Terminology used for C.B

LOW VOLTAGE SWITCHGEAR - TERMINOLOGY DEMYSTIFIED

by: K.Sivakumar, Manager-Training, Larsen & Toubro, Switchgear Training Centre, Coonoor (T.N.)

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Introduction: About 70% of the electrical engineers working in industry happen to work mainly on the Low Voltage Switchgear. But, unfortunately, this subject is not paid enough attention in our electrical engineering academic curriculum. Hence, when the field electrical engineer goes through the manufacturers' catalogues/data sheets of low voltage switchgear, they are bemused. The switchgear specification terminology appears a big jargon for them. An attempt is made in this article to demystify the switchgear jargon.

Every type of switchgear will have certain set of characteristic quantities associated with it. These characteristic quantities are normally stated in form of ratings or specifications in the product catalogue supplied by the manufacturer. A few of the specifications are also marked on the nameplate of the equipment. For a user of switchgear, it is important to know about what these characteristic quantities mean. Following is the brief description for commonly used characteristic qualities.

Voltages: An equipment is defined by the following voltages:

Rated Operational Voltage (U_e): It is that value of voltage, which combined with a rated operational current, determines the application of the equipment and to which the relevant tests and the utilization categories are referred. For single pole equipment, the rated operational voltage is generally stated as the voltage across the pole. For multiple pole equipment, it is generally stated as the voltage between phases.

Rated Insulation Voltage (U_i): It is the value of voltage to which dielectric test voltage and creepage distances are referred. In no case shall the maximum the value of U_e should be more than U_i.

Rated Impulse Withstand Voltage (U_imp): It is the peak value of an impulse voltage of prescribed form & polarity, which the equipment is capable of withstanding without failure under specified conditions of test & to which the values of the clearances are referred. The rated impulse withstand voltage of an equipment shall be equal to or higher than the values stated for the transient over voltages occurring in the circuit in which the equipment is fitted.

Table 1: Preferred values of U_imp for a nominal supply system voltage of 240/415V, in case of over voltage protection by surge arrestors according to IEC:

Rated Control Circuit Voltage (U_c): It is the voltage, which would appear across the "a" contacts in the control circuit.

Rated Control Supply Voltage (U_s): It is the voltage at which generally auxiliary circuits and more precisely the control circuits are operated or to which they must be connected for correct operation. A distinction is made between U_c & U_s. U_s may be different from U_c, due to presence of built in transformers, rectifiers, resisters etc. The rated control circuit voltage and the rated frequency, if any, are the values on which the operating and temperature rise characteristics of the control circuit are based. The correct operating conditions are based upon a value of the control supply voltage not less than 85% of its rated value, with the highest value of control circuit current flowing, nor more than 110% of its rated value.

Currents: An equipment is defined by the following currents:

Conventional free air thermal current (I_th): It is the maximum value of current as stated by the manufacturer to be used for temperature-rise tests of unenclosed equipment in free air. The switching device must be capable of conducting this test current without the temperature rise limits given in the relevant Indian Standard being exceeded (temperature rise limits of terminals as per IS is 70^oC for silver plated copper for all switchgear (except circuit breaker). It must be at least equal to the maximum value of the rated operational current of the unenclosed equipment in eight-hour duty. It is not a rating & is not mandatorily marked on the equipment.

Conventional enclosed thermal current (I_the): It is the value of current stated by the manufacturer to be used for the temperature-rise tests of the equipment when mounted in a specified enclosure. Such tests are mandatory if the equipment is described as enclosed equipment in the manufacturer's catalogues & normally intended for use with one or more enclosure of specified type & size. It shall be at least equal to the maximum value of the rated operational current of the enclosed equipment in eight-hour duty. If the equipment is normally intended for use in unspecified enclosures, the test is not mandatory, if the test for I_th has been made. It is not a rating & is not mandatorily marked on the equipment.

