Overcurrent Protection and Circuit Devices

A collection of flashcards to review key concepts related to overcurrent protection devices and their functionality in electrical systems.

What is overcurrent?

Overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat.

What are the possible causes of overcurrent?

Possible causes for overcurrent include short circuits, excessive load, incorrect design, an arc fault, or a ground fault.

What are common overcurrent protection mechanisms?

Common overcurrent protection mechanisms include fuses, circuit breakers, and current limiters.

What is the purpose of overcurrent protective devices?

Overcurrent protective devices are meant to protect against the potentially dangerous effects of overcurrents such as an overload current or a short-circuit current.

How does a fuse work?

A fuse breaks the circuit by melting and preventing current flow if the current exceeds the fuse's rating.

What is the nominal current rating of a fuse?

The nominal current rating is the maximum current that the fuse can carry without disconnection and without reducing the expected life of the fuse.

What happens when a fuse 'blows'?

When a fuse 'blows', the element in the fuse has melted or ruptured, and it needs to be replaced.

What does breaking capacity (kA) rating indicate?

The breaking capacity rating indicates the current up to which the fuse can safely disconnect fault currents.

What is a key problem associated with rewirable fuses?

The key problem is that the incorrect rating of wire can easily be inserted when replacing the fuse wire, leaving the circuit underprotected.

What modern fuses are incorporated into?

Modern BS 88 fuses are incorporated into sealed cylindrical ceramic bodies (or cartridges).

What are the two main categories of circuit breakers?

The two main categories of circuit breakers are miniature circuit breakers (MCBs) and moulded-case circuit breakers (MCCBs).

What is the advantage of circuit breakers over fuses?

Circuit breakers do not need replacing in the event of a fault; they just need to be reset.

What are the two types of tripping mechanisms in circuit breakers?

The two types of tripping mechanisms are thermal overload and magnetic overload.

What do Type B MCBs typically protect against?

Type B MCBs are used for domestic circuits and commercial applications where there is no inrush current to cause it to trip.

What distinguishes Type C MCBs from Type B MCBs?

Type C MCBs trip between five and ten times the rated current, suitable for applications with small to medium motors or fluorescent lights.

What applications are Type D MCBs used for?

Type D MCBs are used for specific industrial applications with large inrush currents, such as industrial motors, X-ray units, and welding equipment.