Types and Classification of Faults on Electrical Power Systems

Question: How many types of faults exist on the power system and how they are classified?

Answer: Generally four types of faults exist and are classified into two categories.

Symmetrical faults: They give rise to symmetrical equal currents having a displacement of 120°.

Unsymmetrical faults: They gave rise to unsymmetrical currents having unequal displacements.

List contains:

  1. Line to line to line fault
  2. Line to ground
  3. Line to line
  4. Double line to ground

Types of Fault


Severity of Fault

3 Line


Most Dangerous

Line to Line

Unsymmetrical Fault

More Dangerous

Line to Ground

Unsymmetrical Fault

Least Dangerous

Double Line to Ground

Unsymmetrical Fault

Dangerous than Line to Line and Line to Ground Fault


Types of MCB/MCCB


MCB/MCCB are widely used in electrical distribution system for ON/OFF Electrical supply and it also gives over current and short circuit protection. Selection of MCB/MCCB involves technical as well as Mechanical parameters. Not all the parameters are important but careless observation leads to wrong selection of MCB/MCCB or may increase cost unnecessarily.

Specifications of MCB/MCCB:
Current Related:

Frame Size (Inm): Amp
Rated current (In/ Ie): Amp
Ultimate short circuit breaking capacity (Icu): KA
Rated short-circuit breaking capacity (Ics): % of Icu

Voltage Related:

Rated voltage (Ve): Volt
Rated Insulation voltage (Vi): Volt
Rated impulse withstand voltage(Vimp): kV
No’s of Pole : SP,DP,TP,TPN,FP

Application Type:

Utilization Category/ Characteristic : B,C or D curve


Rotary Handle: Extended/ Direct
Alarm Contact
Shunt Trip
Under voltage Trip
Mechanical interlocking
Manual /Auto operation
Motorized Operation

Protection Type:

Protection : Over current / Short circuit
Trip Mechanism: Thermal / Magnetic / Solid / Microprocessor
Trip Mechanism adjustment : Fixed / Adjustable


Reference temperature (if different from 30°C);
Pollution degree;
Suitability for isolation;
Type of Mounting arrangement;
Electrical Life Cycles;
Mechanical Life Cycles;
Dimension: mm
Weight: Kg
Reference Standard: IEC: 60947-1/2, IS: 13947-1/2

Current Ratings:
Frame Size:

Breaker Frame Size indicates the basic framework of the Plastic shell of MCCB that can hold the highest rated current. It is the maximum current value for which the MCCB is designed (upper limit of the adjustable trip current range) and it also a deciding factor for physical dimensions of the device. There are varieties of current rating MCCB for the same series frame Size are available in the market.

For example, DX100 Frame Size MCCB are available in the market for the rated currents of 16A, 20A, 25A, 32A, 40A, 50A, 80A, 100A etc. Also for rated current of 100A MCCB of the frame size DX225 is also available.

Rated Current:

It is the current value above which overload protection is tripped and the circuit is disconnected.

For MCB, rated current is fixed, while in MCCB the rated current is adjustable.

Standard rating of MCB is 1A, 2A, 3A, 4A, 6A, 10A, 13A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, 100A.

Voltage Ratings:
Ultimate short-circuit breaking capacity (Icu):

Breaking capacity can be defined as the maximum level of fault current which can be safely cleared.
It is the highest fault current that the MCCB can trip without being damaged permanently.
The MCCB will be reusable after interrupting a fault, as long as it doesn’t exceed this value.
It is indicate operation reliability of MCCB
This parameter may increase or decrease the cost, so it should be properly decided. Breaking capacity should be higher than the possible fault level. For domestic application fault level may be 10kA.

Operating short-circuit breaking capacity (Ics):

It is expressed as a percentage ratio of Icu and tells you the maximum short-circuit current if a circuit breaker can break three times and still resume normal service.
The higher the lcs, the more reliable the circuit breaker
It is the maximum possible fault current that the MCCB can clear. If the fault current exceeds this value, the MCCB will be unable to trip and another protection mechanism must operate.
If a fault above the Ics but below the Icu occurs, the MCCB can interrupt it successfully but will need a replacement due to the damage suffered.
The Main difference between Ultimate Short Circuit (Icu) and Service Breaking Capacity (Ics) that Icu (Ultimate Braking Capacity) means Circuit breaker can remove the fault and remain usable but Ics (Service Braking Capacity) means Circuit breaker can remove the fault, but it may not be usable afterwards.
For example, if a circuit breaker has an Ics of 25,000 Amperes and an Icu of 40,000


Any fault below 25kA will be cleared with no problem.
A fault between 25kA and 40kA will cause permanent damage when cleared.
Any current exceeding 40 kA can’t be cleared by this breaker.

