FI/LS-Kombischalter oder separate FI + LS — Was ist besser?

RCBO or separate RCD + MCB — Which is better?

Anyone planning or modernizing a distribution board quickly faces the question: RCBOs (FI/LS combination switches) or rather separate RCDs (FI residual current circuit breakers) and MCBs (LS miniature circuit breakers)? Both variants reliably protect against fault currents and overloads — but they differ in space requirements, costs, and handling. This guide shows you when which solution is the better choice.

What is an RCBO (FI/LS combination switch)?

An RCBO (FI/LS combination switch) — also known as a Residual Current Circuit Breaker with Overcurrent Protection — combines two functions in one device: residual current protection (RCD) and circuit protection (MCB). This means a single component performs the task that two separate devices would otherwise handle.

In contrast, the classic variant consists of an RCD protecting several downstream MCBs. Both approaches comply with standards — the differences lie in the details.

Advantages of the RCBO (FI/LS combination switch)

  • Selective protection per circuit: If an RCBO trips, only a single circuit is affected. All others remain active — no more widespread outages.
  • Easy fault finding: When it trips, it's immediately clear which circuit is causing the problem. There's no need for lengthy individual MCB checks.
  • Less wiring: Since the RCD and MCB are in one housing, the wiring effort in the distribution board is reduced. This saves time during installation.
  • Future-proof: New circuits can be added independently without having to redistribute existing RCD groups.

Advantages of the separate RCD + MCB solution

  • Lower acquisition costs: An RCD for several MCBs is often cheaper overall than individual RCBOs for each circuit.
  • Space-saving for few circuits: A 4-pole RCD plus three or four MCBs requires fewer module units than three to four individual RCBOs.
  • Proven principle: The classic combination has been standard for decades and can be found in every distribution board.
  • Easy replacement: In the event of a defect, only the affected individual device needs to be replaced — RCD or MCB, not the entire combination device.

Space requirements in the distribution board

A typical RCBO (FI/LS combination switch) occupies 2 module units (TE) per circuit. The classic variant requires approximately 2 TE for a 2-pole RCD plus 1 TE per MCB. For three circuits, the comparison is as follows:

  • 3× RCBO: 6 TE
  • 1× RCD (2 TE) + 3× MCB (3 TE): 5 TE

For a small number of circuits, the separate solution thus saves space. From about six to eight circuits per RCD group, the difference becomes smaller — and the advantages of selectivity often outweigh the extra space.

Cost comparison

An RCBO (FI/LS combination switch) costs between €30 and €60, depending on the manufacturer and type. A separate RCD costs about €25–€50, and an MCB costs €5–€15. For a group with four circuits, this results in:

  • 4× RCBO: approx. €120–€240
  • 1× RCD + 4× MCB: approx. €45–€110

The separate solution is therefore significantly cheaper for the same number of circuits. However, the RCBO approach offers better availability and easier fault diagnosis — an advantage that pays off in everyday use.

When is which variant worthwhile?

RCBOs (FI/LS combination switches) are ideal when:

  • Maximum availability is important (e.g., home office, servers, refrigerators)
  • Many circuits need to be protected in a small space
  • Fault finding needs to be quick and uncomplicated
  • Individual circuits are to be added later

Separate RCDs + MCBs are sensible when:

  • The budget is limited
  • There are few circuits per group
  • The distribution board is already dimensioned tightly
  • A proven, easy-to-understand solution is desired

Standards and regulations

Both variants are permissible according to ÖVE/ÖNORM E 8001 and DIN VDE 0100-410. The crucial factor is that each circuit has both residual current protection (≤ 30 mA) and appropriate circuit protection. The choice between a combination and single solution is left to the planner.

Important: Since the revision of DIN VDE 0100-530, it is recommended to limit the number of circuits per RCD to a maximum of six. RCBOs automatically meet this requirement.

Conclusion

The decision between an RCBO (FI/LS combination switch) and a separate RCD + MCB solution depends on budget, space conditions, and requirements. Those who want maximum selectivity and easy fault finding will opt for the combination switch. Those who want to save costs and manage with few circuits will do well with the classic variant.

For new installations and modernizations, the trend is clearly towards RCBOs — the convenience in fault finding and high availability speak for themselves. You can find both solutions in our shop from trusted manufacturers such as ABB, Eaton, Hager and Schrack.

Are you planning a new distribution or want to retrofit? Browse our RCBOs (FI/LS combination switches), RCDs (FI residual current circuit breakers), and MCBs (LS miniature circuit breakers) — all from brand manufacturers, delivered quickly.

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