ELECTRIC INDUCTION FURNACE

ELECTRIC INDUCTION FURNACE
The electric induction furnace is a type of melting furnace that uses electric currents to melt metal. Induction furnaces are ideal for melting and alloying a wide variety of metals with minimum melt losses, however, little refining of the metal is possible.

PRINCIPLE OF INDUCTION FURNACE
The principle of induction furnace is the Induction heating

INDUCTION HEATING:
Induction heating is a form of non-contact heating for conductive materials.
The principle of induction heating is mainly based on two well-known physical phenomena:
1. Electromagnetic induction
2. The Joule effect

1) ELECTROMAGNETIC INDUCTION
The energy transfer to the object to be heated occurs by means of electromagnetic induction. Any electrically conductive material placed in a variable magnetic field is the site of induced electric currents, called eddy currents, which will eventually lead to joule heating.

2) JOULE HEATING
Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat.
The heat produced is proportional to the square of the current multiplied by the electrical resistance of the wire.
 Induction heating relies on the unique characteristics of radio frequency (RF) energy - that portion of the electromagnetic spectrum below infrared and microwave energy. Since heat is transferred to the product via electromagnetic waves, the part never comes into direct contact with any flame, the inductor itself does not get hot and there is no product contamination.
 Induction heating is a rapid, clean, non-polluting heating.
 The induction coil is cool to the touch; the heat that builds up in the coil is constantly cooled with circulating water.

FEATURES OF INDUCTION FURNACE
 An electric induction furnace requires an electric coil to produce the charge. This heating coil is eventually replaced.
 The crucible in which the metal is placed is made of stronger materials that can resist the required heat, and the electric coil itself cooled by a water system so that it does not overheat or melt.
 The induction furnace can range in size, from a small furnace used for very precise alloys only about a kilogram in weight to a much larger furnaces made to mass produce clean metal for many different applications.
 The advantage of the induction furnace is a clean, energy-efficient and well-controllable melting process compared to most other means of metal melting.
 foundries use this type of furnace and now also more iron foundries are replacing cupolas with induction furnaces to melt cast iron, as the former emit lots of dust and other pollutants.
 Induction furnace capacities range from less than one kilogram to one hundred tonnes capacity, and are used to melt iron and steel, copper, aluminium, and precious metals.
 The one major drawback to induction furnace usage in a foundry is the lack of refining capacity; charge materials must be clean of oxidation products and of a known composition, and some alloying elements may be lost due to oxidation (and must be re-added to the melt).

CONSTRUCTION OF INDUCTION FURNACE
 There are many different designs for the electric induction furnace, but they all center around a basic idea.
 The electrical coil is placed around or inside of the crucible, which holds the metal to be melted. Often this crucible is divided into two different parts. The lower section holds the melt in its purest form, the metal as the manufacturers desire it, while the higher section is used to remove the slag, or the contaminants that rise to the surface of the melt.
 Crucibles may also be equipped with strong lids to lessen how much air has access to the melting metal until it is poured out, making a purer melt.

TYPES OF INDUCTION FURNACE
There are two main types of induction furnace: careless and channel.
Coreless induction furnaces
 The heart of the coreless induction furnace is the coil, which consists of a hollow section of heavy duty, high conductivity copper tubing which is wound into a helical coil.
 Coil shape is contained within a steel shell.
 To protect it from overheating, the coil is water-cooled, the water being recirculated and cooled in a cooling tower.
 The crucible is formed by ramming a granular refractory between the coil and a hollow internal.
 The coreless induction furnace is commonly used to melt all grades of steels and irons as well as many non-ferrous alloys. The furnace is ideal for remelting and alloying because of the high degree of control over temperature and chemistry while the induction current provides good circulation of the melt.
Channel induction furnaces
 The channel induction furnace consists of a refractory lined steel shell which contains the molten metal. Attached to the steel shell and connected by a throat is an induction unit which forms the melting component of the furnace.
 The induction unit consists of an iron core in the form of a ring around which a primary induction coil is wound.
 This assembly forms a simple transformer in which the molten metal loops comprises the secondary component.
 The heat generated within the loop causes the metal to circulate into the main well of the furnace.
 The circulation of the molten metal effects a useful stirring action in the melt.
 Channel induction furnaces are commonly used for melting low melting point alloys and or as a holding and super heating unit for higher melting point alloys such as cast iron.

ADVANTAGES OF INDUCTION FURNACE:
Induction furnaces offer certain advantages over other furnace systems. They include:
Higher Yield. The absence of combustion sources reduces oxidation losses that can be significant in production economics.
Faster Startup. Full power from the power supply is available, instantaneously, thus reducing the time to reach working temperature. Cold charge-to-tap times of one to two hours are common.
Flexibility. No molten metal is necessary to start medium frequency coreless induction melting equipment. This facilitates repeated cold starting and frequent alloy changes.
Natural Stirring. Medium frequency units can give a strong stirring action resulting in a homogeneous melt.
Cleaner Melting. No by-products of combustion means a cleaner melting environment and no associated products of combustion pollution control systems.
Compact Installation. High melting rates can be obtained from small furnaces.
Reduced Refractory. The compact size in relation to melting rate means induction furnaces require much less refractory than fuel-fired units
Better Working Environment. Induction furnaces are much quieter than gas furnaces, arc furnaces, or cupolas. No combustion gas is present and waste heat is minimized.
Energy Conservation. Overall energy efficiency in induction melting ranges from 55 to 75 percent, and is significantly better than combustion processes.