Rated working voltage (Ve):

It is the continuous operation voltage for which the MCCB is designed.
This value is typically equivalent or close to a standard system voltage.
In three phase it is usually 400V or 415 V. For single phase it is 230V or 240V.

Rated Insulation voltage (Vi):

It is the maximum voltage that the MCCB can resist according to laboratory tests.
It is higher than the rated working voltage, in order to provide a margin of safety during field operation.

Rated impulse withstands voltage (Vimp):

It is the value of transient peak voltage the circuit-breaker can withstand from switching surges or lighting strikes imposed on the supply.
This value characterizes the ability of the device to withstand transient over voltages such as lightning (standard impulse 1.2/50 μs).
Vimp = 8kV means Tested at 8 kV peak with 1.2/50μs impulse wave.

Number of Poles:

No of Pole for MCCB depends on Single Phase & Three Phase Power Controlling /Protection
Single Pole (SP) MCB:
A single pole MCB provides switching and protection for one single phase of a circuit. It is used for Single Phase circuit
Double Pole (DP) MCB:
A two Pole MCB provides switching and protection both for a phase and the neutral. It is used for Single Phase circuit
Triple Pole (TP) MCB:
A triple/three phase MCB provides switching and protection only to three phases of the circuit and not to the neutral. It is used for Three Phase circuit
3 Pole with Neutral (TPN (3P+N) MCB):
A TPN MCB, has switching and protection to all three phases of circuit and additionally Neutral is also part of the MCB as a separate pole. However, Neutral pole is without any protection and can only be switched. It is used for Three Phase circuit with Neutral
4 Pole (4P) MCB:
A 4 pole MCB is similar to TPN but additionally it also has protective release for the neutral pole. This MCB should be used in cases where there is possibility of high neutral current flow through the circuit as in cases of an unbalanced circuit. It is used for Three Phase circuit with Neutral.

Types and accessories of MCB/MCCB


Utilization category / Characteristic (B, C, D, K, Z curve):

Characteristic of Trip curves of MCCB tell about the trip current rating of MCCB.
MCB will trip instantaneously according to their Tripping Characteristic at 0.1 sec.
There are various type of MCCB:

  • Type B MCCB
  • Type C MCCB
  • Type D MCCB
  • Type K MCCB
  • Type Z MCCB

MCB trip curves

Type B MCCB:

Operating Current: This type of MCB trips between 3 and 5 times rated current (In).
Operating Time:04 To 13 Sec
For example a 10A device will trip at 30-50A.
Application: Domestic applications or light commercial applications where connected loads are primarily lighting fixtures, domestic appliances with mainly restive elements.
Suitable for: Restive Load application (Lighting , Small Motor)
Surge Current: The surge current level is relatively low.
Installation at: At Sub feeder of Distribution Board.

Type C MCCB:

Operating Current: This type of MCB trips between 5 and 10 times full load current.
Operating Time:04 To 5 Sec
Application: commercial or industrial type of applications, fluorescent lighting, motors etc where there could be chances of higher values of short circuit currents in the circuit.
Suitable for: Inductive Load application (Pumps, Motor, fluorescent lighting.)
Surge Current: The surge current level is relatively moderate level.
Installation at: At incoming / Outgoing of Distribution Board.

Type D MCCB:

Operating Current: This type of MCB trips between 10 and 20 times full load current.
Operating Time:04 To 3 Sec
Application: specialty industrial / commercial uses (Transformers or X-ray machines, large winding motors, discharge lighting, large battery charging). Where current inrush can be very high.
Suitable for: Inductive- Capacitive Load application (Pumps, Motor)
Surge Current: The surge current level is relatively High.
Installation at: At incoming of Distribution Board / Panels.

