SPDs and AFDDs on a live consumer unitWhat does a red SPD window mean?

Take a live split-load board, the same as the one above, wired to a ring final, a cooker circuit and a lighting circuit. Reading it from the main switch outwards:

• Main switch: the same 100 A on-load isolator as before, with the loose busbars fitted underneath and screwed in.
• SPD (surge protective device): protects the installation against overvoltage, like a lightning strike. Ours is a Type 2 (T2) plug-in cartridge with a little window: green is fine, red = replace. Where it sits in the board varies by manufacturer.
• The SPD doesn't disconnect circuits. Pull it out and everything still works. Think of it as a sponge: it soaks up the surge, turns red, and you replace the cartridge.
• Two RCBOs on the unprotected side. You can tell they're RCBOs from the RCD symbol and the neutral fly leads at the back.
• The first is a B32, 30 mA AFDD/RCBO: a Type B 32 A breaker, a 30 mA RCD and an arc fault detection device in one. It feeds the ring final (two lines in the top, two neutrals). It does more than a plain RCBO, because it will also detect loose connections (arcing).
• The second is an empty B6, 30 mA RCBO, ideal for a domestic lighting circuit needing additional protection. Line in the left-hand terminal, neutral in the far terminal.
• RCD 1 (30 mA Type A) has two CBs, one in use: the cooker circuit. Its neutral goes to RCD 1's neutral bar.
• RCD 2 has three CBs, one in use: the lighting circuit on a 6 A breaker. Its neutral goes to the third neutral bar. All CPCs go to the earth bar.

SPD · Type 2 cartridge · green window OK, red = replace