Protection

Residual Current Device (RCD)

Watches the balance between live and neutral — any leakage to earth trips it in milliseconds.

Real part — tap a marker

Residual Current Device (RCD)

Watches the balance between live and neutral — any leakage to earth trips it in milliseconds.

Terminals

L-in (top)
N-in (top)
L-out (bottom)
N-out (bottom)

Test button 'T' — simulates a leakage
Trip indicator flag — shows the pole that opened
Summation transformer — compares L and N current
Solenoid — releases the mechanism on imbalance
L, N terminals — in (top) and out (bottom)

Cutaway of a residual current device showing the toroidal summation transformer, trip solenoid, latch mechanism and main contacts

Cutaway — tap a marker

⚡ Meet it in a circuit

Terms used here

Defined in the glossary — each one opens at its own entry.

⚠ Safety note

An RCD does not protect against overload — that is the MCB's job. RCD + MCB together (or one RCBO) is the standard way final circuits are built in UK boards today.

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