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What Is a Residual-Current Device?

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  • Written By: Geisha A. Legazpi
  • Edited By: Lauren Fritsky
  • Last Modified Date: 30 November 2016
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A residual-current device is a protection agent that detects any imbalance in the currents in single- and three-phase circuits and shuts down the power if necessary. It detects the sum of electric current at the mains conductors as opposite vectors with same magnitudes. For single-phase connections, the relevant connections are the live wire and the neutral wire. The current that flows through the live wire must be the same as the current that flows in the neutral wire. If that is not the case, trouble in the circuit needs to be solved or there could be lives in danger.

Usually, the residual-current device for single-phase protection makes use of inductive coupling of the net-induced currents of the live and neutral wires. The latter has to be zero if the wiring and insulation are perfect. There are several scenarios where live and neutral current will have a difference.

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In a simple single-phase connection, if the load drains 5 amperes (A), the current on the live wire and the electric current at the neutral should be 5 A. If the residual-current device is connected, it will be able to detect as low as 20 milliamperes (mA) in electrical current difference between the two wires. This could mean that a person has accidentally touched the live wire and is being electrocuted from live into the earth ground. If this person touched the live wire with bare feet touching the earth ground, the residual current circuit breaker (RCCB) will detect this situation, break the circuit, and save the person from further electrocution. In this type of use, the RCCB may also be referred to as a ground fault circuit interrupter or ground fault interrupter where the ground fault could be a leakage current caused by poor insulation, water penetration, or possible electrocution of a person.

Moreover, the residual-current device is also an insulation monitoring device. When the wiring insulation is either on the live or the neutral has been damaged exposing the wire, there is the possibility of leakages to ground or elsewhere. The current leakages go into location unintended, thus the residual-current device will detect it and the RCCB will trip the breaker and de-energize the load. Some RCCBs like the appliance leakage current interrupter are designed to protect appliances that may have insulation or contamination problems.

The arc-fault circuit interrupter (AFCI) is an electrical wiring device that detects and stops the undesired arching in the electrical wiring. These opto-electric arches are usually caused by loose connections that begin with very low-power arching and progress into higher-power arching that generate enough heat to start a fire. The AFCI is able to detect the electric burst patterns associated with an arching and de-energizes the circuit by interrupting it.

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