Winding Resistance

Winding Resistance is an important measurement in electrical machines.  Winding resistance tells us about the condition of the winding.  Any fault in the winding such as an open circuit or an inter-turn short circuit will be reflected in the winding resistance value.  Besides, winding resistance is used to measure I²R losses in the winding.

The Winding Resistance is measured by winding measurement test kits.  In earlier times, winding resistance was measured using the Kelvin Bridge. The Kelvin Bridge is an arrangment of resistors which enables the measurement of very low resistances.  Winding Measurement Kits work by injecting known current through the winding and measuring the voltage drop across the winding.

The machine to be tested is disconnected from the lines and de-energized.  The measurement are usually taken phase-to-phase.  The three readings should be within 1% of the average value.

Winding resistance can change with temperature.  The measurement are usually taken at the cold temperature known as the cold resistance.  The transformer or the motor is allowed to cool for a few hours and the temperature taken.

Based on the measurement taken at a particular temperature, the resistance at any other temperature may be calculated from the following formula

Where

R_s= Resistance value to be calculated at a specific temperature

R_m= Resistance valued measured

T_m= Temperature at which the resistance was measured

T_s= Temperature at which the resistance is to be calculated

T_k= Winding Material Constant ( 234.5 °C for copper or 225 °C for aluminum)

The winding can store a huge amount of electromagnetic energy when a current is passed through them during measurement. When the test current is stopped, there may be a voltage kickback from the winding.  The test equipment should be able to absorb the voltage kick and safely discharge it.

Insulation Resistance Measurement

Insulation Resistance Measurement is an important check in the maintenance of electrical equipment such as motors, transformers. It is estimated that nearly 80% of all maintenance activities in the industry is related to checking the insulation of machines. It is therefore vital that the engineer has a fair idea of the principle behind the measurement of Insulation Resistance and the methods used. Insulation resistance is measured using a meggar.

In the normal operation of machinery, the insulation is subjected to moisture, oil, dust, electrostatic stress due to machine operation and a host of other elements. Hence, insulation ages and deteriorates. It is vital that the health of the insulation be monitored continually to avoid sudden, catastrophic failure of machines.

Principle of Insulation Resistance Measurement

The method used to measure insulation resistance is based on Ohm’s law. A high voltage is applied across the resistance; the current that flows through the insulation is measured. The ratio of voltage and current gives the resistance. The value of the insulation resistance is usually in the order of mega ohms

Instruments used in Measurement

The instrument used to measure Insulation Resistance is known as the Megger. It is similar in principle to the ohmmeter except for the fact that a higher voltage is used. The typical meggars have a test voltage of 500V, 2500V or 5000V. The Meggar has a high internal resistance hence, there it is safe to use despite the high voltage generated. The meggar has 3 terminals. Line, Earth and Guard. 

The test voltage appears on the “Line” Terminal. This terminal is connected to the winding whose insulation needs to be checked. The “Earth” Terminal is connected to the ground. The “Guard” Terminal is connected to the surface of the insulation to measure the surface currents which tends to flow along the surface of the insulation.

Method of Insulation Resistance Measurement

The winding to be tested should first be isolated. The other windings of the machine which are not being tested should be connected to the ground. The voltage is applied to the winding and the reading is taken after about 60 seconds. The reading is noted. After the test is over, the winding needs to be “discharged”. This is because the insulation acts as a dielectric forming a capacitor between the winding and the earth. This can store charge and can deliver a shock if not discharged. Discharging can be done by connecting to the ground.

What should be the value of the Insulation Resistance?

The Insulation Resistance thus measured is usually in the order of mega ohms. A general rule of the thumb is that the minimum value should be greater than 1 mega ohm for every 1kV rating of the machine. Thus, for a machine rated for 11kV, the minimum acceptable value would be 11 mega ohms. Temperature has a direct impact on the value of the Insulation Resistance. The Insulation Resistance decreases with increase in temperature. Thus the values should be normalized for a standard temperature.

That is, a value measured at 20 deg. C cannot be compared with a value measured at 30 deg. C. The value at 30 deg. C needs to be corrected. A general rule of thumb is that the insulation resistance decreases by a factor of 2 for every 10 degree rise in temperature.

Hence, the value taken at 30 deg. C needs to be multiplied by a 2 to get a value corrected to 20 degrees.

How do we ensure a good value of IR?

The Insulation Resistance of a machine depends chiefly on the dryness of the windings. The entry of moisture into the windings lowers the Insulation Resistance. The ingress of moisture can be prevented by ensuring that the windings are kept dry. Special heaters known as anti-condensation heaters are provided in machines to keep them dry. It must be ensured that these heaters are kept on.

How do we improve the Insulation Resistance value?

If machines are found with low Insulation Resistance values below the permissible limits, heating the windings by connecting lamps around them is an effective method of driving moisture from the windings. If no improvement is seen even after heating, other reasons such as insulation wear or deterioration can be suspected.

