Circuit Breaker Operating Time Measurement

The operating time of a circuit breaker is crucial in any protection scheme.  A circuit breaker that takes too long to open will seriously compromise protection causing damage to equipment and people.

Hence, circuit breakers should be periodically tested to see whether they operate at the correct operating time specified by the manufacturer.

Being mechanical devices, circuit breaker are made of numerous springs, washers, and linkages.  These components can get jammed, the lubricating oil in the linkages can evaporate or lose its properties.  The lubricating oil can mix with dust and form a viscous deposit.  This can cause partial seizure and  is particularly true for circuit breakers which are kept in the closed or open position for very long periods of time.   

The Circuit Breaker operating time can be measured by special testing equipment known as the Time interval meters.  These instruments measure the time between the signal to open  and the actual interruption of the current.    The opening time of the contacts belonging to each individual phase is measured.

Arc Flash Protection

An Arc Flash is a serious accident in which a short circuit or an earth fault creates an arc.  The tremendous heat generated can cause severe burns and even death to people who are working nearby. (See Article on Arc Flash).  Arc Flash Relays seek to minimize the damage caused by an arc.  The damage due to an arc can be isolated if the isolating circuit breakers feeding the fault are made to trip at as soon as possible.

An arc which lasts for about 500ms can cause severe damage to people and equipment.  An arc lasting 100ms will cause lesser damage.  An arc lasting just 35 ms will cause far lesser damage. 

Normal protection schemes involving over current and earth fault relays will not be fast enough to isolate these faults.  Besides, making these relays more sensitive can cause undesirable false trippings. 

Arc Flash Relays work by sensing the light intensity caused during the flash over.  An arc causes a tremendous increase in the light intensity, often of the order of several thousand lux.  The light intensity in a normal office lighting will be around 200 or 300 lux.  The light sensed by the sensors is compared with a preset reference level.  The relay activates the trip signal if this reference is exceeded. 

Modern Arc Flash Relays can operate within 2.5 ms of a fault + the operating time of the circuit breaker.  The operating time of the breaker needs to be checked periodically.  Circuit breakers should open within 50ms of the command being given.  Delayed opening will seriously compromise protection. 

Air Break Switches

Air Break Switches are special switches designed isolate a circuit.  The are usually employed in out door installations.  Special Arcing Horns are provided to quench the arc which occurs when the current is interrupted.  These switches are usually operated by a handle which is located at the ground level.  Their operation can also be mechanised. 

Air Break Switches should not be used to interrupt load currents. They are isolating devices.   They can, however, be used to interrupt small currents such as the exciting current of a transformer or the capacitive charging current of a long transmission line.  A variation of the Air Break Switches is the Load Switch which can interrupt current on load as it has special arc quenching device. 

Air Break Switches can be mounted both horizontally and vertically.  These switches usually have a rugged construction as they are exposed to the weather.  Air Break Switches are usually found in groups of three, one for each phase.  These groups are opened together.  Hence, they are also known as gang-operated switches. 

Air Break Switches have been designed upto 135 kV

Jogging and Inching in Motors

Jogging refers to repeated starting and stopping of a motor in short bursts to perform a particular movement such as moving a crane to a particular location. The motor is operated on full power.  Special Jogging circuits are available. 

Inching is similar to Jogging and the two are often confused with each other.  Inching involves short thrusts of motion at reduced power.  The motor is made to run at a reduced voltage. 

Jogging should not be done more than 5 times in a minute.  This is because jogging can cause the motor to overheat.  If Jogging is to be done frequently, the starter of the motor should be derated. 
 
Special Jogging Blocks are also available which prevent the motor from being jogged more than 5 times an hour by logging the number of starts per hour. 

A simple jogging Circuit

Line Chokes

Line chokes are inductances which are connected in series to devices such as inverters and rectifiers.  Lines chokes are used widely in Drive systems, battery chargers and UPS.  They are available in both single phase and three phase designs.

Line Chokes reduce harmonics and limit surge currents.  They also protect rectifiers from voltage fluctuations in the mains supply (dv/dt phenomena).  Line chokes also reduce the peak currents drawn by inverters and reduce the starting current drawn by motors.

Non- Contact High Voltage Detector

The Non Contact High Voltage Indicator is a safety device which indicates with a buzzer sound and flashing light whether a conductor or an equipment is live or not. 

When working with High Voltages, it is extremely important to ensure that we know which equipment is live and which equipment is dead.

Too often, engineers and workers are exposed to severe injuries or even death when someone accidentally gets exposed to live conductors or tries to connect a test equipment to a conductor which they thought was dead.

Hence, it is absolutely essential that we ensure that equipment does not have any voltage and is discharged before we begin work.

The Non Contact High voltage indicator works by sensing the electric charge lines emanating from a point at high potential.  Since, these lines of electric charge can be detected at a distance from the conductor, contact with the conductor is not necessary.  The Non contact high voltage indicators available in the market can be used to detect a wide range of voltage from 80V to nearly 500kV.  These indicators are usually powered with alkaline batteries.

Each High voltage detector has a inbuilt test function which tests the device for functionality prior to use. 

The Non Contact High Voltage Indicator is held near the live conductor using a hot stick.  Gloves of the appropriate rating should be worn when using the equipment. 

Harmonic Mitigating Transformers

Harmonics Mitigating transformers are special kind of transformers which are used to mitigate harmonics in the connected loads. They are often confused with K Transformers. K Transformers(Refer Article) is a type of transformer which is over sized to cater to loads which generate harmonics. A Harmonics Mitigating Transformer, however, works by passively suppressing the harmonics.

Harmonics Mitigating Transformers are designed like ordinary transformers except that they have specific phase shifts (+15 deg, -15 deg, 30 degs). The primary winding is usually delta while the secondary winding is a zig zag winding. The zig zag winding of the secondary cancels a particular order of harmonics depending on the phase shift.

A Harmonics Analysis is conducted to identify the order of the harmonics generated before choosing the type of Harmonics Mitigating Transformer

The harmonics are prevented from reaching the primary and affecting transformer output. Harmonics Mitigating Transformers fall under the passive methods of harmonic suppression.