How to wire an RCBO

Written by autumn st. john
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How to wire an RCBO
Only a qualified electrician can wire RCBOs, which include the operations of a mini-circuit breaker. (Jupiterimages/ Images)

Use of the RCBO electrical safety device has been on the rise since the introduction of the 17th edition of the IEE Wiring Regulations, with the devices getting increasing publicity both in the traditional press and online. The RCBO combines the operations of a miniature circuit breaker (MCB) and a residual current circuit breaker (RCCB). This means that RCBOs act as a back-up if a circuit or appliance demands an excess of current, and also protect against the risk of fault currents flowing to earth, reducing the risk of fire and electric shock. The process of installing a RCBO consists of wiring the device correctly. UK law specifies that only trained, qualified electricians can install RCBOs.

Skill level:

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Things you need

  • Flat-head screwdriver
  • Wire cutters/strippers

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  1. 1

    Check that the RCBO is switched off and that the rated voltage is suitable.

  2. 2

    Take off the unit’s front cover by turning both its fasteners 90 degrees and gently lifting the cover away from the rest of the device.

  3. 3

    Fully open the RCBO’s bottom clip and screw the incoming terminal onto either the busbar system or the line phase cable. This ensures the RCBO is in place and secure. Make sure the tooth of the busbar is within the device’s terminal cage and close the unit’s bottom clip back up. Hold the RCBO firmly onto the busbar system and tighten the low terminal screw as much as you can.

  4. 4

    Cut the blue/black flying neutral supply lead to the appropriate length and connect it to the neutral terminal block.

  5. 5

    Cut the cream flying earth lead to the appropriate length and connect it to the earth terminal block.

  6. 6

    Connect the load connections to the unit’s outgoing terminals. Ensure you don't connect the load neutral wire to the distribution board’s neutral terminal block.

  7. 7

    Press the “Test” button on the front of the unit after installation to test the RCBO. Check that all of the cable connections are tight enough. This includes factory made connections, as these may have become loose during the process of installation or during transit. The factory made connections should be correctly torqued according to the appropriate standard for RCBOs. All other connections should have had a minimum of 2 Nm applied to them.

Tips and warnings

  • Perform a monthly functional test of the RCBO by pushing the “Test” button. If the RCBO trips, it is functioning as it should be. You can tell if the RCBO has tripped by checking whether or not it’s gone to the “Off” position.
  • An RCBO should only be installed by a trained, qualified electrician, as per legal requirements in the UK. By ignoring this requirement you are not only in breach of the law, but also putting your safety at risk.

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