Other parameters related to the health of the Insulation are the Polarization Index(PI), tan delta, hipot test, step test, etc

Measurement of Earth Resistance

Measurement of Earth Resistance is a vital part of the maintenance of any electric installation. The function of a sound earthing system is to ensure that all electric equipment are connected to the ground potential. Hence, a well-maintained earthing system ensures the proper functioning of protection systems, absorbs electrical noise and provides safety to operating personnel. The earth resistance is measured using an earth meg-gar.

“Fall of Potential” Method:
The Earth resistance is measured using the “Fall of Potential” Method. The method works by injecting a constant current between two spikes which are inserted into the ground and measuring the voltage at points between them (as shown in the figure)

The “Fall of Potential” Method is a three terminal test. The electrode whose earth resistance is to be measured is disconnected from the system or earthing grid. The earth meg-gar has a current terminal, a voltage terminal and a common terminal. The common terminal is connected to the electrode,

how to Hi-pot Test

Hi-pot Test is a high voltage test that is used to check the integrity of insulation for high voltage equipment such as bus bars, cables, motors etc. The term 'Hi-pot' is the shortened form of High Potential. The Hi-pot test is used to ensure that an insulation can withstand a high potential without risk of failure.

However, the hi-pot test carries with it the potential failure of the insulation during the testing process itself. Weak insulation can fail during the test. Hence, many equipment owners avoid conducting this test. The hi-pot test certifies that the insulation is sufficient to withstand excess voltage during operation. This is significant in situations where the failure of a machine in service can cause serious damage or downtime as compared to a failure during the testing procedure.

The Hi-pot test is alternatively known as Dielectric Withstand test. The test involves the application of a high voltage usually about two times the
operating voltage. Thus a 6.6kV equipment will be tested at a voltage of 13kV.

The test is conducted for 1 minute or five minute. If the hi-pot test is conducted on a transformer winding or an alternator winding, the test is conducted on individual phases. The phases are separated and those phases which are not subjected to the hi-pot voltage are grounded.

Test Procedure
Prior to commencing the hi-pot test, it is necessary to get the Insulation Resistance and the Polarization Index values for the insulation. This ensures that the winding are free of any moisture or contamination. A wet or contaminated winding is more vulnerable to fail during the test.

The hi-pot test voltage is applied to the winding terminals to be tested. The voltage is sustained for one minute or five minute and then reduced. The current during the test period is also studied. Should there be a failure during the testing. There wile be a surge in the current which will cause the MCBs in the hi-pot test kit to trip.

There are two methods of raising the voltage to the value of the test voltage. They are

Step Test
In this method, the test voltage is raised gradually in small incremental steps. This enables the tester to abandon the test if he suspects that any current increase which may indicate a weak winding.

Ramp test
In this method, the test voltage is raised gradually or ramped up at a specific rate. The voltage can be increased to the rated voltage along with constant monitoring of the current. The ramp method is the most effective test as it can avoid any insulation failure during the test by identifying potential weaknesses in the winding early on.

The Hi-pot test does not offer scope for analysis such as the Insulation Resistance or the Polarization Test. It is simple a pass-fail kind of test. It is significant in that it gives operators the confidence that the equipment is strong enough to withstand the operating voltage and transient over voltages in the system.

The high voltage used during the test calls for high standards of safety. The area around the test location should be cleared of all items not related to the test(clutter). The area needs to be cordoned off to prevent the entry of unauthorized persons. Personnel should be stationed at the main power switch so that the switch can be turned at the first sign of any abnormality. The personnel conducting the test should be properly trained with awareness of emergency first-aid procedures in the event of an electric shock. The device which is being tested should be grounded after the test to discharge the capacitance.

How to Tan Delta Method

Tan Delta is a a diagnostic test conducted on the insulation of cables and winding. It is used to measure the deterioration in the cable. It also gives an idea of the aging process in the cable and enables us to predict the remaining life of the cable. It is alternatively known as the loss angle test or the dissipation factor test.

Principle

The Tan Delta test works on the principle that any insulation in its pure state acts as a capacitor. The test involves applying a very low frequency AC voltage. The voltage is generally double the rated voltage of the cable or winding.

A low frequency causes a higher value of capacities reactanc which leads to lesser power requirement during the test. Besides, the currents will be limited enabling easier measurement.

In a pure capacitor, the current is ahead of the voltage by 90 degrees. The insulation, in a pure condition, will behave similarly. However, if the insulation has deteriorated due to the entry of dirt and moisture. The current which flows through the insulation will also have a resistive component.

This will cause the angle of the current to be less than 90 degrees. This difference in the angle is known as the loss angle. The tangent of the angle which is Ir/Ic (opposite/adjacent) gives us an indication of the condition of the insulation. A higher value for the loss angle indicates a high degree of contamination of the insulation.

Method of Testing

The cable or winding whose insulation is to be tested is first disconnected and isolated. The test voltage is applied from the Very Low Frequency power source and the Tan delta controller takes the measurements. The test voltage is increased in steps upto the rated voltage of the cable. The readings are plotted in a graph against the applied voltage and the trend is studied. A healthy insulation would produce a straight line.

