page_title: Residual-current device
section_title: Typical design
hierarchical_section_title: Residual-current device / Typical design
caption_attribution_description: Deutsch: Innenleben eines Fehlerstromschutzschalters English: inside of a Residual Current protective Device
context_page_description: A residual-current device, residual-current circuit breaker, or ground-fault circuit interrupter is a device that quickly breaks an electrical circuit to prevent serious harm from an ongoing electric shock. Injury may still occur in some cases, for example if a human falls after receiving a shock, or if the person touches both conductors at the same time.
  These electrical wiring devices are designed to quickly and automatically disconnect a circuit when it detects that the electric current is not balanced between the supply and return conductors of a circuit. Any difference between the currents in these conductors indicates leakage current, which presents a shock hazard. Current of around 30 mA through the human body is potentially sufficient to cause cardiac arrest or serious harm if it persists for more than a small fraction of a second. RCDs are designed to disconnect the conducting wires quickly enough to prevent serious injury.
  RCDs are testable and resettable devices. A test button safely creates a small leakage condition, and a reset button reconnects the conductors after a fault condition has been cleared.
context_section_description: The diagram depicts the internal mechanism of a residual-current device (RCD). The device is designed to be wired in-line in an appliance power cord. It is rated to carry a maximal current of 13 A and is designed to trip on a leakage current of 30 mA. This is an active RCD; that is, it latches electrically and therefore trips on power failure, a useful feature for equipment that could be dangerous on unexpected re-energisation. Some early RCDs were entirely electromechanical and relied on finely balanced sprung over-centre mechanisms driven directly from the current transformer. As these are hard to manufacture to the required accuracy and prone to drift in sensitivity both from pivot wear and lubricant dry-out, the electronically-amplified type with a more robust solenoid part as illustrated are now dominant.
  In the internal mechanism of an RCD, the incoming supply and the neutral conductors are connected to the terminals at (1), and the outgoing load conductors are connected to the terminals at (2). The earth conductor (not shown) is connected through from supply to load uninterrupted.
  When the reset button (3) is pressed, the contacts ((4) and another, hidden behind (5)) close, allowing current to pass. The solenoid (5) keeps the contacts closed when the reset button is released.
  The sense coil (6) is a differential current transformer which surrounds (but is not electrically connected to) the live and neutral conductors. In normal operation, all the current down the live conductor returns up the neutral conductor. The currents in the two conductors are therefore equal and opposite and cancel each other out.
  Any fault to earth (for example caused by a person touching a live component in the attached appliance) causes some of the current to take a different return path, which means that there is an imbalance (difference) in the current in the two conductors (single-phase case), or, more generally, a nonzero sum of currents from among various conductors (for example, three phase conductors and one neutral conductor).
  This difference causes a current in the sense coil (6), which is picked up by the sense circuitry (7). The sense circuitry then removes power from the solenoid (5), and the contacts (4) are forced apart by a spring, cutting off the electricity supply to the appliance.
  The test button (8) allows the correct operation of the device to be verified by passing a small current through the orange test wire (9). This simulates a fault by creating an imbalance in the sense coil. If the RCD does not trip when this button is pressed, then the device must be replaced.
caption: Opened 3-phase residual-current device
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