02_Overcurrent Protective Devices - Sem 1 2024-25_unlocked

Both overload and short-circuit conditions.

Magnetic action of the electromagnet.

To protect electrical circuits by preventing overloads or faults that can cause fires or other safety hazards.

To make the necessary re-ignition voltage too high to be supplied from the power source.

It shows its short-time thrust current/thermal/electromagnetic stress ability.

Assumptions help in accurately calculating the required fuse ratings under specific conditions.

At the origin of the final circuit.

Overload, short circuit (line-to-line), earth fault, and voltage dip (or momentary loss of supply).

Current operated protective devices.

It senses a fault and trips a primary isolation device.

By selecting appropriate protective devices and adjusting their settings to reduce interruptions during fault conditions.

The current rating needs to be determined based on the circuit conditions.

Yes, a single device at the origin may serve both purposes.

Fuses often break within the first half cycle of the prospective fault current.

It heats up and bends to trip contacts.

To prevent nuisance trips by switch-on surges.

Heat generated = I²R, where I is the current and R is the resistance of the fuse wire.

I is the fault current, R is the contact resistance, and t is the duration of the short circuit current.

Relatively low-magnitude currents, such as transformer magnetizing currents or line and cable charging currents.

Lower current results in a slower trip response, while higher current leads to a faster trip response.

By making the arc interact with a transverse magnetic field generated by the arc current.

The I-t curves are parallel to each other, showing discrimination between fuses of different ratings when used in series.

Typically ± 5% or ± 10% tolerance on the resistance of a current-limiting fuse element.

The rating of the HRC must be greater than the start-up current of the load.

Heat loss = 2kdl, where k is a constant and d is the diameter of the fuse wire.

The amount of energy generated from the arcing point to the point at which the overcurrent is safely interrupted.

By thermal action, exhibiting an inverter I-t characteristic.

800 A.

The effects of arcing at its contacts and the electromagnetic forces and thermal conditions from short-circuit currents.

Long time thermal trip response and short time magnetic trip response.

To increase the arc length and improve interruption efficiency.

It shows its maximum impulsive stress ability and is the peak value of the calculated fault current.

It helps quench the arc by creating a high resistance substance through a chemical reaction with silver vapor.

To rapidly interrupt a short circuit fault.

They need replacement after blowing, may dissipate heat to adjacent components, may not be sensitive enough for earth faults above 300 A, and have fixed fuse link settings.

I²t is the same in major and minor fuses if they are connected in series.

Bimetal switch and electromagnetic switch.

By using compressed air or retracting contacts into a sealed chamber to blow out the arc.

Inverse Definite Minimum Time Lag.

The fuse will not cut-off and may need more than one cycle to break the circuit.

It closes the gap and trips contacts on a short circuit.

It is not easy to achieve full-range discrimination due to their overlapping I-t curves.

The trip characteristic is a superposition of the inverse curve and the instantaneous curve.

Each fuse can have slight differences in total operating time at a particular overcurrent value.

The amount of energy required to melt the fuse element before it begins to arc.

The current increases, strengthening the electromagnet, which pulls the soft iron armature towards it.

At the natural current zeros in AC arcs when the power input is zero.

The current rating needs to be determined based on the circuit conditions.

The maximum to which the fault current reaches before the fuse melts.

On-board adjustable trip responses by electronic means.

Type D MCB.

Long time thermal trip response, short time magnetic trip response, and instantaneous magnetic trip response.

To protect an electric circuit from overload or fault conditions.

The maximum value of current it can carry indefinitely without exceeding temperature limits.

To ensure the HRC can clear any short-circuit fault.

Heat loss is proportional to the surface area of the fuse wire.

The total energy the fuse will let through during an operation, calculated as Prearcing I²t + Arcing I²t.

Total I²t is also known as Clearing I²t.

To disconnect circuits and equipment experiencing continuous overloads before overheating occurs.

To carry current of short-circuit magnitude until the fault is cleared by another breaker or fuse closer to the fault.

Total load of the circuit, diversity of the total load, rating of the circuit conductors, operating time, possibility of upstream/downstream discrimination, and energy-let-through.

At any point along the conductor.

Overload discrimination and fault discrimination.

Adjustment to the magnetic trip.

