Understanding Residual Current Devices (RCDs) - What do they do, how do they do it?
RCDs are protective devices used in electrical installations. They are designed to quickly break electrical circuits, and this prevents the user of the device from any serious harm as a result of an electrical shock. They have no doubt saved plenty of lives throughout the years.
There are a couple of different kinds that you’ll either find, purchase or install yourself. There are ‘plug in’ types that act as a safety adapter for appliances that do not have a circuit breaker in built, and there are other options that sit on fuse boards or boxes.
Historically, BS7671 recognised two types of earth-leakage circuit breakers. As well as what we understand to be an RCD today, there used to also be an older voltage-operated device. Nowadays we only deal with current-operated devices.
What do RCDs do?
When understanding what an RCD does, it helps to envision a simple electrical circuit.
Electricity flows through live cables and, then, through an appliance where it gives power to a particular function. For example, a switched-on kettle will begin to heat water up. The electricity will then flow back through the neutral cable to complete the full electrical circuit.
RCDs are designed to check that the current flowing through the live wire is equal to the current flowing back through the second part of the circuit, aka the neutral wire.
If, for any reason, some of the electricity flowing into that kettle was to flow out through the earth cable, the residual current flowing through the neutral cable would be less than the current flowing through the live cable.
If the current is not the same, the RCD will simply disconnect the supply.
How do RCDs work?
Inside every RCD is an iron core.
The electricity that flows into the RCD from the live cables passes through a switchgear and a coil that goes around the iron core. When this electricity passes around the coil it generates a magnetic field.
The current that flows back into the RCD, from the neutral cables, passes around a coil that is wired the opposite way around to that of the live wire.
So, you get two opposing magnetic fields. The current returning through the neutral wire should be the same as the current that was supplied, and therefore, the two opposing magnetic fields should be equal, and cancel each other out.
With no magnetic field overall, nothing will happen to the switchgear. The current will continue to run around the appliance as intended.
If there is a fault in an appliance some electricity can leak away to ground. In this circumstance the current that is supplying the device would still be the same, so, the magnetic field in the first coil would still be the same too. However, less current would return to neutral coil as some is leaking away. There would be an imbalance between the two magnetic fields.
That leaves a net magnetic field which triggers the switchgear. How? The switchgear possesses an iron panel which is attracted to the iron core in the event of a net magnetic field. This breaks the circuit.
A reset button is then pressed to push the circuit back into place.
Types of RCD
Residual current can take various waveforms depending on the load characteristics, so there are a variety of RCD types to ensure appliances are equipped to deal with any issues.
Electric Installation Wiki indicates that “the following types of RCDs are defined in IEC 60755, for suitable protection of different forms of residual current”:
Type AC RCDs are there to detect alternating currents. They’re suitable for general use and cover the vast majority of applications.
Type A RCDs also residual current as well as alternating. Type A RCs are specifically intended to be used for single phase class 1 electronic loads.
In addition to the detection characteristics of type A RCDs,type F RCDs are specially designed for circuit protection where single phase variable speed drivers could be used.
Type B RCD are intended to be used for loads with three-phase rectifiers, such as variable speed drives, PV system, EV charging station and medical equipment.
You may also come across the following phrasing with regards to RCDs:
These are installed inside a consumer unit, or fusebox. They are installed to provide protection to both the individual who will be interacting with the unit, and the group of circuits that are involved. Fixed RCDs provide a high level of protection as they protect the wiring and the sockets on a circuit.
Specially designed socket-outlets with an RCD inbuilt, used in place of a standard socket to provide protection to the person in contact with any equipment used.
Finally, portable RCDs are designed to plug into pretty much any standard socket-outlet. Appliances are then plugged into the portable RCD. They provide protection on demand.
How do you test RCDs?
To make sure that you have fitted RCDs into your property or circuit board correctly, you’ve got to make sure they’re being tested at regular intervals.
The good news is, RCDs are easily testable and re-settable devices.
Each RCD will be fitted with a test button. This button creates a safe, small leakage. Each RCD will be fitted with a reset button. This button reconnects the conductors once a fault has been cleared.
Testing an RCD: Step by Step Guide
- Plug a small lamp appliance into a socket and make sure it is working. Leave it turned on.
- Ensure that the electricity is connected to the RCD securely, and that the main switch is in ‘on.’
- Turn off all of your other electronic appliances such as televisions and radios.
- Press the test button on your RCD, but do not hold your finger down on the test button. At this point, your RCD should operate, ie, turn off the appliance.If it does not operate, you should have it checked out by a professional.
- After pressing the test button, and the RCDs have turned off, check that your small lamp is now off. You’ll need to move the lamp around several power points and ensure that it does not turn on when plugged into any of them.
- Once you have completed your testing, you can turn your RCD back on and check that the lamp works when plugged back into a power point.
Double pole RCDs disconnect both the energised and return conductors when a fault occurs, but single pole RCDs only disconnects the energised conductor.
Ageing electrical appliances with old wiring may have a small amount of earth leakage, and while this is normal, this current imbalance can trip an RCD.
It is also worth remembering that earth leakage increases with each additional electrical appliance you have plugged in. A single meter box RCD protecting all household wiring is more likely to experience nuisance tripping, as a result.
If you have fixed RCD protection, it will reduce the risk of electric shock to you and the people who use your equipment.
LED Controls supplies a range of different electrical circuit protection methods including Miniature Circuit Breakers (MCB), Moulded Case Circuit Breakers (MCCB), Residual Current Devices (RCD) and Residual Current Breakers with Overload protection (RCBO).
We stock products from a number of leading brands including ABB, Chint and IMO. Shop our range of electrical circuit protection here, but remember, although RCD protection reduces the risk of injury from electric shocks - it does not reduce the need to take care.
If you have any questions or queries about LED Controls’ services and products, don’t hesitate to get in touch with the team by emailing [email protected], calling 08450 756230, or clicking here to fill in a contact form.