| 1. Read the name | Both devices respond to residual current, which can occur when current leaving on the live conductor does not return through the neutral conductor. | Residual current protection only. | Residual current protection combined with overcurrent protection. | Check the device marking and manufacturer’s documentation; appearance alone may not identify its protective functions. |
| 2. Separate the protections | Overcurrent includes overload current and short-circuit current. Residual current protection addresses a different electrical condition. | Does not provide overload or short-circuit protection. It must be used with suitable overcurrent protection in the installation. | Provides residual current protection and overcurrent protection for the circuit, subject to its ratings and installation requirements. | Confirm that appropriate overcurrent protection is provided and coordinated with the device and circuit. |
| 3. Know what the ratings mean | Rated current and rated residual operating current describe different characteristics. | Its rated current indicates the current it is designed to carry under specified conditions; its residual-current rating indicates a separate operating characteristic. | Also has ratings for its overcurrent function, as well as residual-current characteristics. | Do not interpret an RCCB’s rated current as overload protection. Select ratings according to the circuit design and applicable requirements. |
| 4. Consider circuit coverage | A residual-current device can serve one or more circuits, depending on the design. | One RCCB may protect several downstream circuits when correctly designed with suitable overcurrent protection. Leakage on one circuit can affect the shared device. | An RCBO is commonly used to provide residual-current and overcurrent protection for an individual circuit. | Consider how many circuits will be disconnected by a trip and how faults will be located. |
| 5. Use the test button correctly | The test button checks the device’s tripping mechanism by creating a test condition specified by the manufacturer. | Testing does not add overcurrent protection and does not replace required inspection or verification. | Testing does not replace inspection or verification of either protective function. | Follow the manufacturer’s instructions and local rules. Have selection, installation, and testing carried out by a qualified person. |