The test should be continued only if the graph is a straight line. A rising trend would indicate weak insulation which may fail if the test voltage is increased beyond the rated voltage of the cable.

Interpretation of the test data

There are not standard formulae or benchmarks to ascertain the success of a tan delta test. The health of the insulation which is measured is obtained by observing the nature of the trend which is plotted. A steady, straight trend would indicate a healthy insulation, while a rising trend would indicate an insulation that has been contaminated with water and other impurities.

How to Live Tank circuit breaker

Live Tank circuit breakers are circuit breakers in which the interrupting chamber is at the line potential. The interrupting chamber should therefore be provided with insulated supports. The center of gravity of these circuit breakers is higher, hence live tank circuit breakers need extra support for seismic capability (ability to withstand earthquakes)


In dead tank circuit breakers, the interrupting chamber is at ground potential.  The conductors enter the interrupting chamber through insulated bushings.  Maintenance activities are easier to conduct as the interrupting chamber is at ground level. Seismic capability is higher as the interrupting chambers are at ground level.  

How to Float & Boost Charger

Float charging is used where the battery rarely gets discharged.  A typical application where float charging can be used would consist of the float charger, battery and the load in parallel.  During normal operation, the load draws the power from the charger.  When the supply to the charger is interrupted, the battery steps in.

Float charging of a battery involves charging the battery at a reduced voltage.  This reduced voltage reduces the possibility of overcharging.The Float charger ensures that the battery is always in the charged condition and is therefore considered "floating".  The Float charger starts by applying a charging voltage to the battery.  As the battery gets charged, its charging current reduces gradually.  The float charger senses the reduction in charging current and reduces the charging voltage. 

If the battery gets drained, the float charger will again increase the charging voltage and process continues.  Float chargers can be connected indefinitely to the batteries.

Boost charging involves a high current for short period of time to charge the battery.  It is generally if the battery has been discharged heavily.  Boost charge enables the quick charging of depleted batteries.

For instance, a two volt lead acid battery which has been discharged will initially be boost charged with a charging voltage of around 2.35-2.4 volts.  However, as the battery voltage rises, the charger will switch over to the float charge mode with a float voltage of 2.25 volts.

Most battery chargers come equipped with provisions for both boost and float charging.

How to Voltage Supervision Relay

The Voltage Supervision Relay is an integral part of any  protection system.  The voltage supervision relay protections systems from under-voltage and over-voltage.  Over voltage in a system can result in serious damage to insulation and equipment while under-voltage can cause motors to draw more current and reduce the speed of the motors, disturbing the process.  


Besides protecting against over-voltage, the voltage supervision relay can also be used to detect earth faults as the phase to earth voltage is distorted when there is an earth fault in one of the phases.  Voltage supervision relays can generate alarms when the voltage is low or high in only one phase.  This is also known as phase asymmetry.  

In motor circuits, the voltage supervision relay protects against single phasing.  Single phasing can cause serious damage to motors.

A simple auxiliary relay can also be used to generate alarm for under-voltage.  When the voltage drops, the relay can drop off thus generating an alarm or a shutdown.

How to Trip circuit supervision

Trip circuit supervision in Circuit breakers is an vital part of any protection scheme. If the trip relay fails to operate, it may result in upstream tripping or even in damage to equipment.  Trip circuit supervision makes sure that the tripping coil of a circuit breaker is always in the healthy condition.  

The Trip circuit supervision is particularly important in breakers which have only one trip coil.    The Trip circuit supervision relay continually measures the resistance of the trip coil of circuit breakers.  It also measures the control voltage of the trip coil and gives and alarm when the control voltage falls to low levels.

The Trip circuit supervision relay injects a constant current through the trip coil of the breaker and measures the voltage drop across the coil.  Thus, the relay is able to measure the resistance of the coil.  

The Trip circuit supervision relays can also monitor more than one breaker coil.

If the Trip circuit supervision Relay detects a fault, it activates the breaker failure logic which can activate a backup breaker if installed or cause the tripping of upstream breakers.

Thermal Magnetic Trip (MCB)

Thermal Magnetic Trip is a method of over current protection which is widely used in LV switch gear such as MCBs and circuit breakers.  The Thermal Magnetic Trip works by sensing the current and tripping the breaker. 

The Thermal magnetic element, as the name suggests, has two units viz. the thermal unit and the magnetic unit.  The thermal unit is used to sense the current using the heating effect of current.

The Thermal unit consists of a bimetallic strip which bends due the heat produced by high current passing through it.  The bimetallic strip takes time to bend.  This enables the element to have a time delay feature.  The time delay depends on the magnitude of the over current protection. 

The Magnetic Trip element comes into play in case of severe faults.  When the current is extremely high, say 400 %, the current causes the magnetic trip element to attract a trip element which causes the unit to trip.