Rated Operational Current (I_e): It is stated by the manufacturer and takes into account the rated operational voltage, the rated frequency, the rated duty, the utilization category & the type of protective enclosure, if applicable. Switchgear may have several rated operational currents. In the case of equipment for direct switching of individual motors, the indication of a rated operational current may be replaced or supplemented by an indication of the maximum rated power output, at the rated operational voltage considered, of the motor for which the equipment is intended.

Rated uninterrupted current (I_u): It is a value of current, stated by the manufacturer, which the equipment can carry in uninterrupted duty (a duty in which the main contacts of equipment remain closed, whilst carrying a steady current without interruption for periods of more than eight hours i.e. weeks, months or even years).

Rated current (I_n): This rating is defined for circuit breaker & fuses. For circuit breaker, the rated current is the rated uninterrupted current (I_u) & is equal to the conventional free-air thermal current (I_th). For fuse-link, the rated current is that value of current, which it can carry continuously without deteriorating, under specified condition.

Rated Frequency: It is the supply frequency for which equipment is designed & to which the other characteristic values correspond. The same equipment may be assigned a number or a range of rated frequencies or be rated for both A.C. and D.C.

Utilization Category: It is a combination of specified requirements related to the conditions in which the switching device or the fuse fulfills its purpose, selected to represent a characteristic group of practical applications. The specified requirements may concern, for example, the values of making capacities (if applicable), breaking capacities & other characteristics, the associated circuits & the relevant conditions of use and behavior.

Normal Load & Overload Characteristics: The ability to make & break currents under normal load & overload conditions is stated in terms of following two ratings:

Rated Breaking Capacity: The rated breaking capacity of an equipment is a value of current, stated by the manufacturer, which the equipment can satisfactorily break under specified breaking conditions. The breaking conditions which shall be specified are: a) the phase-to-phase voltage across the supply terminals of the equipment after the breaking of the current & b) the characteristics of the test circuit. The rated breaking capacity is stated by reference to rated operational current, according to relevant Indian Standard (IS) on the product. An equipment shall be capable of breaking any value of current up to and including its rated breaking capacity. For A.C., it is expressed by the r.m.s. value of the symmetrical component of the current.

Rating Making Capacity: The rated making capacity of an equipment is a value of current, stated by the manufacturer, which the equipment can satisfactorily make under specified making conditions. The making conditions which shall be specified are: a) the phase-to-phase voltage across the supply terminals of the equipment just before the making of the current & b) the characteristics of test circuit. The rated making capacity is stated by reference to rated operational voltage & rated operational current, according to relevant Indian standard (IS) on the product. An equipment shall be capable of making any value of current up to and including its rated making capacity. For A.C., it is expressed by the r.m.s. value of the symmetrical component of the current.

Short Circuit Characteristics: Short circuit currents may be encountered during current making, during current carrying in closed position & during current interruption. The ability of the equipment to make, carry & break short circuit currents is stated in terms of one or more of the following ratings:

Rated Short-Circuit Making Capacity (I_cm): The rated short-circuit making capacity of an equipment is the maximum value of short circuit current, stated by the manufacturer, which the equipment can satisfactorily make at the rated operational voltage, at rated frequency and at a specified power factor of A.C. It is expressed as the maximum prospective peak current, under prescribed condition.

Rated Short Time Withstand Current (I_cw): It is the current that an equipment can carry without damage during specified short time under prescribed conditions of use and behavior.

Rated Short-Circuit Breaking Capacity (I_cn): The rated short-circuit breaking capacity of an equipment is the maximum value of short circuit current, stated by the manufacturer, which the equipment can satisfactorily break at the rated operational voltage, at rated frequency and at a specified power factor of A.C. It is expressed as the r.m.s. value of the a.c. component, under prescribed condition.

Rated ultimate short circuit breaking capacity (I_cu): It is the value of ultimate short circuit breaking capacity (a breaking capacity for which the prescribed conditions according to a specified test sequence do not include the capability of the circuit breaker to carry its rated current continuously) assigned to that circuit breaker by the manufacturer for the corresponding rated operational voltage, under specified conditions. It is expressed as the value of the prospective breaking current, in kA (r.m.s.).

