RCDs, RCBOs and Type A Protection Explained for Landlords

What an RCD Does

An RCD monitors the current flowing in the live and neutral conductors of a circuit. Under normal conditions, the same current that flows out through the live conductor returns through the neutral. If these values differ — even by a very small amount — it means current is taking a different path, which could be through a person.

A 30mA RCD will detect a difference of 30 milliamps between the live and neutral current and trip the circuit within about 40 milliseconds. This response is fast enough to prevent a fatal electric shock in most circumstances, and is the standard for protection in domestic and residential installations.

The Difference Between an RCD and an MCB

An MCB (miniature circuit breaker) protects against overcurrents — if too much current flows, it trips. An RCD protects against earth faults — if current leaks to earth, it trips. They address different types of fault and a circuit needs both types of protection.

On older consumer units, MCBs and RCDs were separate devices. The RCD would protect a group of circuits (a 'split-load' arrangement), while each circuit had its own MCB for overcurrent protection. This arrangement has limitations: if the RCD trips, it takes all the circuits on that half of the board off supply simultaneously.

What an RCBO Is

An RCBO (residual current circuit breaker with overcurrent protection) combines the functions of an RCD and an MCB in a single device. Each circuit has its own RCBO, providing both overcurrent protection and 30mA earth fault protection on an individual circuit basis.

Modern consumer units are typically fitted with RCBOs for each circuit. If an earth fault occurs on the ring main, only the ring main trips — not every other circuit in the property. This is both safer and more convenient than a split-load arrangement where multiple circuits share a single RCD.

Type AC vs Type A Protection — Why It Matters Now

Here is where it gets more detailed, and where the requirements have changed in recent years.

Standard RCDs and RCBOs in the UK have historically been Type AC — they detect sinusoidal (AC) fault current. This covers the type of leakage current produced by most traditional electrical appliances.

However, modern electronic equipment — washing machines with variable speed motor drives, EV chargers, heat pumps, inverter-driven appliances — can produce DC components in their leakage current. Type AC devices cannot reliably detect DC leakage current. A Type AC RCD connected to a modern washing machine with a variable speed motor may fail to trip even if the appliance has an earth fault producing DC leakage.

Type A RCDs: The Current Requirement

Type A RCDs and RCBOs detect both AC and pulsating DC fault current. They are now the minimum standard required by BS7671 for circuits supplying socket outlets in most domestic locations — because those sockets may be used to supply modern appliances that produce DC leakage.

In practice, this means that where a consumer unit upgrade is being carried out, or where RCBOs are being installed on a circuit that supplies socket outlets, Type A devices should be used rather than Type AC. This is not a matter of choice — it is what BS7671 requires.

On an EICR, an inspector may record the presence of Type AC protection on circuits where Type A is required as an observation. The coding applied will depend on the specific risk and context.

What This Means for Landlords in Practice

For landlords reviewing an EICR that references Type A protection, the key question is which circuits are affected and what the inspector has recommended. If the consumer unit is otherwise in good condition and the observation relates to a small number of socket circuits, it may be possible to replace the relevant devices without a full board replacement.

If the consumer unit is older and a replacement is already under consideration, specifying Type A RCBOs from the outset is the appropriate approach — it avoids the need to revisit the board again within a short period as the regulations continue to be enforced more consistently.