Choose the right RCD for your plug-in solar kit. This guide explains Type A, Type B, and Type F in plain English. 

The Big Change: Why Old RCDs Can’t Keep Up

In the summer of 2026, the UK officially adopted BS 7671 Amendment 4. This law changed the game for "Plug and Play" solar. While it made these kits legal and easier to use, it also introduced a strict "hard cutoff" for older safety devices.

The main reason you can’t use an old RCD is a phenomenon called "Blinding."

1. The "Blinding" Problem (Type AC vs Type B)

Most older UK homes have Type AC RCDs. These were designed decades ago to detect simple leaks from "standard" appliances like kettles or heaters.

• The Issue: Solar panels produce Direct Current (DC). When this flows through a solar inverter and into your home, it can leak small amounts of DC into your wiring.

• The Danger: This DC leakage "blinds" an old Type AC RCD. It essentially "freezes" the device so it cannot "see" a dangerous fault. If someone got an electric shock, the RCD would stay stuck and wouldn't trip, making it useless.

• The 2026 Rule: All solar installations now require a Type B (or sometimes Type A) RCD that is specifically built to detect DC leaks without getting blinded.

2. Bidirectional Power Flow

Old RCDs were designed for one-way traffic: electricity coming from the grid into your house.

• The Issue: Plug-in solar is different. Power flows "backwards" from your panels into your sockets.

• The Danger: Many old safety devices are "unidirectional." If power flows through them the wrong way, their internal testing components can burn out or fail to trip during a surge.

• The 2026 Rule: Safety devices must now be "Bidirectional." This ensures the device can protect you regardless of whether the power is coming from the sun or the street.

RCD vs RCBO: Which one for 2026?

While both provide safety, the industry has moved toward RCBOs for the 2026 standards.

• RCD (The Old Way): Usually protects a "bank" of 5 or 6 different circuits. If your solar panel has a minor issue, it will trip the RCD and turn off half your house (fridge, lights, and TV).

• RCBO (The 2026 Standard): This is an RCD and a Fuse combined into one small switch for one specific circuit.

  • Why it’s better: If the solar panel trips the RCBO, only that socket goes off. The rest of the house stays on. It makes finding faults much easier and prevents "nuisance tripping" which is common with solar inverters.

Summary.

If you are plugging in a solar kit in 2026, your home's "fuse box" (consumer unit) must be up to date. Using an old RCD is like having a smoke alarm with no batteries—it looks like it's protecting you, but the DC power from the solar panels has effectively turned it off.

For a safe, legal installation, you need a Type B, Bidirectional RCBO.