Last Updated on October 22, 2023 by Kevin Chen
You may not know about residual current devices(RCDs), but these devices are important for protecting you and your family from electric shock. There are different types of RCDs on the market, each with its benefits.
So which one should you buy? And how do they work?
This article offers everything you need to know about the residual current device to make the best decision for your home.
What is a Residual Current Device?
Let’s start with the definition. A residual current device (RCD) is an electrical safety device that trips when a person or product’s amount of current being drawn exceeds a safe limit.
These devices are designed to protect people from electrical shock, and they may also protect your property from fire and other damage.
Residual current devices are ground fault circuit interrupters (GFCIs).
Residual Current Device Basics
An RCD is designed to detect small amounts of electric current that leak into an area where they aren’t supposed to be. These currents are called “leaks” because they flow through the ground rather than through a circuit breaker or fuse as normal electricity would do.
The RCD detects leaks by measuring the difference between the amount of current flowing into a system and the amount of current flowing out of it. If there’s a difference, the RCD will trip if more current is flowing out than in.
If you’ve ever used a circuit breaker, then you can understand how this works. A circuit breaker detects overloads by comparing the number of currents entering and exiting a power source. If too much current flows into a device, the circuit breaker will trip to prevent damage to your electrical system.
Residual Current Device Installation & Testing
How do I know if my RCD is bad? You are likely to ask this question once you have purchased.
The surest way to know is by testing it.
Testing your residual current devices regularly will make sure they’re working properly.
The good news is the testing process is simple: you simply run an electrical load – like a light bulb – through the RCD and see if it trips. If it doesn’t trip when you’re testing, you’ll need to reset it. If it still trips after the reset, this could be a sign that it is in bad condition.
Most modern RCDs have a test button that makes it easy to do the testing. Connect the RCD to the load, then press the ‘Test’ button.
The button will simulate the current leakage of a circuit, and from there, you will know whether your RCD is in perfect condition.
How does RCD work?
Whether you are deep into the technicalities of the device or not, it will be smart to know the working principle of the RCD.
A typical RCD has an iron core in it. This core is surrounded by a magnetic field that is produced by a current flowing through the RCD.
The electric current creates the magnetic field, and it acts as a magnet.
When there is a flow of current in the circuit, the magnetic field will be disturbed, and this will cause the core to move.
A typical RCD has two poles: one is connected to neutral while another one is connected to the earth. The current flowing back to the RCD from the neutral also passes through the coil even though its wiring is opposite to that of the live wire.
So, this means that we have two magnetic fields that are opposing each other. The two currents should still be the same; hence the magnitude of the two magnetic fields should also be equal.
Since we have two magnetic fields that are equal two each other, the result will be zero. Nothing will happen to the switchgear, and the current will continue flowing as required. This is what happens under normal circumstances.
What happens when you have a faulty appliance or any other fault on the circuit? The current returning to the neutral will be less than the one that is flowing through the live wire.
Since the current is not flowing as required, there is a difference between the two currents. This causes a difference in the magnetic field (magnetic imbalance), and this will cause the core to move. The movement is facilitated by switchgear.
The movement of the core has two effects: firstly, it breaks the circuit and secondly, it sends a signal to the RCD, which sends an alarm to alert you of the fault.
You can reset the RCD by pressing the “reset” button. This will normalize the current flow, that is if there is no major fault on your electric device or electric circuit in general.
Typical design and components of RCD
A typical RCD is designed to be lined alongside a power cable of an appliance. The arrangement allows it to trip in case there is a current leakage.
The components that make up the RCD include:
-Terminals: The terminals provide attachment points for the incoming supply and the neutral conductors on one side, and the other side has the terminals for the earth conductors.
-Reset button: This button is used to reset the RCD after it has tripped due to the current leakage. You simply need to press it, and the RCD will go back to its normal functioning.
-Solenoid: A solenoid is a device that comprises a coil and a plunger. When a voltage is applied to the coil, it causes the plunger to push against the coil, closing the circuit, thus causing an electric current to flow through the appliance.
-Sense coil: The sensing coil is used to detect the current in the circuit and then send it to the RCD.
-Trigger wire: The trigger wire is connected to the RCD so that when it trips, it sends a signal to a control unit.
