Otis Singleton
Electrical protective devices are essential safety features that prevent electrical circuits from damage caused by overloading or short circuits. They work by interrupting the flow of current when there is an overcurrent or excessive current flow. This is achieved through the use of fuses, circuit breakers, and other protective devices. While it may be tempting to bypass a circuit breaker in certain situations, it is generally not recommended due to the potential risks and safety hazards involved, such as increased fire hazards and legal and insurance implications.
Characteristics and Values of Electrical Protective Devices
| Characteristics | Values |
|---|---|
| Function | To protect electrical circuits from extreme voltages or currents, overloading, short circuits, and potential hazards |
| Types | Fuses, circuit breakers, RCCB, gas discharge tubes, thyristors, SPD, ESD protection electronic fuses, etc. |
| Working | Fuses contain a metal strip or wire that melts when the flow of current is high, breaking the circuit. Circuit breakers act as switches that open when excessive current is detected, interrupting the flow of current. |
| Safety | Essential to prevent electrical fires, damage to appliances and wiring, and to ensure consumer safety. |
| Back-up | In a radial system, each overcurrent protection device backs up the devices downstream from it. |
| Resetting | Fuses need to be replaced, while circuit breakers can be manually reset without damage. |
| Cost | Fuses are cheaper and easily repaired, while MCBs are more expensive and require regular testing. |
| Usage | Fuses and circuit breakers are used in every conceivable electrical system where there is a possibility of overcurrent damage. |
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What You'll Learn
Fuses
In the context of industrial control systems, fuses are often used to protect higher SCCR (Short Circuit Current Rating) values over 10kA. Fuses are also used in residential applications, and to protect semiconductor devices.
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Circuit breakers
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by overcurrent or excess current. It is a type of over-current protection device (OCPD) that automatically removes power from a faulty system. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset manually or automatically to resume normal operation. Circuit breakers are commonly installed in distribution boards. They are also used as a main switch to manually disconnect ("rack out") and connect ("rack in") electrical power to a whole electrical sub-network.
In a radial system, each overcurrent protection device backs up the devices downstream from it. If the computer power supply fuse fails to function properly, the plug strip thermal breaker will respond after a certain coordination delay. If it also fails, the branch breaker will back them both up, again after a coordination delay. This delay is needed by the back-up device to give the primary protection device – the device closest to the overload or fault – a chance to respond first.
The circuit breaker contacts must carry the load current without excessive heating and must withstand the heat of the arc produced when interrupting (opening) the circuit. Contacts are made of copper or copper alloys, silver alloys, and other highly conductive materials. Service life is limited by the erosion of contact material due to arcing while interrupting the current. When a high current or voltage is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way so that the gap between the contacts can again withstand the voltage in the circuit.
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Overcurrent protection
An overcurrent occurs when the normal load current in an electrical circuit is exceeded. This can be caused by a short circuit or an overload situation. In a short circuit, the current takes a shortcut around the normal path of current flow. Overloads can occur when too many loads are connected to a single power source, causing the overall current to exceed the rated value. Sustained overloads can be destructive and must be prevented.
The voltage rating of an OCPD determines its ability to suppress internal arcing during overcurrent or short-circuit conditions. The voltage rating must be equal to or greater than the circuit voltage. Low-voltage circuit breakers protect circuits using less than 1000 V of electricity. The interrupting-current rating, or short-circuit rating, of a fuse or circuit breaker is the maximum current it can safely interrupt. If a fault current exceeds the interrupting capacity of the device, it may rupture and cause additional damage.
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Residual current devices
A residual-current device (RCD) is a life-saving electrical safety device designed to prevent fatal electric shocks and provide protection against electrical fires. RCDs are also known as residual-current circuit breakers (RCCBs) or ground fault circuit interrupters (GFCIs). RCDs are designed to be wired in-line with an appliance's power cord and constantly monitor the electric current flowing through the circuit.
RCDs work by detecting an imbalance between the supply and return conductors of a circuit, indicating leakage current, which can cause electric shock. When an imbalance is detected, the RCD quickly interrupts the circuit by disconnecting the conducting wires, a process known as "tripping". This tripping occurs within 10-15 milliseconds, preventing serious injury or damage to electrical devices. RCDs are rated to carry a maximum current of 13 A and are designed to trip on a leakage current of 20-30 mA, with trip times ranging from tens to hundreds of milliseconds.
The sensitivity of RCDs is classified into three groups based on their rated residual operating current (IΔn): high, medium, and low sensitivity. High sensitivity RCDs offer direct-contact and life injury protection, medium sensitivity RCDs provide fire protection, and low sensitivity RCDs are typically used for machine protection. RCDs can be tested using a pushbutton marked 'T' or 'Test', and if the device does not trip when this button is pressed, it needs to be replaced.
While RCDs provide essential protection against electric shocks and fires, they do not offer safety against short circuits or overload conditions in a circuit. Therefore, it is important to combine RCDs with other protection devices, such as fuses or circuit breakers, to ensure comprehensive safety in electrical circuits.
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Surge protection devices
SPDs are used in electrical circuits to protect against extreme voltages or currents. They are allied in parallel to the power supply circuit and can be used on all stages of the power supply system. SPDs are capable of repeating their functions as specified. They are permanently connected and intended for installation between the secondary of the service transformer and the line side of the service disconnect overcurrent device.
There are three types of SPDs:
- Type 1: This type of SPD is intended to be installed between the secondary of the service transformer and the line side of the service disconnect overcurrent device. Its main purpose is to protect the insulation levels of the electrical system against external surges caused by lightning or utility capacitor bank switching.
- Type 2: Type 2 SPDs are also permanently connected and are installed on the load side of the service disconnect overcurrent device, including brand panel locations. They protect sensitive electronics and microprocessor-based loads against residual lightning energy, motor-generated surges, and other internally generated surge events.
- Type 3: Point-of-utilisation SPDs are installed at a minimum conductor length of 10 meters (30 feet) from the electrical service panel to the point of utilisation.
Overall, SPDs are an essential component of electrical systems, providing protection against voltage surges and ensuring the safe and reliable operation of sensitive equipment.
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Frequently asked questions
A protective electrical device is a device that protects an electrical circuit from damage. This includes damage from overcurrent, overload, or short circuits.
Protective electrical devices work by interrupting the flow of current when there is an overload or fault current. This is done by adding a 'weak link' to the circuit, which will break in the event of a fault.
Examples of protective electrical devices include fuses, circuit breakers, RCDs, RCCBs, and ELCBs.
While it is generally not recommended, it is possible to bypass a protective electrical device in certain instances. For example, electricians may temporarily bypass a circuit breaker to diagnose a circuit or during power supply testing. Bypassing a protective device removes a layer of safety and can increase the risk of damage, electrical faults, and fires. It may also have legal, insurance, and warranty implications.
