swapping a breaker, make sure to:
- Check cable sizes and calculate the design current
- Confirm Zs values and ensure compliance with disconnection times
- Label all circuits clearly
- Issue updated EIC or Minor Works Certificates as needed
- Explain your choices to the client — especially if you’re recommending an upgrade from MCBs to RCBOs or adding surge protection
Dan’s Final Thoughts
Choosing the right circuit protection is about more than convenience — it’s about getting it right. As SPDs become the norm, RCBOs replace traditional split-load boards, and AFDDs emerge in high-spec work, your approach to board builds needs to evolve too.
Stay current. Stay compliant. And always choose protection that suits the install — not just what’s left in the van.