Thus, the thermal element is used to trigger a delayed response to minor overcurrents while the magnetic element is used to swiftly respond to high overcurrents

Transformer Magnetic Balance Test

The Magnetic Balance test is conducted on Transformers to identify inter turn faults and magnetic imbalance.  The magnetic balance test is usually done on the star side of a transformer.  A two phase supply 440V is applied across two phases, say, 1U and 1V.  The phase W is kept open.  The voltage is then measured
between U-V and U-W.  The sum of these two voltages should give the applied voltage.  That is, 1U1W + 1V1W will be equal to 1U1V.
For instance, if the voltage applied is 440V between 1U1V, then the voltages obtained can be

1U1V = 1U1W  + 1V1W
440V =  260V +  180V
The voltages obtained  in the secondary will also be proportional to the voltages above.
This indicates that the transformer is magnetically balanced.  If there is any inter-turn short circuit that may result in the sum of the two voltages not being equal to the applied voltage.
The Magnetic balance test is only an indicative test for the transformer. Its results are not absolute.  It needs to be used in conjunction with other tests.

How to Core Balance Transformer (CBCT)

The CBCT or the Core Balance Current Transformer is a current transformer is used for earth fault protection in grounded three phase systems. It is also known as the zero-sequence current transformer. The CBCT is a current transformer through all the three phases are made to pass as in the diagram. Thus the magnetic fluxes caused by the three phase currents cancel each other. The net resultant flux being zero does not induce any current in the secondary of the transformer. Thus the secondary current of the core balance current transformer when all the three phases are healthy is zero.

When an earth fault occurs in one of the phases, the zero-sequence fault current which flows is not cancelled by the flux of the other two phases and hence induces a current in the secondary.

The core balance current transformer can be connected to an earth fault relay which can be used to generate the tripping signal.

What is Current Transformer

A current transformer (CT) is a type of transformer that is used to measure AC Current. It produces an alternating current (AC) in its secondary which is proportional to the AC current in its primary. Current transformers, together with voltage transformers (VTs) or potential transformers (PTs), which are designed for measurement, are known as an Instrument transformer.

The main tasks of instrument transformers are:

− To transform currents or voltages from a usually high value to a value easy to handle for relays and instruments.

− To insulate the metering circuit from the primary high voltage system.

− To provide possibilities of standardizing the instruments and relays to a few rated currents and voltages.

When the current to be measured is too high to measure directly or the system voltage of the circuit is too high, a current transformer can be used to provide an isolated lower current in its secondary which is proportional to the current in the primary circuit. The induced secondary current is then suitable for measuring instruments or processing in electronic equipment. Current transformers have very little effect on the primary circuit.

Current transformers are the current sensing units of the power system. The output of the current transformers are used in electronic equipment and are widely used for metering and protective relays in the electrical power industry.

What is a plug setting multiplier

Plug setting multiplier of relay is referred as ratio of fault current in the relay to its pick up current. Suppose we have connected on protection CT of ratio 200/1 A and current setting is 150%. Hence, pick up current of the relay is, 1 × 150 % = 1.5 A.

What is meant by IDMT relay

Definition: IDMT (relay) stands for Inverse Definite Minimum Time (relay). In IDMT relay, its operating is inversely proportional to fault current. and also a characteristic of minimum time after which this. relay definitely operates.

Vacuum Circuit Breaker Technology

• The continuing use of vacuum interrupter technology

1. Breaker Overview

The structure of an outdoor vacuum circuit breaker (VCB)
is shown in Fig.1. The three-phase ganged interrupter assembly
is mounted in a self-contained breaker module
containing the operating mechanism, auxiliary switch,
mechanical linkage, and vacuum interrupter (VI)
mounted in its own support insulator. The mechanical
linkage and supporting framework are isolated from
the high voltage interrupter by an insulated drive rod
and the interrupter support insulators.

• 2. Features of Vacuum Interrupters

2.1 The Property of Vacuum
The dielectric strength of a vacuum is superior to
other dielectric mediums such as air and SF6 gas as
shown in Fig. 2. Due to high dielectric strength of a
vacuum environment, it is possible to reduce the arcing
time of the interrupter as the dielectric capabilities of
a vacuum can better withstand the generated transient
recovery voltage (TRV). The net result of this is the the magnetic force generated by the current flow, this will
cause the arc to be forced from the point of origin across the
spiral petal to the outer diameter of the contact. Once the
arc is on the outer edge of the contact, the magnetic force
along the outer diameter will cause the arc to rotate around
the outside diameter of the contact until it is extinguished.
The AMF contact structure will generate an axial magnetic
field caused by the current flow through the contacts which
in turn will create a diffuse arc (low energy arc) over the
entire contact area dispersing the arc energy over the contact
surface area. The diameter of spiral contacts can be smaller
than that of the AMF contact structure for the same fault
current rating due to its ability to rapidly move and control
the arc energy. In comparison the AMF contacts are superior
for applications which require long arcing times and require
a high number of full fault interruptions.minimum arcing time of a 15 kV vacuum circuit breaker
is approximately 1 to 2 ms. The higher dielectric capability
of the vacuum will yield a lower arc voltage and therefore
reduce the amount of arc energy to be dissipated across the
contacts in comparison with other interrupting mediums.
This results in vacuum interrupters being able to break
large fault currents with a short interrupting time and with
minimal contact erosion. These properties allow vacuum
interrupters to have a very compact design.

