What Is a Type B RCD and Why Do You Need One for Plug-In Solar?

If you have asked an electrician about adding solar, you have probably heard about Type B RCDs. Most people’s first question is: what is wrong with the ones I already have? Here is the answer.

What an RCD does

A Residual Current Device monitors the balance of current flowing through the live and neutral wires. In a healthy circuit these are equal. If current is leaking to earth — through a fault or a person — the balance shifts and the RCD trips within milliseconds, disconnecting the circuit before serious harm occurs.

Type A versus Type B

Type A RCDs detect alternating (AC) residual currents and half-wave rectified AC currents. These cover the fault types produced by most domestic appliances. Type A is the standard in UK consumer units and is adequate for almost all conventional loads.

Type B RCDs detect everything Type A detects, plus smooth DC residual currents. Solar microinverters converting DC to AC can, in certain fault conditions, produce a smooth DC leakage current to earth. A Type A RCD does not detect smooth DC — a fault of this type goes undetected with standard protection. A Type B device catches it.

When is Type B required?

For a hardwired solar installation, BS 7671 Amendment 4 requires Type B RCD or RCBO protection on the dedicated circuit feeding the microinverter. The electrician supplies and fits this as part of the job — a Type B RCBO in the consumer unit costs £40 to £80 for the device plus fitting.

For a socket-connection installation under SI 2026/848, the IPS v2 requires the certified inverter to limit its own DC leakage current to a level where a Type A RCD will still operate reliably. This is a design requirement on the inverter, not on the building wiring. A socket-connected certified install does not legally require you to upgrade to Type B — but if an electrician is already on site for any reason, fitting Type B protection on the circuit costs little extra and is always the safer choice.

For the consumer unit safety checklist before installing solar, see our consumer unit article.