Type K MCCB:

Operating Current: This type of MCB trips between 8 and 12 times full load current.
Operating Time:04 To 5 Sec
Application: Suitable for inductive and motor loads with high inrush currents.
Surge Current: The surge current level is relatively High.
Installation at: At incoming of Distribution Board / Panels.

Type Z MCCB:

Operating Current: This type of MCB trips between 2 and 3 times full load current.
Operating Time:04 To 5 Sec
Application: These types of MCBs are highly sensitive to short circuit and are used for protection of highly sensitive devices such as semiconductor devices.
Surge Current: The surge current level is relatively too low.
Installation at: At Sub feeder of Distribution Board for IT equipment.

MCB/MCCB Accessories:
Rotary Handle:
  • It is used to extend ON/OFF handle of MCCB when Panel Door is closed.
  • It is also used to indicate ON/OFF or Trip Position
Shunt Trip:
  • Used for Remote Tripping
Alarm contact:
  • It gives Tripping Indication when MCCB Trip.
  • It does not give when MCCB is in normal condition (either ON or OFF)
Auxiliary contact:
  • It used for remote signaling and control purpose.
  • It is also give ON/OFF indication of MCCB at remote location.
Under Voltage Tripping:
  • It used to trip MCCB in under voltage condition (70 to 35% of rated Voltage).
Mechanical Interlocking:
  • It used to mechanical interlock of two MCCB on the same Panel.
Manual / Auto:
  • MCCB may have provision for Auto /Manual operation.
  • An “auto/manual” switch in front of Panel.
  • When set to the “Manual” position, lock out electrical control and when set to “auto”, lock out the manual control; remote indication
Motorized operation:
  • MCCB may have option for manual operation or with a motor mechanism for electrically controlled

MCB is designed and used in AC power system of 50 to 60Hz.
Electromagnetic force of magnetic release is related with power supply frequency so If Frequency is changed than electromagnetic fore of Magnetic element is changed hence MCCB tripping current will be different.
If we used MCCB for protection in DC circuits than specially design DC circuit MCCB should be used rather than normal type of MCCB.


MCCB is suitability for visible isolation. It is particularly important.
If a circuit breaker is turned off, it should indicate so visibly.
It should not be able to indicate otherwise if the contacts are not open. In other words, it offers proof of isolation.

Type of Mounting Arrangement:

According to mounting arrangement, MCBs can be divided into two categories.

DIN Rail Mount MCB:

  • The main advantage of this type of MCB is versatility
  • DIN rails are used by many different types of electrical and communications equipment, and they are mainstream in industrial settings.
  • They can be easily integrated into nearly any control or protection system.
  • A disadvantage of this type of MCB is that more work is required for installation, and plug-in MCBs may be a better choice for simple installations.

Plug-In MCB:

  • These MCBs are easy for installation. As name indicates, they just have to be plugged into a compatible electric panel.
  • Plug-in MCBs are suitable for applications that use circuit breakers exclusively- typically residential and commercial electrical distribution systems.
  • When using plug-in MCBs it is important that the breakers and the panel must match. It is not an issue when both are of the same brand.
Pollution degrees:
  • It determines in what kind of environment circuit breakers can be installed.
  • In a Domestic purpose where there is no dust no humidity, the circuit breaker is comfortable.
  • For Domestic purpose pollution degree 2 is suitable.
  • But in an outdoor public installation where there may be dust which cause leakage currents and lead to dangerous arcs.
  • For dusty pollution, humidity environment or outdoor type heavy-duty applications (incoming switchboards) pollution degree 3 is suitable.
Energy Class:
  • MCB need some time for tripping, In this time, fault current will create some energy which will exist in system. This energy is termed as release energy. For efficient MCB operation it should be in within limited.
  • On basis of amount of release energy it is classified in class 1, class 2 and class 3. Class 3 is best which allows maximum 1.5L joule/second.

MAX44211 Output Protection


Unwanted transients emanating from the power line back to the line-driver’s output and back-EMF generated by the coupling circuit when the load is disconnected and connected, and vice versa, create undesirable over-voltage conditions. Back-EMF is also generated when the line driver outputs are toggled from high to low impedance state, if significant current was flowing at the disconnect time. The MAX44211 power line communications driver most often is placed facing the power line with an isolating stage between them. In case of such over-stress conditions, external protection is required to protect the MAX44211 and downstream circuits.

