Residual current devices and why they should be installed
Often called safety switches, residual current device (RCD) is the original generic term that also includes various devices such as residual current circuit breaker (RCCBs) and residual current breakers with overload (RCBOs). RCDs are now established as an acceptable means of providing additional protection against electrocution and reducing some risks of ﬁres caused by electrical faults. They do not eliminate the need for basic insulation principles to maintain separation from conductive components.
The safety of an electrical installation is reduced and the risk of a person receiving an electric shock is increased by substandard, incorrectly wired, or in various situations unearthed equipment. While fuses and circuit breakers provide protection against short-circuits and current surges, they do not adequately protect against current ﬂowing through the human body. RCDs will detect small leakage currents and greatly reduce the risk of death due to electrocution by cutting the ﬂow almost instantaneously. Essentially the longer the current ﬂow and the larger the current, the greater the probability of heart ﬁbrillation caused by the muscles surrounding the heart pulsing at different rates to the heart, which can result in death. An RCD with a trip setting of 30mA greatly reduces the possibility of heart ﬁbrillation and a fatality from an electric shock. In a healthy electric circuit, the active and neutral conductor currents should be practically identical, while the earth current may be a few milliamps. In the case of a fault (for instance somebody touching an active wire), a current will ﬂow from the active to earth. This will make the current in the active greater than the current in the neutral, and the RCD will trip.
As little as 250mA can be enough leakage current from a faulty installation to generate sufﬁcient heat to start a ﬁre. Many electrical ﬁres start by earth leakage current due to faulty wiring or equipment. A conventional protection device will not detect fault current in this very low range, whereas RCDs are typically designed to trip at 30mA, greatly reducing the risk of an electrical fault becoming an electrical ﬁre.
Residual current breakers with overcurrent protection (RCBOs).
RCBOs combine in one single device the residual current function and the over-current protection function typical of a miniature circuit breaker (MCB). RCBOs are tripped by either current leakage to earth, overloads, or short circuits, and are self protecting up to a designated maximum short-circuit current. By combining these features, an RCBO is considered an optimal device for achieving both overload protection and earth leakage detection.
Operating principles of RCD operation
An RCD operates by constantly comparing the balance between the current ﬂowing in the active and neutral conductors in a circuit. The incoming supply, phase and neutral, passes through a transformer core, which acts as the primary winding. The secondary is the winding wrapped around the core through the magnetic ﬂux ﬁeld to the trip mechanism. Under normal conditions the phase and neutral currents are equal and opposite, so no ﬂux is induced in the transformer core and no current ﬂows in the secondary winding. When current leaks to earth through a fault in the circuit or an accident with the equipment, an imbalance is detected by the RCD, so the phase and neutral currents will no longer be balanced. A magnetic ﬂux will be induced in the core and a current will ﬂow in the secondary winding that will activate the trip mechanism and isolate the incoming supply before injury or damage can occur. For this to be effective, the RCD must operate very rapidly from a low earth leakage current. Those designed to protect humans operate with leakage current of 30mA within 200 milliseconds or at a higher earth current of 150mA, but tripping in less than 30 milliseconds.