Residual-current circuit breaker Guide, Meaning , Facts, Information and Description
In electrical engineering, residual current circuit breakers (RCCB) or residual current devices (RCD) are circuit breakers that operate to disconnect their circuit whenever they detect that current leaking out of the circuit (such as current leaking to earth through a ground fault) exceeds safety limits. These devices can be tested to see if they are operational and/or they have been wired correctly.The terms ground fault circuit interrupter (GFCI) or ground fault interrupter (GFI) are used in the United States and Canada, but these terms are not, strictly speaking, correct (or at least complete), because a RCCB will trip if current leaks anywhere, not just to ground. One might more properly call the device a Balance Fault Interrupter (BFI), rather than GFI, because it will trip if current, for example, leaks to or from another phase of the electrical supply, or to/from another circuit such as either the "hot" or "cold" side of a nearby 12 volt DC renewable energy system, or a nearby ethernet jack, etc.. The device will trip on any balance fault, not just a balance fault to ground.
The term earth leakage circuit breaker (ELCB) is also (incorrectly) used, though strictly speaking this refers to a different type of device.
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Single phase RCCBs operate by measuring the balance between in-bound and out-bound currents using a differential current transformer, and operating the circuit breaker if there is a balance fault (i.e. a difference in current between the "hot" and "neutral"). More generally (single phase, three phase, etc.) RCCBs operate by detecting a nonzero sum of currents, i.e. the current in the "hot" or "hots" plus that in the "neutral" must equal zero (within some small tolerance), otherwise there is a leakage of current to somewhere else (to ground, or to another circuit, etc.).
RCCBs are designed to attempt to prevent electrocution or fire by detecting the leakage current, which can be far smaller (typically milliamperes) than the trigger currents needed to operate conventional circuit breakers, which are typically measured in amperes. RCCBs are intended to operate quickly, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock.
Residual current detection is complementary to, rather than a replacement for, conventional over-current detection, as residual current detection cannot provide protection for faults which do not involve an external leakage current, for example faults that pass the current directly from one side of the circuit through the victim to the other. Notably, RCCBs do not provide protection against overloads or short circuits between phase (live, hot, line) and neutral or phase to phase.
The photograph depicts the internal mechanism of a RCCB. The device pictured is designed to be wired in-line in an appliance flex. It is rated to carry a maximum current of 13 amps and is designed to trip on a leakage current of 30mA.
The incoming supply live (US: hot) and neutral conductors are connected to the terminals at (1) and the outgoing load conductors are connected to the terminals at (2). The earth (US: ground) conductor (not shown) is connected through from supply to load uninterrupted.
When the reset button (3) is pressed the contacts (4 and 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 flowing 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.
However 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 there is an imbalance (difference) in the current flowing in the two conductors (single phase case), or, more generally, a nonzero sum of currents from among various (e.g. 3phases and neutral) conductors.
This difference causes a current to flow 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 device is designed so that the current is interrupted in a fraction of a second, greatly reducing the chances of a dangerous electric shock being received.
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 device does not trip when this button is pressed then a fault has developed and the device must be replaced.
In most houses, only some (if any) circuits are protected by RCCBs. German law, for example, requires the installation of RCCBs only for circuits leading to bathrooms (due to the highly increased danger of leakage currents when operating electrical devices in a wet environment: a hair dryer falling into a bathtub might otherwise be fatal). US law (the National Electrical Code) requires RCCBs in bathrooms, kitchens, garages, exterior areas, crawl spaces, unfinished basements, near wet bars, swimming pools, and spas. Additionally, it might be a good idea to protect circuits leading to outlets in reach of children, or outlets that are indoors but near a door (where people are likely to plug something in while working outside) by RCCBs.
Sometimes a single RCCB is installed covering the entire electrical installation in a property. However this is considered bad practice by some because any fault will cause all power to be cut to the premises including to devices such as freezers, fire alarms etc. and injury may be caused by occupants being suddenly plunged into darkness. A more common practice is to link all the outlets in a room to a single RCCB. More than one RCCB on a single circuit is unnecessary, provided they have been wired properly.
It is important to check RCCBs regularly. To do so, press the button labelled "Test" or "T" on the RCCB unit (which will simulate a ground fault by bypassing some current) and see if the RCCB reacts by opening the cuircuit (and, of course, get it replaced immediately if it does not).
In Europe, one sometimes installs a RCCB next to the circuit breaker in the distribution board, protecting an entire circuit. Hence, they are built in the same shape to fit on the same rail. Alternatively, an RCBO (Residual-current Circuit Breaker with Overcurrent protection) can be used which incorporates an RCCB and circuit breaker in one device.
Electrical plugs which incorporate a RCCB are sometimes installed on appliances which might be considered to pose a particular safety hazard, for example long extension leads which might be used outdoors or garden equipment.
Hubbell also makes a number of extension cords that include an RCCB. These are useful when running a long extension cord out a door or window, from an indoor outlet that's not protected by an RCCB. Thus, for example, when using a shopvac to vacuum up rainwater from a front or back porch, driveway, rooftop, or the like, protection is still provided when running from an otherwise ordinary outlet.
Electrical sockets with included RCCBs are becoming common. In the US these are required by law in wet areas (See electrical socket#In the United States for details.)
RCCB sockets are usually of the decora size (a size that harmonizes outlets and switches, so that there is no difference in size between an outlet cover and a switch cover). For example, using the decora size outlets, RCCB outlets can be mixed with regular outlets or with switches, ethernet jacks, etc., in a multigang box. Many companies such as Hubbell, provide multigang boxes that facilitate the mixing of RCCB sockets, switches, ethernet jacks, phone jacks, video connectors, and the like, in various permutations, that can be accept a standardized cover plate solution.
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Example
Use
Types
See also
External links