Types Of Circuit Protection Devices
All electrical circuits have a maximum amperage. If this amperage is exceeded, wires within the installation can overheat and cause damage. Circuit protection devices are vital to protect installations and ensure user safety if a fault occurs.
Intro
Circuit protection devices are essential for electrical installations. They protect devices from damage by disconnecting the power when overcurrents and earth faults are detected. This helps prevent fires and keeps consumers safe from electrocution.
Protection devices such as RCDs are also mandatory in domestic properties for electrical circuits to comply with 18th Edition Wiring Regulations.
Overcurrents
What is an overcurrent?
A flow of electricity larger than the normal and safe level for a circuit is called an overcurrent. These can be the result of overloads, short circuits, and ground faults. Without an overcurrent protection device, such as a circuit breaker, they can lead to fires, electric shocks, and damage to electrical equipment.
What is an electrical overload?
Overloads can be caused by a faulty device or by connecting too many devices to a circuit. This increases the current demand to a level the electrical installation cannot handle. As a result, wires may overheat and cause fires, which is why you should never overload an electrical socket. To check whether a circuit is overloaded, take a look at Electrical Safety First’s socket overload calculator.
What is a short circuit?
Short circuits occur when a live conductor touches a conductive object that is not part of the intended circuit and current flows between them back to the source. The diverted charge flows with less resistance than it did in the original circuit. This causes the wires in the installation to heat up. Occurring in both AC and DC circuits, these faults can be caused by damage, loose connections, and much more. Short circuit protection devices like miniature circuit breakers trip circuits when this type of fault occurs.
What is a ground fault?
A ground fault also called an earth fault, is commonly caused by damage to the electrical installation. These are currents that can unexpectedly flow from a healthy circuit down to earth which can cause damage to cables and the insulation if the circuit is not interrupted by a ground fault circuit interrupter. An earth fault is when electricity travels to the ground through a faster route outside of the intended circuit. This uncontrolled flow heats up cables causing burns, fires, and electric shocks.
Circuit Breakers
What are circuit breakers?
Circuit breakers are electrical protective devices found inside consumer units. Their role is to automatically disconnect the current flow in a circuit when there is a fault. They can also be manually controlled. Unlike fuses, they can be reset, making them a recommended alternative.
How does a circuit breaker work?
Circuit breakers have a switch connected to an electromagnet or a bimetallic strip.
Electromagnet - In a circuit breaker with an electromagnet, the current flowing through the circuit affects the electromagnet's magnetic force. When the current increases, the magnetism also increases. If the current flowing is larger than the safe limit for the installation, the electromagnet’s force becomes strong enough to pull down a metal lever, which breaks the circuit. This type of circuit breaker is resilient to small increases in amperage but will quickly trip when faults occur
Bimetallic Strip - A circuit breaker with a bimetallic strip contains two metals that heat up as current flows through them. As the strips of metal grow hotter, they curl. If there is an overcurrent, the strips will curl so much that they trip the switch. This breaks the circuit.
They are not suitable for applications where short circuit currents could be large enough to weld the bimetallic strips together. As the strips curl slowly, only long-term overloads will cause this type of circuit breaker to trip.
Thermal-Magnetic - Thermal-magnetic circuit breakers contain both electromagnets and bimetallic strips. They protect against overloads and short circuits.
Single pole circuit breakers
Single pole circuit breakers are commonly used for domestic applications, they will only disconnect the live cable from the circuit in the event of a fault. They monitor the current flow in a single wire and trip when there is an overcurrent. An example of where a single pole circuit breaker is used is in the lighting of a house. These breakers usually only occupy one way of a consumer unit.
Double pole circuit breakers
A double pole circuit breaker, measures the current through two wires simultaneously and trips if there is an overcurrent in one or both wires. If an overcurrent occurs in either of the poles, the breaker will trip both circuits. These are usually used in properties where the live and neutral need to be isolated for maximum protection.
Fuses
A fuse does the same job as a circuit breaker, only fuses have to be replaced. A fuse is a simple circuit protection device that can be used for appliances. Inside a fuse is a thin wire, usually made of a tin and bismuth alloy, which charge flows through when it is fitted into a circuit. When an overload occurs, exceeding the limit of the fuse, the wire inside heats up and melts. This breaks the circuit and keeps consumers and appliances safe.
A fuse box that houses rewireable fuses can still be found in some buildings, but it is outdated and would benefit from being replaced with modern alternatives. A rewireable or BS 3036 semi-enclosed fuse can only be used on a.c circuits.
MCBs
What is an MCB?
An MCB is an isolating device which protects electrical installations from short circuits and overloads. The abbreviation stands for miniature circuit breaker. They are automatic electrical switches that trip and disconnect the power to a circuit when a fault is detected. In a consumer unit, there may be multiple MCBs for different circuits. MCBs come in a range of poles including single, double, triple, triple and neutral, and four-pole. They are a reusable alternative to fuses.
How do MCBs work?