• 2.2 Basic Structure of Vacuum Interrupters

The basic structure of a vacuum interrupter is shown in
Fig. 3. The main contacts are installed in a ceramic insulator
in which high vacuum approximately 10-5 Pa is maintained.
The movable terminal is connected to the breaker mechanism
through an insulated drive rod which will open and
close the interrupter contacts. A stainless steel bellows is
used to allow contact travel and still maintain the integrity of
the vacuum in the interrupter. The contact material strongly
affects the interrupting capability, service life, and reliability
of the vacuum interrupter. There are three types of contact
designs used in vacuum interrupters, and they are applied
based on their application as shown in Table 1. Generally,
in applications with fault currents below approximately 20
kA, a flat butt type of contact design is sufficient. In applications
where fault currents can exceed 20 kA, the design
limitation of the contact will manifest itself as a localized
high energy arc which results in large amounts of metal
vapor being placed in the contact zone and thereby limiting
the fault rating of the interrupter. In order to improve the
interrupter’s fault capacity either the spiral or axial magnetic
field (AMF) contact design can be applied. These contacts
are used in higher fault ratings as they are able to utilize the
force created by the magnetic fields associated with the fault
current. This enables the interrupter to effectively control
the generated arc and therefore create a more efficient interrupter
design. The spiral contacts will generate a magnetic
field caused by the current flow through the contact. Due to

• 2.3 Arc Control During Interruption

By using a high-speed video camera and a specially
adapted vacuum chamber, the motion of the arc movement
can be observed. Fig. 4 shows the movement of the
arc for a spiral contact configuration during fault current
interruption. At contact part plus 1.5 ms, the arc has been
established between the contacts. At contact part plus 2 ms
the arc is beginning to rotate around the outer edge of the
contacts as a result of the magnetic forces that are being
generated from the flow of the fault current through the
contacts. This movement of the arc around the outer diameter
of the contact surface provides the greatest amount of
surface for the arc to travel over thus providing a larger area
to distribute the heat generated from the arc. The net result
of this movement of the arc is less localized overheating of
the contact surfaces and therefore less metal vapor being
introduced into the area between the contact surfaces. This
greatly improves the interrupting capability of the interrupter
and also results in less erosion of the contact surfaces,
improving the life of the interrupter.
Fig. 5 shows the arcing that takes place in an AMF
contact configuration during fault interruption. At contact
part plus 1 ms, the arc is beginning to be dispersed as a
result of the axial magnetic fields generated by the contact
geometry. This dispersal will generate numerous low energy
arcs (macroscopically one diffused arc) which will spread
out across the entire contact surface by contact part plus 3
ms. As the contacts continue to separate, the density of the
diffuse arc will continue to decrease so that at contact part
plus 5 ms interruption will occur when the next current zero
is encountered. By dispersing the arc energy into numerous
low energy arcs spread across the entire contact surface,
the amount of contact erosion and thus metal vapor in the
contact area can be controlled. This will result in greater
interrupting ratings for the contacts and longer contact life.

• 2.4 Short Circuit Fault Interruption

Fig. 6 shows a typical oscillograph during short-circuit
current interruption. When the contacts separate during
fault interruption an arc appears between the contacts and
is maintained until the next current zero crossing is encountered.
At the same time, an arc voltage is present across the
contacts. Once the current flow is interrupted a TRV will
appear across the contacts. If at the time the TRV appears,
the dielectric withstand of the contact gap is greater than the
TRV value, the current will be successfully interrupted. If the
TRV is greater than the dielectric withstand of the contact
gap, the arc will reappear and will be successfully interrupted
at the next current zero crossing. This condition will occur
when the time to the first zero crossing event is less than
the designed minimum arcing time of the interrupter.

• 3. Design Test

The capability of a medium-voltage circuit breaker is
verified by design tests specified in the IEEE/ANSI standards,
C37 series. In the design tests, a series of capabilities
(dielectric, current carrying, short-circuit current interruption,
and other current switching) are tested to meet the
required ratings. The following items, based on the design
test requirements, need to be taken into consideration
when demonstrating the circuit breaker’s capability and
application.

• 3.1 Asymmetrical Short-Circuit Current

Interruption and the Effect of Various X/R
Ratios
3.1.1 The rated interrupting time of a circuit breaker is
defined as the maximum permissible time interval
between the energization of the trip circuit (at rated
control voltage), and the interruption of the current
in the main circuit in all three phases. The standards
allow for ½ cycle of relay time prior to the trip circuit
energizing. This ½ cycle of relaying time is not
allowed to be counted as part of the interrupting
time of the circuit breaker. A circuit breaker rated
for three cycles must completely interrupt all current
flowing in the main circuit by the time three cycles
of 60 Hz current has passed (50 ms) after the trip
circuit is energized. If the current continues to flow
beyond the 50 ms, the breaker cannot be rated as a
three-cycle breaker. A five-cycle breaker is one that
interrupts the current between three and five cycles
(50-83 ms) after the trip circuit is energized. If there
is any current flow after 83 ms, the breaker cannot be
rated as a five-cycle breaker. The interrupter must be
able to interrupt current flow on the first or second
current zero after contact part. See Fig. 7.

