RCBO vs RCD + Separate MCB: Which Combination Do You Need?
An RCBO gives every circuit its own combined earth-fault and overcurrent protection in one device; a shared RCD feeding several MCBs protects those same circuits against earth faults collectively, with each MCB handling its own overcurrent protection individually. Both are compliant, widely used approaches — the difference is what happens when one circuit develops a fault, and how much of the board goes down with it.
This guide compares both arrangements directly, including the discrimination and cost trade-offs that actually decide which one belongs on a given board.
One Fault, Two Very Different Outcomes
With a shared RCD feeding a row of MCBs, an earth fault on any one of those circuits trips the RCD — which takes out every circuit sharing it, not just the faulty one, because the RCD has no way to know which downstream circuit the leakage came from. With RCBOs, each circuit has its own dedicated earth-fault sensing built into the same device as its overcurrent protection, so a fault on one circuit trips only that RCBO. Everything else on the board keeps running. This is the entire argument for RCBOs in one sentence: better discrimination between circuits, at a higher per-circuit cost.
Side-by-Side Comparison
| Feature | Shared RCD + MCBs | RCBOs |
|---|---|---|
| Fault discrimination | Whole RCD group trips together | Only the faulty circuit trips |
| Cumulative leakage risk | Higher — many circuits share one threshold | Lower — each circuit has its own threshold |
| Board space | More compact per circuit | Each RCBO typically wider than an MCB |
| Cost per circuit | Lower — one RCD serves many MCBs | Higher — one RCBO per circuit |
| Fault diagnosis | Must isolate circuits individually to find the fault | Tripped RCBO identifies the faulty circuit immediately |
Three Questions That Decide the Right Layout
How Costly Is It if the Wrong Circuits Go Down Together?
If a board mixes critical loads — server racks, refrigeration, life-safety systems — with general circuits, losing all of them together because of a fault on one unrelated circuit is a real business or safety cost. RCBOs remove that risk entirely by isolating faults to a single circuit; a shared RCD accepts it as a trade-off for lower cost.
How Many Filtered Electronic Loads Are on the Board?
Boards with lots of IT equipment, LED drivers or VFDs are prone to cumulative leakage nuisance tripping when several small "healthy" leakage currents share one RCD threshold. Splitting those circuits across individual RCBOs — or at least across more, smaller RCD groups — reduces how often that adds up to a trip.
What's the Budget and Board Space Available?
RCBOs cost more per circuit and are typically wider than an MCB, so an all-RCBO board needs more enclosure space and budget than a shared-RCD layout for the same number of ways. On smaller domestic and light-commercial boards where discrimination matters less, a well-sized split across two or three RCD groups is often the pragmatic middle ground.
Frequently Asked Questions
Is an RCBO board always the "correct" or "better" choice?
Both approaches are recognised, compliant designs when applied correctly — RCBOs simply trade higher cost and board space for better discrimination. Many well-designed boards use a mix: RCBOs on critical or leakage-prone circuits, and shared RCD groups on straightforward, low-risk circuits.
Can I retrofit RCBOs into a board that currently has a shared RCD?
Usually yes, provided the board and busbar system support it — RCBOs are designed to fit the same DIN rail and busbar arrangement as MCBs on most modern consumer units and distribution boards. Confirm compatibility with the specific board manufacturer before ordering.
Do RCBOs still need the same RCD type (AC/A/B) consideration as standalone RCDs?
Yes — an RCBO's earth-fault sensing element needs the same Type AC/A/B consideration as a standalone RCD for the load it protects. A circuit feeding a VFD or EV charger needs an RCBO with Type A or B sensing, not the basic Type AC found on many standard RCBOs.
Why would anyone still choose a shared RCD given RCBOs exist?
Cost and board space, mainly — on a board with many low-risk final circuits where an occasional shared trip is an acceptable inconvenience rather than a real problem, a shared RCD delivers full compliance at meaningfully lower cost than an all-RCBO layout.
How many circuits is it reasonable to put on one shared RCD?
There's no single fixed number — it depends on the cumulative leakage of what's actually connected. As a practical guide, keep an eye on how many filtered electronic loads share a group and split further if nuisance tripping starts appearing as more equipment is added.
Related Reading
Shop the Parts
| Product | Use | Link |
|---|---|---|
| ABB RCBO, 10A, 1-Pole, 30mA Trip Sensitivity | Individual final circuit protection, combined RCD + MCB | View product → |
| ABB RCBO, 32A, 4-Pole, 30mA Trip Sensitivity | Higher-current three-phase final circuits | View product → |
| Full RCBO range | Browse by current rating, pole count and trip sensitivity | Browse RCBOs → |
| Full RCD range | For shared-group layouts feeding separate MCBs | Browse RCDs → |
| Full MCB range | Overcurrent protection to pair with a shared RCD group | Browse MCBs → |
| All genuine ABB products | RCBOs, RCDs and MCBs from the same manufacturer | View all ABB products → |
Disclaimer: ABB, and any other manufacturer or brand names referenced in this article, are used solely for identification and compatibility purposes. Z&F Corporation is not affiliated with, authorised by, or an official representative of these manufacturers; all trademarks are the property of their respective owners.