-Control unit: The control unit contains circuits and other electronic components that are used to determine if there is any leakage of current in the circuit and then send a signal to the trigger wire so that it can trip.
-Test wire: The test wire is connected to the RCD so that when it trips, it sends a signal to a control unit. It also stimulates the fault in an appliance by creating an electric imbalance in the sense coil.
-Control Unit: This is the main component of the RCD. It contains circuits and other electronic components that are used to determine if there is any leakage of current in the circuit and then send a signal to the trigger wire so that it can trip.
Types of residual current devices
RCD are not all the same. They come in different makes, forms and designs. Let’s look at the most common types of RCDs.
As the name suggests, these RCDs are fixed and have no moving parts. They are capable of functioning for a long period. This type of RCD is suitable for applications where the circuit is not prone to short circuits, and there is no need to test the RCD often. The drawbacks with this type of RCD include maintenance, cost and size.
RCD with thermal overload protection
This type of RCD can detect faults due to overloading or overheating. They can also detect faults due to voltage fluctuations (voltage overshoot and undershoot). These types of RDCs are suitable for applications where there is a possibility of short circuits low power loss, and there is a need to test them regularly.
These RCDs are made up of a battery and a small circuit board. A rechargeable battery or mains electricity can power them. This type of RCD is suitable for applications where the circuit is not prone to short circuits, and there is a need to test it regularly. The drawback with this type of RCD is maintenance, their size, and they are not suitable for testing continuously.
These RCDs are used to protect socket outlets. There are two types of socket outlets: the standard threaded socket and the double-ended, which can be wired in any way. These RCDs have a transformer to convert the mains voltage so that it is suitable for protecting both types of sockets. This type of RCD is suitable for applications where there is a possibility of short circuits low power loss, and there is a need to test it regularly.
These are digital RCDs that combine two or more input sources into one output signal, with an optional extra output for monitoring purposes. They differ from analogue outputs in that they can operate at higher frequencies (up to 10 kHz). Binary outputs tend to be more accurate than other types of outputs because they do not introduce any distortion or noise into the voltage output, but this also makes them more expensive than other types of RCDs.
Other types of RCDs
Here are other types of RCDs that you should know:
Type AC: This is the type of RCD designed to detect the alternating currents. They are the most common RCD as they fit in many applications.
Type Type A RCD: These are used on both the residual and alternating currents.
-Type F RCDs: These RCDs are just like type A, only that they are specially designed for the single-phase variable speed drivers.
Technical Features of RCD
There are a number of technical features that you should know about residual current devices. Most of these features affect the device’s performance and general working. They include;
Number of poles
The number of poles refers to the number of conductors interrupted when the RCD is triggered. Most RCDs have one or two poles. They are usually referred to s either single-pole or double-pole RCD.
Number of elements
The number of elements is the number of conductors used to sense the current. It also affects the sensitivity and the accuracy of the RCD.
The sensitivity is the amount of current required to trigger the device. It is measured in milliamperes. The higher the sensitivity, the less current will be required for triggering. The sensitivity is usually categorized into high sensitivity, medium sensitivity and low sensitivity.
Accuracy refers to the accuracy and precision of working of the RCD. It is measured in percentage points and percentage of range.
The proximity effect refers to a phenomenon that happens when an RCD is placed close to a conductor or circuit breaker. The device will not work properly because it will get triggered by other conductors nearby that are not connected to any load.
This is also referred to as the response speed of the RCD. It is measured in the time it takes for the RCD to open once triggered.
This refers to the amount of current passing through the RCD when it is triggered. The amount of current passing through the device will trigger a circuit breaker.
This refers to the amount of voltage that is required to trigger a device. It is measured in volts and volts per kilometre (V/km).
Limitations of RCD
At this point, you can easily claim that RCD is the best safety device for your electric circuit. Right?
While this may be true, RCDs also have some limitations. For instance, a Residual current device does not cancel out all the risks associated with the current leak. An RCD alone is incapable of detecting overload problems in circuits. You will need to add other devices, such as a circuit breaker.
I hope that from this guide, you have learned important details about RCDs. RCDs are the best devices you can use to protect your electric circuits and appliances. With proper installation, maintenance, and testing of RCDs, your safety will be ensured.
And if you want to buy residual current devices in China or you want to get its components, let us help you. ICRFQ is a reliable sourcing agent for electrical and electronic components in China.
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