• 3.1.2. A typical short-circuit asymmetrical wave is shown in

Fig. 8. For a three-cycle circuit breaker, it is necessary
to complete the interruption where the interrupting
time is equal to or less than three cycles. Moreover,
the test procedure in the IEEE standards requires
demonstrating that an interrupter can reliably
interrupt current under the most severe switching
conditions with the maximum arcing energy (longest
arcing time and a major loop of asymmetrical current).
Symmetrical interruptions must be performed
to meet the ANSI/IEEE standards. The test duties
included in Table 1 of IEEE C37.09-1999 demonstrate
the interrupter’s capability to interrupt current
flow, both symmetrical and asymmetrical.

• 3.1.3 The current peak value and major loop duration

are controlled by the value of X/R. The X/R ratio
is the ratio of the main circuit’s inductance divided
by the resistance. The higher the ratio is, the slower
the decay of the dc component of the asymmetrical
fault current. Fig. 8 shows the decay characteristic
of an asymmetrical current. Notice the peak current
decreasing with time. A standard value of X/R is
given as 17 at 60 Hz in IEEE Std C37.09-1999.
However, the purchaser of the breaker needs to evaluate
his system and verify that the X/R of the system
is less than 17. When the circuit has an inherently
high X/R, the interrupting time must still meet
the interruption rating of the breaker (three or five
cycles); however, the arcing time will increase due to
a larger major current loop. The major loops will not
only have an increased time between current zeros, it
will also have a higher peak current due to a higher
X/R ratio. Very high X/R ratios may not even have
a current zero for the first couple of cycles. For example,
if you compared an X/R=30 with an X/R=17
(see Fig. 8), the arcing time increases by a factor of
1.1, and the current peak value increases by a factor
of 1.4.

• 3.1.4 Symmetrical current interruptions must also be performed

according to IEEE C37.09-1999, Table 1.
The symmetrical interruptions are required for design
testing and to verify that the contact serviceability
as stated in IEEE C37.04-1999 section 5.8.2.5 is
achieved. The standards require a minimum of 800
percent of the required asymmetrical interrupting
capability of the circuit breaker be accumulated on
a single interrupter. Contact serviceability is the
amount of contact erosion that the interrupter must
be able to withstand and still be able to perform a
successful interruption.

1. 3.1.5 There are several design features that cause a vacuum

interrupter to be superior to other types of interrupters
for this application. The AMF style of contact
inside the interrupter allows for a longer arcing time
due to the low current density of the diffused arc. This
reduced energy will keep the surface of the contacts
from overheating and thus reduce the possibility of
restrikes. The reduction of restrikes is due to reduced
hot spots and reduced metal vapor in the contact gap. For circuit breakers rated below 100 kV, in IEEE Std
C37.04-1999, the rated TRV is represented by a 1-cosine
wave, while on the other hand, in IEC 62271-100, 2003
the TRV is represented by an exponential-cosine wave as
shown in Table 2 and Fig. 9. These ratings are for shortcircuit
tests, duty 4 and duty 5 in IEEE C37.09 and for
terminal fault test, duty T100a, in IEC. It is worth noting
that in the IEEE standards the TRV specifications are
different for outdoor and indoor applications. The outdoor
specification is more severe than the indoor. Breakers tested
and rated for indoor use, may not be suitable for outdoor
applications. Also, the differences between the IEC and
the IEEE TRV requirements are significant enough that
breakers tested to the IEC standard may not pass testing
for IEEE. The steeper slope of the recovery voltage will
change the requirements of the contact design to achieve a
greater rate of dielectric recovery inside the interrupter. The
capability of the interrupter to successfully interrupt this
high rate of rise of recovery voltages (RRRV) is dependent
upon the internal geometry of the interrupter. The type of
contact and the surrounding dielectric fields produced by
the internal shield all have a large impact on the capability
of the interrupter. The AMF style of contact is well suited
for this purpose due to reduced hot spots and metal vapor
in the contact gap. The smooth contact surface allows for
a very rapid dielectric recovery of the gap which allows
the vacuum interrupter to withstand the TRV better than
other types of medium. The rapid recovery allows for small
contact gaps, which translates to a very short stroke of the
interrupter and operating mechanisms.