It uses variable time delay settings in circuit breakers, allowing downstream devices to trip earlier than upstream ones.

Let-through energy can be expressed as ∫ I² R dt.

MCBs are responsible for clearing overloads in minor circuits.

It helps in understanding the operational characteristics and response times of the devices.

An electrical breakdown of a gas that produces a prolonged electrical discharge.

Short-circuit.

An automatically operated electrical switch that uses air to protect an electric circuit during abnormal conditions.

They are definite minimum time devices for short-circuit protection and inverse devices for overload protection.

General purpose use (Residential & light commercial installations).

Fuses need to be replaced after melting, while circuit breakers can be reset after tripping.

It shows its maximum thermal stress ability and is expressed as an RMS value.

The highest possible amount of power that may be dissipated in a short circuit fault.

The selection of an upstream device with a higher current rating and instantaneous pickup level than the next downstream device.

It is measured in terms of the let-through energy (or I²t).

Breaking Capacity, Making Capacity, Rated current, Short-time current rating, Time-current curve, I2t let-through energy.

They could exceed the heat dissipation rate, leading to overheating of conductors and devices.

Fuses and Circuit Breakers.

To ensure proper protection against overload and short-circuit conditions.

To minimize disturbance in the installation by ensuring the downstream OCPD operates before the upstream device during a fault.

The current carrying capacity of a fuse wire.

Air Circuit Breaker (ACB), Miniature Circuit Breaker (MCB), and Moulded Case Circuit Breaker (MCCB).

The area between the minimum melt characteristic and the total clear characteristic, accounting for variations in operating time.

1. The fuse element carries the fault current until it reaches the cut-off current; 2. The fuse carries the arcing current until it is fully suppressed.

Carry full load currents, open automatically for small overloads, rapidly interrupt short-circuit currents, withstand voltage across open contacts, and close on existing faults.

Overload discrimination deals with excessive current over time, while short-circuit discrimination addresses immediate high fault currents.

Six major types.

An arc appears between the opening contacts.

It shows its indefinite current stress ability.

The relationship between current and time for operation.

Time discrimination and current discrimination.

250 A to around 800 A.

R = ρl/A, where ρ is the resistivity, l is the length, and A is the cross-sectional area.

Yes, in emergency situations or maintenance operations.

Pre-arcing time plus arcing time.

The heat generated is far greater than the heat dissipated.

Load currents in other minors have negligible I²t effects on the major, fuses are of the same type from the same manufacturer, and tolerance of a fuse is between ± 5% to ± 10%.

4 ×.

A fuse is made up of a piece of metal that melts when overheated, requiring replacement.

Characteristics of OCPDs, let-through energy, discrimination, and I-t curves.

The maximum fault current that a circuit breaker can interrupt without being destroyed.

The enclosure is made of glass or other chemical compounds, with a ceramic body and metal-end caps welded by a silver current carrying element.

It defines the ability of the circuit breaker to withstand rated short-time current for a specific duration.

They work in unison to cater for all fault conditions.

To carry short circuit heavy current for a known time interval and blow off if the fault is not removed.

To determine the range of fault current that allows for discrimination.

A temperature-sensitive component (e.g., bimetal strip).

They can clear both high and low fault currents, do not deteriorate with age, enable fast operation, provide reliable discrimination, require no maintenance, and are cheaper than other circuit protection devices.

95% Prearcing I²t of the major fuse must be greater than 105% Total I²t of the minor fuse.

They are in phase, making the arc purely resistive.

Moulded Case Circuit Breaker.

A bimetal strip deforms, releasing a lock and separating the contacts.

The maximum fault current a circuit breaker can withstand while being closed.

It is the energy in joules liberated in one ohm of resistance in a circuit protected by a fuse, expressed as I²t.

To set circuit breakers to appropriate limits that prevent unnecessary trips during faults.

Time discrimination and current discrimination.

It indicates how the fuse responds to continuous current ratings over time.

It is greater than the peak of the normal 50 Hz system voltage.

Current flows from terminal A to B via the contact and the electromagnet.

It pulls the contact apart, disconnecting the circuit instantly and stopping the flow of fault current.

Heat must be removed from the arc more quickly than it is generated by ohmic heating.