Choosing the Right RCBO for a TT System

William Winter
May 1
4 min read

Choosing the Right RCBO for a TT System

TT systems are widely used across the UK, but they behave very differently to TN systems. That difference has a direct impact on how protective devices operate, and how circuits behave when a fault occurs.

Understanding this is key when selecting an RCBO.

How a TT System Behaves

In a TT system, the earth path is provided by an earth electrode rather than a low resistance metallic path.

This means:

Fault current is lower
The return path is less predictable
Overcurrent devices cannot be relied on for disconnection

Instead, protection relies on residual current detection.

Why RCD Protection Is Critical

On a TT system, disconnection is achieved by detecting an imbalance between line and neutral.

When a fault to earth occurs, some current leaves the intended path and returns through the earth. The RCD detects this imbalance and disconnects the supply.

This is why a 30mA RCD is typically used to meet the requirements of BS 7671.

Disconnection vs Isolation

It is common to assume that once a protective device has operated, the circuit is safe. In many cases this is true, but it depends on what the device has actually done.

Disconnection refers to stopping the flow of current. Isolation means fully separating the circuit from the supply so that no part of it remains connected.

These are not always the same outcome.

A device may interrupt current by disconnecting the line conductor, which removes the load and stops normal operation. However, if the neutral remains connected, part of the circuit is still electrically linked back to the supply.

On a TT system, this becomes more relevant. Protection relies on RCD operation rather than high fault current, and the neutral conductor is not guaranteed to remain at zero potential under all conditions. Even after the device has operated, the circuit may not be fully isolated.

This does not mean the installation is unsafe or non-compliant. However, it does mean that the condition of the circuit after disconnection depends on how the device switches the conductors.

Understanding this distinction is important when assessing how an installation behaves in practice, not just whether it meets the minimum requirements.

Neutral Behaviour on TT Systems

The neutral conductor is connected back to the supply and referenced to earth.

Under real conditions, it is not always at zero potential. It can carry voltage due to load imbalance, voltage drop, or system conditions.

If only the line is disconnected, the neutral remains connected to the installation.

RCBO Types and Their Effect

Why RCBOs effect the install differently

All RCBOs are designed to detect an imbalance between the current in the line and neutral conductors and disconnect the supply when that imbalance exceeds the operating threshold. In that respect, they perform the same protective function and can all be used to achieve compliance with BS 7671.

Where they differ is not in how they detect a fault, but in how they interrupt the circuit.

Some RCBOs disconnect only the line conductor. When these devices operate, current flow is stopped and the load is de-energised. However, the neutral conductor remains connected to the installation and back to the supply.

Other RCBOs disconnect both the line and neutral conductors. When these devices operate, the circuit is fully separated from the supply, and no part of it remains connected.

This difference does not affect whether the device will trip under fault conditions, but it does affect the condition of the circuit after disconnection.

On a TT system, this becomes more relevant. Protection relies on residual current detection rather than high fault current, and the neutral conductor is not guaranteed to remain at zero potential under all conditions. As a result, a circuit that has had only the line conductor disconnected may still have a connection back to the supply through the neutral.

In contrast, a device that disconnects both conductors removes that connection entirely.

Both approaches can meet the requirements of BS 7671. However, they do not leave the installation in the same state after operation.

Understanding this difference allows for more informed design decisions, particularly on TT systems where system behaviour under fault conditions is more dependent on correct device operation.

Why It Matters

TT systems rely on residual current protection rather than high fault current to achieve disconnection. As a result, the correct operation of the RCD is critical to the safety of the installation.

However, disconnection is only part of the outcome.

Once a device has operated, the condition of the circuit depends on how the conductors have been interrupted. On a TT system, where the neutral is not guaranteed to remain at zero potential, a circuit that is no longer carrying current may still have a connection back to the supply.

This is why what happens after a device operates is just as important as whether it trips.

Understanding how different RCBO designs behave allows for more informed selection, ensuring the installation performs as intended under fault conditions, not just that it meets the minimum requirements.