• 3.3 Continuous Current Carrying Capability of

Vacuum Interrupters
3.3.1 The rated continuous current of a breaker is determined
by the current carrying capability without
exceeding the temperature limitations as listed in
Table 2 of IEEE C37.04-1999. This temperature
limit takes into consideration the types of materials,
plating, and atmosphere that surround the point
where the temperature is being measured.The TRV characteristics depend on the system in which a
circuit breaker is installed. The TRV is controlled mainly by
the amount of inductance and/or capacitance on each side
of the breaker. However, to provide a general application
guide, the TRV values under most system circuit conditions
are specified in the following standards. The most common spot in a VCB where temperature
rise is a concern is the moving stem of the vacuum
interrupter. It is necessary to monitor the temperature
very closely at this critical connection point where the
current path must allow for the motion of conductors.
A typical temperature rise graph is shown in Fig. 10.
In a medium-voltage VCB, the thermal time constant
is usually small. If a VCB is required to carry overload
current for several hours, the temperature rise on a
VCB will approach the saturation values. This temperature
may be over the thermal limits of materials.
Whether or not the breaker will exceed any critical
temperature limits will need to be evaluated carefully.
The IEEE standards cover standard overload
requirements; refer to IEEE C37.10-1999 section
5.4.4. If the breaker will be required to exceed these
limits, the purchaser and manufacturer will need to
review the continuous current test data to verify the
breaker’s capability for higher overload ratings.temperature rise associated with the
higher resistance. The spiral or butt
type contact is generally better suited
for high continuous currents than the
AMF style of contact. Consideration
must also be given to the location
where the circuit breaker will be installed.
If the installation site has an
ambient temperature rating of 50°C,
as compared to the 40°C used in
the IEEE standards, then the total
temperature rise must be decreased accordingly (by
10°C in this case). The overall performance of the
interrupter needs to be considered to determine the
best contact design, material, and size for any given
application. To ensure proper application of the circuit
breaker, the purchaser and manufacturer should
review the test reports.

• 3.4 Dielectric Capabilities of Vacuum

Interrupters.
3.4.1 Vacuum interrupters are well suited to withstand
voltage surges due to the rapid recovery of the dielectric
strength of the contact gap caused by the high
level vacuum. Other types of interrupter medium
take much longer to recover due to the need to remove
hot gas (in SF6 for example) from the gap. The
smooth contact surfaces and surrounding geometry
(primarily the internal shield), as well as the high
level of vacuum, all contribute to the interrupter’s
voltage withstand capability.
3.4.2 Lightning Impulse Withstand Voltage (BIL)
Vacuum is very reliable during lightning impulse
withstand voltage. This is demonstrated by testing
performed per section 4.4. of IEEE C37.09-1999.
The flat design of contacts and the voltage profiling
of the surrounding components (for example,
the internal shield, bellows, moving contact stem,
etc.) make this a very impulse friendly interrupter.
Another factor that allows the small contact gap to
withstand voltage surges is the high level of vacuum
inside the interrupter. A high level vacuum eliminates
all foreign particles within the interrupter. This results
in having no impurities and a very low molecular
count across the contact gap. The absence of foreign
particles and molecules will not allow the voltage to
jump across the internal gap.
There are several ways of improving the external
dielectric capabilities of a vacuum interrupter. Many
manufacturers will use various types of minor modifications
to improve the dielectric capabilities, or
margins, within a particular interrupter design. Some
examples are the use of an externally contoured, or
shedded, porcelain body to increase the creepage
distance. The increase in creepage distance allows for
more margin in a high contamination environment 3.3.2 The size and design of the main contacts and conductors
contribute to the continuous current rating
of the interrupter. The larger the contact diameter,
the better the current carrying capability. The contact
design and material also play a part in the continuous
current rating. The different contact materials may
have different resistance values. A higher resistance
value may limit the amount of current due to the
Another example would be to use a heat shrink material
on the ends to increase creep and strike. The use
a potting material to completely cover the outside of
the interrupter to increase the dielectric strength has
also been used for increased margins or capabilities
of a given interrupter design.
Vacuum interrupters are ideal for use in low-, medium-,
and even the lower end of high-voltage applications. Vacuum
will eventually be used in high-voltage applications
as technology keeps improving the capability of vacuum
interrupters. The internal design of the vacuum interrupter,
especially the main contacts, greatly impacts the capability of
the interrupter. There are different types of contact designs
that will utilize different materials to achieve different ratings
and capabilities. The internal design of the interrupter
will depend on the rating of the circuit breaker where the
interrupter will be installed. Some of the items to consider
on the internal design are not only the contact shape but also
the internal shield shape, the moving stem shape, blending
radii, and even the mounting configuration of the vacuum
interrupter. Whether the breaker will be primarily used for
capacitor switching duty, transformer magnetizing, or just
fault interruptions, will affect the design. It is the responsibility
of the end user of the circuit breaker to verify that
the breaker (ultimately the interrupter) is suited to meet the
specific needs of the system. The ability of the circuit breaker
to function properly on a given system will ultimately come
down to the review of the test data to ensure that the breaker
has been tested and verified to function correctly under the
duties in which it will be asked to perform.



in the power distribution market as replacements
for oil circuit breakers has exposed this technology
to people who have not been previously exposed to it. This
article covers the features of the vacuum interrupter and how
they correspond to the ability of the interrupter to perform
according to its ratings.
Vacuum interruption technology has been used for
many years and has proven itself to be a reliable means for
interrupting fault currents in distribution switchgear. The
application of vacuum circuit breakers as replacements for
oil circuit breakers offers many advantages to the user in
areas such as maintenance, performance, and environmental
concerns. From a maintenance point of view, with a vacuum
breaker there is no oil handling required with the associated
clean up as well as long contact life and lower mechanism
operating forces due to the small size and stroke required
by a vacuum interrupter. A typical vacuum interrupter as
applied in a distribution breaker will easily handle 10,000
operations at rated continuous current and more than 20 full
fault operations without the need for contact replacement.
Also, as a result of the lower mechanical requirements the
mechanisms in today’s vacuum breakers are much simpler
in design with fewer moving parts and lighter loads being
applied.