Electrical Protective devices are weak link devices use in protecting the electrical project from overcurrent, transient surge, related power issues, and electrical users against shock.
Over-current is when the current flow through a circuit is above what is rated for. It can be an overload which implies an excessive demand from the utilization point or an electrical stray outside its pathway called Shortcircuit.
Shortcircuit are fault due to electricity stray outside its pathway. This may be as a result of faulty insulation, loose connection or fault wiring from the appliances end making hot wire to touch the neutral or grounded system.
The transient surge is a sudden increase due to a spike in nominal voltage. This does happen when switching ON a device or due to the lighting effect.
The related power issues, on the other hand, maybe a reverse phase rotation in a 3 phase system.
Electrical users also need protection against shock which can only be ensured by proper grounding or the type of devices use in protecting the electrical circuits.
An efficient electrical design must be able to protect against fault, adequate personnel protection, and be able to minimize disruption.
Critical understanding of these devices, operations, and strategies are thus required because of the position they hold in every electrical project.
Based on this, every electrical protective device must have the following technical characteristics;
1- Rated current
2- Breaking value and characteristic
3- Shortcircuit capacity
The rated current of a protective device is the maximum value the device can withstand. This value must be equal to the rating of the circuit it is protecting. It is a measure of standard values such as 10A, 20A, 32A, etc.
The breaking value of any electrical protective device is the peak value of the short circuit current that will not get the device burn. This value varies widely based on construction types and material. Although not the main label of a device, it also exists for a device as standard values such as 60A, 100A, 1kA 4kA, 16.5kA, etc.
Goals of using protective devices
- To localize and isolate electrical fault
- To prevent and minimize any power loss
Classification of protective devices
Protective devices use in the building industry can be categorized into four
- Over-current protective devices
- Ground fault protective devices
- Combination (over-current and Ground fault) protective devices
- Special protective devices
Over-current protective devices
The overcurrent protective devices operate less than 5ms and do not need replacement other than resetting. This category includes the miniature circuit breaker (MCB) which is a bi-metallic strip that becomes deformed by heating before will eventually displace from the latch point, the molded case circuit breaker (MCCB) which are being used for the main feeders because of the ability to withstand higher current.
The overcurrent protective devices includes the circuit breakers, fuses, isolators, and relays.
A fuse is a wire place in a fuse holder designed to melt above certain amperage so as to disconnect the supply from the circuit. They can be of a Cartridge type (General purpose) which are being used in the electrical building services industry or the HRC (high rupture capture) types which has time delay and are specifically designed for high current.
The isolators are used as supplemental to the circuit breakers to isolate the system completely. They can only be operated when the current is not flowing through the circuit to isolate the system entirely. This implies that they can only be open after the circuit breakers have been opened and closed before the circuit breakers are closed.
The relays evaluate the circuit operations constantly and trip the instant the circuit condition changes. They are thus efficient in protecting against transient surge and related power issues.
The Ground fault protective devices
The Ground fault protective devices are of two types;
The ground fault protection of equipment (GFPE) such as the deprecated ELCB is designed to sense and ground fault at equipment level i.e lesser than the conductor level up to 30mA and are mainly for domestic safety.
The Ground fault protection interrupters (GFPI) are designed to limit electrical shock when the hot and the neutral are unbalanced. An example of a device in this category is the RCBB and is mainly for human being protection as they senses fault down to 5mA where the current finds its path to earth. Their main limitation is their unwanted tripping due to any changes.
Combination (over-current and Ground fault) protective devices
The combination protective device is a combination of MCB and RCCB in a single device which protect against over-current, and earth leakage simultaneously. A device example is RCBO.
Special protective devices
The special protective devices include the one we do use to prevent current leaks called GFCI and those for deteriorated cables called AFCI.
The GFCI protect shocks or hazard that may occur when a body comes in contact with small leakage current usually between 5-30mA. The human body does serve as a ground path to this leak and can lead to cardiac arrest. They are thus recommended to be used in areas within 2m moisture range or prone to water such as the bathroom, kitchen, and outdoor areas.
The AFCI, on the other hand, protects against arcing fault caused by deteriorated cables.