The internal ESD clamping structure is present inside MAX44211 (see Figure) to protect the part from ESD or sub-microsecond events. When the application has hazards where these events can occur frequently and for comparatively longer time, it is recommended to have external protection circuitry. Failure to accommodate an output protection can result in over-stressing the outputs (OUT+/ OUT-) and eventually damage the part.

This application note provides insights and information on how to add external protection circuitry at the outputs of the MAX44211. A circuit using VAVDD = 15V is used in this application note.

The figure shows the internal ESD clamping protection structure.

Download PDF: MAX44211-Output-Protection

Source: Maxim Integrated

High-Speed Over-current Detection


This high-speed, low-side over-current detection solution is implemented with a single zero-drift fast-settling amplifier (OPA388) and one high-speed comparator (TLV3201). This circuit is designed for applications that monitor fast current signals and over-current events, such as current detection in motors and power supply units.

The OPA388 is selected for its widest bandwidth with ultra-low offset and fast slew rate. These parameters allow the circuit to be a well-balanced, high-speed solution in order to accurately detect high frequency current components. In applications that only require average current detection, devices with less bandwidth can be used like the LPV821. In applications that require faster response time, devices with larger bandwidth can be used like the THS4521.

The TLV3201 is selected for its fast response due to its small propagation delay of 40 ns and rise time of 4.8 ns. This allows the comparator to quickly respond and alert the system of an over-current event all within the transient response time requirement. The push-pull output stage also allows the comparator to directly interface with the logic levels of the micro-controller. The TLV3201 also has low power consumption with a quiescent current of 40 µA.

Typically for low-side current detection, the amplifier across the sense resistor can be used in a non-inverting configuration. The application circuit shown, however, uses the OPA388 as a differential amplifier across the sense resistor. This provides a true differential measurement across the shunt resistor and can be beneficial in cases where the supply ground and load ground are not necessarily the same.

Dedicated current sense amplifiers can also be an option for high-speed current detection. The trade-offs for each option should be taken into account when deciding on a current sense solution. Integrated solutions can provide larger bandwidth and savings on board space, but can be more costly and offer limited customization. Discrete solutions can be lower cost and easily customizable, but could require precision resistors to increase measurement precision.

Download PDF: High-Speed-Overcurrent-Detection

Source: Texas Instrument

Motion Sensing & LED Control

Reference Design Description

The Motion Sensing & LED Control Reference Design demonstrates how to use Zilog’s Z8FS040 ZMOTION MCU with the IXYS LDS8710 High-Efficiency LED Driver and explains how to implement features such as ambient light detection and LED dimming in addition to passive infrared motion detection. It also illustrates how the combined capabilities of both products are an exceptional choice for energy management functions in applications ranging from low-power displays and backlighting to ambient room lighting control.


  • Integrated passive infrared (PIR) motion sensing and LED control combined in a single 8-pin Zilog MCU
  • Self-contained, single-board solution for motion sensing and LED control
  • Motion detection adapts to various lenses with simple header file changes
  • Selectable power source of either two AA batteries or an external power supply with operating voltage in the 2.7 V to 3.6 V range
  • Ability to switch high-intensity LEDs on and off in response to motion
  • Controls LED brightness from three sources:
    • Console command
    • Potentiometer (control source depends on software build)
    • Ambient light

Source: EEWeb Design Library

Reference Design

Portable Variable Power Supply

Portable Variable Power Supply.jpg
Photo taken from http://www.Instructables.com website

In many of the project applications and testing applications, Variable DC Power supplies are used. Sometimes it happens that one can not find an AC power socket nearby testing circuit. Also, Power supplies are usually AC powered and are bulky, because of their voltage transformers, to bring anywhere near testing circuit or equipment. Hence, Portable Variable DC Power Supply is required which helps people become mobile in doing electronic projects that will need power supplies.

This project uses a voltage regulator and a potentiometer that allows the user to change the output voltage between 2 volts to 25 volts. It is also equipped with a small seven segment display which acts a voltage meter which lets the user to know how much voltage is being supplied by the regulator. Powered by a 9V battery, this project gives mobility in making circuit projects. It also includes a banana jack that accommodates plugs that can help attach it to different applications.


Source: Portable Variable Power Supply