Rated service short circuit breaking capacity (I_cn): It is the value of service short circuit breaking capacity (a breaking capacity for which the prescribed conditions according to a specified test sequence include the capability of the circuit breaker to carry its rated current continuously) assigned to that circuit breaker by the manufacturer for the corresponding rated operational voltage, under specified conditions. It is expressed as the value of the prospective breaking current, in kA (r.m.s.), corresponding to one of the specified percentages of the rated ultimate short circuit breaking capacity.

Table 3: Standard ratios between I_cu & I_cs

Time Intervals: Closing & opening operations of a switching device may be defined by following times:

Opening Time: It is the interval of time between the specified instant of initiation of the opening operation & the instant when the arcing contacts have separated in all poles. The instant of initiation of the opening operation means the instant of the application of the opening command, e.g. energizing the release etc. In case of a directly operated circuit breaker, the instant of initiation of the opening time is the instant of initiation of a current large enough to cause the circuit breaker to operate. In the case of a circuit breaker operated by any form of auxiliary power, the instant of initiation of the opening time is the instant of application of the auxiliary power to the opening release. For circuit breakers opening time is commonly referred to as tripping time, although, strictly speaking, tripping time applies to the time between the instant of initiation of the opening time and the instant when the opening command becomes irreversible.

Arcing Time: It is the interval of time between the instant of initiation of the arc in a pole or a fuse & the instant of final arc extinction in that pole or that fuse. In a multiple-pole switching device, it will be the interval of time between the instant of the first initiation of an arc & the instant of final arc extinction in all poles.

Break Time: It is the interval of time between the beginning of the opening time of a mechanical switching device (or the pre-arcing time of a fuse) & the end of the arcing time.

Make Time: It is the interval of time between the initiation of the closing operation and the instant when the current begins to flow in the main circuit.

Closing Time: It is the interval of time between the initiation of the closing operation and the instant when the contacts touch in all poles.

Pre-arcing Time: It is the time between the commencement of a current large enough to cause the fuse element(s) to melt & the instant when an arc is initiated.

Operating Time: It is the sum of pre-arcing & the arcing time.

Operational Life: It is the duration for which electrical equipment operates reliably (or is expected to operate reliably) under normal specified operating conditions. The operational life of a switching device is expressed as the number of make-break operations the device will perform.

Electrical Life: It is the number of on-load switching operations, which the device can perform without any repair or replacements. The electrical life depends on the utilization category, the rated operational current & the rated operational voltage.

Mechanical Life: It is the number of no-load switching operations (i.e. with no current flowing through the poles) which the switching device can perform without mechanical failure.

Service Conditions: The switchgear manufacturer claims the ratings after testing as per the procedures specified in the standards. The standards also specify the service conditions under which the tests are to be carried out. Following are the service conditions considered:

Ambient air temperature: It is the temperature existing in the vicinity of the equipment if supplied without enclosure or in vicinity of the enclosure if supplied with an enclosure.

Altitude: It is the height above the sea level where the equipment is going to be installed.

Humidity: It is the moisture content in air expressed in percentage (i.e. relative humidity).

Pollution degree: The pollution degree refers to the environmental conditions for which the equipment is intended. For equipment intended for use within an enclosure or provided with an integral enclosure, the pollution degree of the environment in the enclosure is applicable. Unless otherwise stated by the relevant product standard, equipment for industrial application is generally for use in pollution degree 3 environment according to which conductive pollution occurs, or dry non-conductive pollution occurs which becomes conductive due to condensation.

Table 4: Normal service conditions in terms of Indian Standards.

Switchgears intended to be used in conditions other than normal service conditions, should be designed accordingly or used according to the relevant product standard, where applicable, or according to agreement between manufacturer & user. Many times certain correction factors are employed in case actual service conditions differ from normal.

Conclusion: It is hoped that at least some of the switchgear jargons are clarified above. Further reference can be made to IS 13947 (Specification for Low Voltage Switchgear & Controlgear) & IS 13703 (Specification for Low Voltage Fuses).

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