Residual Current Device & How It Will Save Your Life!
What is it?
An RCD is a safety device that has become an electrical standard in the United States and around the world. The RCD is designed to protect you from any stray currents that would otherwise pass through you, to ground from a faulty component or wiring. As little as 30mA is enough to kill a human in the wrong circumstances, so RCD’s need to be correctly installed and verified for operation from time to time. Any new installation, commercial or residential must have RCD’s installed and any older installation must have the circuits modified to contain them.
How Does it Work?
Other common circuit protection devices such as fuses and Miniature Circuit Breakers, MCB’s, generally only protect circuit components and cabling from damage during an overload. In general, the current flowing from the active, through the components and returning via the neutral will be the same. The RCD will monitor the current flow between the active and the neutral and if a slight change occurs it will trip the power immediately.
Inside the RCD is a ferromagnetic core that has both the active and neutral wires spiralled around it. In a normal situation when both currents are equal they will both produce a similar magnetic field. The wires are turned around the ferromagnetic core so that the current flows in opposite directions, therefore, the magnetic fields will oppose each other and cancel out. In the case a human is exposed to a live circuit, downstream of the RCD, then the magnetic field will change and this, in turn, will trip the continuity of the circuit.
The process to trip the RCD is by a 3rd winding around the ferromagnetic core, when the magnetic field becomes polarised in a certain direction, current will flow in this coil. The coil will then pull a set of switches that open the circuit, removing the power.
When They Won’t Work.
RCD’s have limitations and all have different specifications. It is important to check what RCD’s are installed in your circuits at your home or business. Some RCD’s are only designed to protect in the event of a fire and are not sensitive enough to protect from an electric shock. An RCD rated to trip ~30mA is required to protect a human from electric shock. If a person is to touch both the active and neutral wires at the same time then the current will remain the same and then RCD will not detect a change in magnetic field.
How To Test RCD’s?
While most RCD’s do have test buttons built in, the main tripping coil circuit is not correctly tested. A specialised tool is required for testing. Most electricians have the capability to test RCD’s that may be present in your electrical circuits. According to the National Electrical Code RCD’s & Ground Fault Circuit Interrupters, GFCI’s, requires testing every 12 months to remain compliant. If you are unsure if your RCD’s will work call your local electrician far peace of mind.