Type AC RCD — circuits serving modern appliances

The find

Same property as Find 003. The split-load consumer unit had an original Type AC RCD protecting one bank of circuits. Those circuits included a washing machine and a dishwasher — modern appliances with electronic motor controllers. The RCD label said “Type AC.” The appliances needed at minimum a Type A.

What you’re looking at

RCDs work by measuring the difference between current flowing into a circuit and current flowing back. If those values don’t match — if current is leaking somewhere it shouldn’t — the RCD trips.

But fault current doesn’t come in just one shape. It depends on what’s producing it:

Type AC — detects smooth, alternating fault current. This is what simple older appliances (basic heaters, incandescent lighting, old motors) produce. It’s the original RCD type and the cheapest to manufacture.

Type A — detects smooth AC fault current plus pulsed DC fault current. Modern appliances — washing machines, dishwashers, heat pumps, EV chargers — use electronic controllers that produce this pulsed DC component. A Type AC RCD simply cannot detect it. Not slowly. Not partially. Not at all.

Type F — as Type A, plus higher frequency fault currents from variable speed drives. Required for EV chargers and certain industrial loads.

Why it matters — the analogy

The fault code

C2 — Potentially Dangerous. The RCD is present and functional — for smooth AC fault current. But the appliances on those circuits produce pulsed DC fault current. If an earth fault develops in the washing machine or dishwasher, the Type AC RCD will not respond. The circuit protective function it’s supposed to provide simply does not apply to the fault type most likely to occur.

The fix

Replace the Type AC RCD with a Type A RCD of the same rating and pole configuration — or, better, replace the whole consumer unit with a modern board using RCBOs, which gives individual circuit protection. If an EV charger is present or planned, a Type F device is required for that circuit specifically. The Type AC RCD costs slightly less. The difference in protection is not slight.