RCD vs MCB vs RCBO: What’s the Difference?
RCDs, MCBs and RCBOs may sit side by side in a consumer unit, but they do different jobs. The quickest way to compare them is to ask which fault each device detects and whether it protects one circuit or a group of circuits.
RCD vs MCB vs RCBO comparison
| Feature | RCD / RCCB | MCB | RCBO |
|---|---|---|---|
| Residual-current protection | Yes | No | Yes |
| Overload protection | No | Yes | Yes |
| Short-circuit protection | No | Yes | Yes |
| Usual circuit coverage | May protect a group of circuits | One circuit | One circuit |
| Common markings | Type AC, A, F or B; IΔn; current rating | Current rating; B, C or D curve; breaking capacity | Both residual-current and overcurrent markings |
| Fault effect | May disconnect several downstream circuits | Disconnects its circuit | Disconnects its circuit |
RCD is the generic family name. In this comparison, “RCD” refers to the common RCCB arrangement used with separate MCBs. For the terminology and ratings in more depth, read what an RCD is.
What does an RCD protect against?
An RCD monitors the balance of current flowing through the live conductors. An imbalance indicates that some current is taking an unintended path. The device disconnects when the residual current and operating time reach its designed threshold.
An RCCB can provide residual-current protection to several downstream circuits, but it does not detect overload or short circuit. Each circuit therefore needs suitable overcurrent protection, commonly an MCB.
RCD type—AC, A, F or B—must suit the residual-current characteristics of the connected equipment. Electronic loads can influence selection, so manufacturer requirements and the current edition of BS 7671 need to be considered.
What does an MCB protect against?
An MCB protects a circuit and its conductors against overcurrent. Its thermal element responds to sustained overloads, while its magnetic element responds rapidly to high short-circuit current.
The B, C or D curve marking relates to the magnetic trip characteristic, not residual-current detection. Loads with higher starting current may require a different curve, but fault-loop conditions and disconnection requirements must still be satisfied.
An MCB does not provide residual-current protection. Where that protection is required, the MCB must form part of an arrangement with an appropriate RCD, or an RCBO can provide both functions.
What does an RCBO protect against?
An RCBO combines the functions of an RCD and MCB in one device. It responds to residual current, overload and short circuit on its individual circuit.
Because each circuit has its own residual-current device, a fault can be isolated without automatically disconnecting every circuit that would otherwise share one upstream RCCB. This can improve continuity and make fault identification easier.
RCBOs have more selection variables than the front current rating alone: residual-current type, sensitivity, trip curve, breaking capacity, poles, neutral switching, directionality and board compatibility can all matter. See our RCBO guide for a full explanation.
Grouped RCD and MCBs or individual RCBOs?
| Arrangement | Advantages | Points to consider |
|---|---|---|
| RCCB with several MCBs | Familiar layout and fewer residual-current devices. Can be cost-effective in suitable designs. | One residual-current fault may disconnect several circuits. Leakage from multiple loads can accumulate. |
| RCBO on each final circuit | Individual residual-current protection, better fault localisation and usually improved continuity. | Higher device cost and more characteristics to coordinate. Physical and manufacturer compatibility must be checked. |
| Mixed arrangement | Can allocate individual protection to higher-priority circuits while grouping others where appropriate. | Requires a clearly coordinated design and correct labelling. |
There is no universal answer based only on the property type. Circuit purpose, leakage characteristics, continuity requirements, load type, board capacity and manufacturer system all affect the choice.
Key selection checks
- Required protection: overcurrent, residual current or both.
- Circuit current and conductor capacity: the protective rating must suit the complete circuit design.
- Load characteristics: inrush, electronic controls and possible DC components affect curve and RCD type.
- Fault-current conditions: breaking capacity and required disconnection need verification.
- Continuity: consider the consequences of a single device disconnecting multiple circuits.
- System compatibility: use the enclosure manufacturer’s approved devices and busbar arrangement.
Browse dedicated ranges of RCDs, MCBs and RCBOs to compare the relevant markings and product data.
RCD, MCB and RCBO FAQs
Can an MCB replace an RCD?
No. An MCB responds to overload and short circuit; it does not provide residual-current protection.
Can an RCD replace an MCB?
An RCCB does not provide overload or short-circuit protection, so it must be coordinated with a suitable fuse, MCB or other overcurrent device.
Do you need an RCD if you have RCBOs?
Each RCBO already incorporates residual-current protection for its circuit. Whether any further upstream residual-current protection is required depends on the overall installation design and discrimination requirements.
Why might an RCBO be preferred?
It gives each circuit both forms of protection and can limit the impact of one circuit fault. That does not remove the need to select the correct type, ratings and compatible board system.
Compare our complete circuit protection range, including RCDs, MCBs and RCBOs. Match every device to the circuit design and manufacturer-approved enclosure system.
This guide is for general product-selection information. Electrical work should be designed, installed, tested and certified by a competent person in accordance with the current edition of BS 7671 and relevant manufacturer instructions.