RCCB/RCD

As the development of modern industry, electricity becomes amazingly useful. However, it also becomes amazingly dangerous too. Do you know that it takes only a few milliseconds for a strong electric current flowing through your heart to harm you? Think about this, if you’re using an electric cutter and you accidentally chop through the cable. Besides, the tool has a metal case, you’re holding on it and you accidentally touch the peeled wire. What’s more, you’re standing on the ground but the electricity has to go somewhere. At this time, there’s a very high risk that your body will form a “short circuit”. So how to prevent electricity through our body? One way to reduce the risk is to use a clever protective device called a residual current device (RCD), or residual-current circuit breaker (RCCB). The RCD/RCCB is a device that instantly breaks an electric circuit to prevent serious harm from an ongoing electric shock. In Europe, these are better known by their initials RCD. Also, a combined RCD+MCB (miniature circuit breaker) is known as RCBO (residual-current circuit breaker with overcurrent protection). In America, the device is more commonly known as a ground fault circuit interrupter (GFCI), ground fault interrupter (GFI) or an appliance leakage current interrupter (ALCI). In Australia, they are sometimes known as safety switches or an RCD. An earth leakage circuit breaker (ELCB) may be a residual current device, although an older type of voltage-operated earth leakage circuit breaker also exists.

The figures vary from year to year for a different country. However, global statistics reveal that electrocution is one of the top five causes of death at work. There is a lot of electric shock accidents at home. In one country, electrical accidents cause several hundred deaths and several thousand injuries each year. How and why do people get an electric shock? Essentially, as they accidentally touch leakage current equipment or as an electrical appliance fails in such a way that its exposed metal parts temporarily become energized conductor and dangerous. Many electrical appliances will fuse to protect against excessive currents. However, they don’t any helps in this leakage current situation. Even more, some appliances also have “earth” or “ground” cables to protect us when live cables touch things they shouldn’t. The “earth” or “ground” isn’t part of the normal power carrying circuit, it’s just a spare cable. It attached to the exposed metal parts of an appliance. Eventually, it connected to the earth through your household wiring. Then through a metal spike or water pipe that enters the ground outside your home. Basically, the idea is that if the live cable fails and touches something like a toaster or distribution metal box, the ground cable carries the current safely away.But what if the earth/ground cable fails as well? The circuit will through your body, how to protect against electric shock? That’s where RCD, GFCI, ELCB come to our rescue.

The image depicts the internal mechanism of a residual current device (RCD). The RCD consists of five parts. There are Magnetic relay (1), Mechanism (2), Incoming line terminals (3), Loading terminals (4) and Sense coil (5). The live wire connects the incoming line terminal (1). Then plug to power to your electrical appliances from loading terminals (4). Turn on the handle of mechanism (2) to make connection copper contact parts of Incoming line terminals (3) and Sense coil (5). The sense coil 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 to 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). 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 (5), which is picked up by the magnetic relay (1). The sense circuitry then removes power from the mechanism (2), and the contacts of Incoming line terminals (3) are forced apart by a spring, cutting off the electricity supply to the appliance. The RCD is designed so that the current is interrupted in milliseconds, greatly reducing the chances of a dangerous electric shock being received.

In our modern household consumer unit (switchboards), like the image as below, have built-in RCDs on some (but not all) of the household circuits. That means if you accidentally cut through a cable or get it wet, the trip switch on the consumer unit will interrupt the power and save your life. It is doing the same job as a plug-in RCD. Vitally, it’s vitally important to understand how your consumer unit is set up. In this box, for example, only the upstairs power outlets have RCD protection. So if you’re using an upstairs plug to power your toaster, you’re protected. However, if you were using power tools plugged into a downstairs outlet, you’d need to use a plug-in RCD to protect yourself because the upstairs circuits don’t have RCDs fitted.

The electrical engineer Charles Dalziel (1904–1986) from US, who was a professor of electrical engineering and computer sciences at UC Berkeley. He saved countless lives when he came up with the brilliant idea of the ground-fault circuit interrupter around 1960. He began patenting the idea in January 1961 (in application number 85,364) and was finally granted a patent in October 1965. Among the benefits of the invention that Dalziel listed were high enough current sensitivity to protect people against electrocution, low operating current and energy consumption.