Unidirectional and bidirectional RCBOs

How it works

Inside an RCD — and why direction can matter

Every RCD and RCBO measures the same thing: the difference between the current going out on the line conductor and coming back on the neutral. Both conductors pass through a toroidal — doughnut-shaped — magnetic core. When the currents balance, their magnetic effects cancel and nothing happens. When they differ by more than the rated residual current (usually 30 mA), the imbalance induces a signal in a sensing winding around the core, and that signal trips the device.

There are two ways that sensing signal is turned into a trip, and both constructions exist in both directional classes:

• Electromechanical devices use the sensing signal directly to trip a high-precision relay, with no electronic amplification. Typical RCCBs are electromechanical — though electronic ones exist.
• Electronic devices amplify the sensing signal with a small electronic circuit before tripping. Many compact RCBOs use electronics — and compact bidirectional RCBOs are on the market too, so "electronic" does not mean "unidirectional".

What goes wrong with a unidirectional device is specific and, in the worst case, sneaky. Some electronic RCD circuits can be damaged when voltage remains on their outgoing terminals after the device has operated — the IET identifies operation following an earth fault, or simply pressing the test button, as relevant circumstances. Backfeed a unidirectional device from an inverter or battery and it can be left with its residual-current function permanently inoperable while still looking and switching perfectly normally. For directionally marked MCBs, reverse connection can separately impair the arc-extinguishing and short-circuit performance. This is the whole argument for Regulation 530.3.201: selection must take the appropriate use of unidirectional and bidirectional devices into account.

Getting it right

How to check, specify and install correctly

1. 1

   Check the exact device — catalogue number, not brand or look

   BEAMA is explicit: suitability must be confirmed from the catalogue number, version, markings, connection diagram and the manufacturer's instructions or declaration — not from the brand, the module width, the pole count or a Type A marking. A device from a brand that makes bidirectional RCBOs elsewhere in its range is not automatically bidirectional itself, and a bidirectional current product does not establish that earlier versions of the same product were.

2. 2

   Look for the direction markings — they can be subtle

   Where supply and load must be distinguished, the product standards require clear marking — examples include 'in/out', 'line/load' and power-flow arrows, and BEAMA notes the marking may be on the side of the device rather than the face. Line and neutral symbols on their own identify the conductors, not the supply direction. Where a device is marked, connect it exactly as marked; if it is unmarked, the datasheet decides, not a guess.

3. 3

   Never connect a source to the load side of an RCD

   Amendment 4's redrafted 551.7.1 restricts connecting a source to the load side of an RCD under certain conditions. A battery or PV inverter should connect through its own suitably rated device arrangement — and where an RCD provides additional protection on a generator connection, remember it must disconnect all live conductors including the neutral, which rules out solid-neutral RCBOs for that duty.

4. 4

   Separate the direction question from the waveform question

   A bidirectional device is not automatically a Type B device, and a Type B device is not automatically bidirectional. Direction (where the supply may be connected) and waveform (what shape the residual current is — Type AC, A, F or B) are two independent selection decisions, and a PV or battery installation usually needs both answered correctly.

5. 5

   Record it at design, on the certificate and at inspection

   At design and installation, record the device selection for the source circuits and the manufacturer declaration that supports it. At periodic inspection, follow the Electrical Safety First / WRAG route: obtain the manufacturer's confirmation for the device actually fitted, keep it with the EICR if it confirms bidirectional suitability, and code only where no confirmation exists — C3 for additional-protection reliance, C2 where fault-protection disconnection relies on it.