MCBs contain a bimetallic switch or an electromagnet, which causes the switch to trip when there is an overload. For more information on how this works, check out the circuit breaker section on this guide page.
How to choose the correct current rating for an MCB
MCBs come in a range of amperages, but those used in domestic installations usually have a breaking capacity of 6kA. When choosing the current rating for an MCB, ensure it is equal to or greater than the current the installation is expected to carry, but no larger than the maximum current capacity for the circuit.
Using an MCB with a lower current rating than the total current in a circuit may cause it to trip unnecessarily. For example, if the total current in a circuit is 12A, then an MCB of 16A would be needed.
What are the different types of MCB?
MCBs come in different types, including A, B, C, D, K, and Z. MCB types refer to the device’s trip curve, which is the maximum current the circuit breaker can withstand before it trips.
RCDs
What is an RCD?
RCD stands for residual current device, and these circuit breakers are vital for preventing fatal electric shocks. They detect differences in incoming and outgoing current. If the RCD detects an imbalance, it will disconnect the circuit. This protects electrical installations and consumers from ground faults. If an RCD trips, it will shut off the power to all of the circuits it is responsible for (not just the faulty circuit). It is normal to see two RCDs in consumer units.
RCDs have a breaking capacity which refers to the maximum amount of current the device can take before it trips. Their sensitivity is measured in milliamps, and they usually have a rating of 30mA. This is because currents above 30mA can cause electrocution.
They also come in many forms. Fixed RCDs are those found in consumer units which protect single or multiple circuits. RCD sockets are plug sockets with residual current devices built into them. There are also portable RCDs which are plug adaptors that contain an RCD. These are particularly useful when using current electrical equipment in outdoor environments.
To comply with 18th Edition Wiring Regulations all new domestic electrical installations must be fitted with RCD protection.
How does an RCD work?
Inside an RCD current from a live wire passes into a coil wrapped around an iron core called a toroid. This generates a magnetic field. As the current flows back from the circuit into the RCD through a neutral wire, it passes through a coil wired the opposite way to the live cable. This creates another magnetic field. When the currents from both are equal, the magnetic fields cancel each other out.
If there is an earth fault, the magnetic fields become unbalanced. This causes net electromagnetism which generates a current. The current then passes into a detector coil connected to the toroid, causing the circuit breaker to trip.
What are the main types of RCD?
There are three main types of RCD. These are Type AC, Type A, and Type B.
When should you use an RCD with an extension cable?
If an extension lead is less than 15m, used indoors, and plugged into a socket already protected by an RCD, then an additional residual current device is not required. However, if an extension lead is used outside, it is highly recommended to fit it with an RCD. Especially when it is unclear if the appliances are RCD protected.
RCBOs
What is an RCBO?
RCBO stands for residual current circuit breaker with overcurrent. They are used to protect against overcurrents and earth faults. This means an RCBO does the job of both an MCB and an RCD and can be fitted into a consumer unit. These, therefore, offer full circuit protection.
Consumer Units
What is a consumer unit?
Also called a fusebox, consumer units are a form of distribution board that provides electricity to individual circuits from the main power supply. They control the amperage through the circuits to prevent damage and maintain user safety.
A metal consumer unit comprises of a fireproof casing, a main switch, a busbar, a DIN rail, RCDs, and circuit breakers. They can come populated, where they contain MCBs, RCBOs, and RCDs, unpopulated, or empty with only a main switch.
Do plastic consumer units need to be replaced?
The enclosure of a fusebox must be made of fireproof material or placed within a non-combustible casing to comply with 18th Edition Wiring Regulations. This makes steel a popular choice of material as it stops fires from spreading if they occur due to an electrical fault. Plastic fuseboxes are less safe than metal alternatives but are compliant with regulations as long as they are housed in a non-combustible unit.
SPDs
Surge protection devices protect installations from power surges called transient overvoltages. These power surges can be caused by faulty wiring, electrical overload, lightening strikes, power outage, and switching transformers and motors. An SPD works by directing any overvoltages to the earth, limiting the surge conducted into circuits. This protects the circuit from overloading and damaging appliances and cables.
SPDs are commonly used to protect installations with sensitive equipment such as safety circuits, washing machines, lighting, computers, and TVs. Whilst they are not mandatory to install, they are highly recommended.
What are busbars and DIN rails?
Busbars are inside consumer units to help distribute power. They are used to connect isolation devices to the circuit. Made of conductive metals such as aluminium and copper, they transmit high levels of current. When installed, they are cut to the correct size and plugged into the bottom of the required devices.
A DIN rail is a metal bar, often made of steel, on which circuit breakers and other electrical equipment are mounted. They do not conduct current and are found at the back of the unit.
What are the ways on a consumer unit?
The amount of ways on a consumer unit are the number of outgoing paths for current to flow into circuits. Therefore, larger properties are likely to need more ways. To work out the number of useable ways on a consumer unit, take away the number of ways the main switch, any SPDs, and RCDs take up from the total number of ways.
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