Short-circuit The accidental or intentional connection, by a relatively low resistance or impedance, of two or more points in a circuit which are normally at different voltages. Rated performances Voltages and frequencies Rated operational voltage Ue A rated operational voltage of an equipment is a value of voltage which, combi- ned with a rated operational current, determines the application of the equipment and to which the relevant tests and the utilization categories are referred to.
Rated insulation voltage Ui The rated insulation voltage of an equipment is the value of voltage to which dielectric tests voltage and creepage distances are referred. In no case the maximum value of the rated operational voltage shall exceed that of the rated insulation voltage. Rated impulse withstand voltage Uimp The peak value of an impulse voltage of prescribed form and polarity which the equipment is capable of withstanding without failure under specified conditions of test and to which the values of the clearances are referred.
Rated frequency The supply frequency for which an equipment is designed and to which the other characteristic values correspond. Currents Rated uninterrupted current In The rated uninterrupted current for a circuit-breaker is a value of current, that the circuit-breaker can carry during uninterrupted service.
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Performances under short-circuit conditions 1. ABB Protection and control devices 33 1 Protection and control devices Rated 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. Rated breaking capacity The rated breaking of an equipment is a value of current, stated by the manu- facturer, which the equipment can satisfactorily break, under specified breaking conditions.
Rated ultimate short-circuit breaking capacity Icu The rated ultimate short-circuit breaking capacity of a circuit-breaker is the maximum short-circuit current value which the circuit-breaker can break twice in accordance with the sequence O — t — CO , at the corresponding rated operational voltage. After the opening and closing sequence the circuit-breaker is not required to carry its rated current. Rated service short-circuit breaking capacity Ics The rated service short-circuit breaking capacity of a circuit-breaker is the maximum short-circuit current value which the circuit-breaker can break th- ree times in accordance with a sequence of opening and closing operations O - t - CO - t — CO at a defined rated operational voltage Ue and at a defined power factor.
After this sequence the circuit-breaker is required to carry its rated current.
Rated short-time withstand current Icw The rated short-time withstand current is the current that the circuit-breaker in the closed position can carry during a specified short time under prescribed conditions of use and behaviour; the circuit-breaker shall be able to carry this current during the associated short-time delay in order to ensure discrimination between the circuit-breakers in series. Rated short-circuit making capacity Icm The rated short-circuit making capacity of an equipment is the value of short- circuit making capacity assigned to that equipment by the manufacturer for the rated operational voltage, at rated frequency, and at a specified power-factor for ac.
Category A - Circuit-breakers not specifically intended for selectivity under short-circuit conditions with respect to other short-circuit protective devices in series on the load side, i. Category B - Circuit-breakers specifically intended for selectivity under short- circuit conditions with respect to other short-circuit protective devices in series on the load side, i. Such circuit-breakers have a short-time withstand current rating.
Electrical durability The electrical durability of an apparatus is expressed by the number of on-load operating cycles and gives the contact resistance to electrical wear under the service conditions stated in the relevant product Standard. ABB Protection and control devices 35 1 Protection and control devices 1. A circuit-breaker must control and protect, in case of faults or malfunctioning, the connected elements of a plant.
In order to perform this function, after detection of an anomalous condition, the release intervenes in a definite time by opening the interrupting part.
The protection releases fitted with ABB SACE moulded-case and air circuit- breakers can control and protect any plant, from the simplest ones to those with particular requirements, thanks to their wide setting possibilities of both thresholds and tripping times. The choice and adjusting of protection releases are based both on the requi- rements of the part of plant to be protected, as well as on the coordination with other devices; in general, discriminating factors for the selection are the required threshold, time and curve characteristic.
The signal coming from the transformers and from the Rogowsky coils is processed by the electronic component microprocessor which compares it with the set thresholds. When the signal exceeds the thresholds, the trip of the circuit-breaker is operated through an opening solenoid which directly acts on the circuit-breaker operating mechanism. ABB Protection and control devices 41 1 Protection and control devices 1. L - Overload protection in compliance with Std. IEC Function of protection against overloads with inverse long time delay and trip curves complying with IEC ; applicable in the coordination with fuses and with medium voltage protections.
S - Short-circuit protection with adjustable delay Function of protection against short-circuit currents with adjustable delay; thanks to the adjustable delay, this protection is particularly useful when it is necessary to obtain selective coordination between different devices. S2 - Double S This function allows two thresholds of protection function S to be set indepen- dently and activated simultaneously, selectivity can also be achieved under highly critical conditions.
D - Directional short-circuit protection with adjustable delay The directional protection, which is similar to function S, can intervene in a different way according to the direction of the short-circuit current; particularly suitable in meshed networks or with multiple supply lines in parallel. I - Short-circuit protection with instantaneous trip Function for the instantaneous protection against short-circuit. EFDP - Early Fault Detection and Prevention Thanks to this function, the release is able to isolate a fault in shorter times than the zone selectivities currently available on the market.
Rc - Residual current protection This function is particularly suitable where low-sensitivity residual current protection is required and for high-sensitivity applications to protect people against indirect contact. G - Earth fault protection with adjustable delay Function protecting the plant against earth faults. U - Phase unbalance protection Protection function which intervenes when an excessive unbalance between the currents of the single phases protected by the circuit-breaker is detected. OT - Self-protection against overtemperature Protection function controlling the opening of the circuit-breaker when the temperature inside the release can jeopardize its functioning.
UV - Undervoltage protection Protection function which intervenes when the phase voltage drops below the preset threshold. OV - Overvoltage protection Protection function which intervenes when the phase voltage exceeds the preset threshold. RV - Residual voltage protection Protection which identifies anomalous voltages on the neutral conductor. RP - Reverse power protection Protection which intervenes when the direction of the active power is oppo- 1. UF - Under frequency protection This frequency protection detects the reduction of network frequency above the adjustable threshold, generating an alarm or opening the circuit.
OF - Overfrequency protection This frequency protection detects the increase of network frequency above the adjustable threshold, generating an alarm or opening the circuit. M - Thermal memory Thanks to this function, it is possible to take into account the heating of a component so that the tripping is the quicker the less time has elapsed since the last one.
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R - Protection against rotor blockage Function intervening as soon as conditions are detected, which could lead to the block of the rotor of the protected motor during operation. Iinst - Very fast instantaneous protection against short-circuit This particular protection function has the aim of maintaining the integrity of the circuit-breaker and of the plant in case of high currents requiring delays lower than those guaranteed by the protection against instantaneous short-circuit.
Dual setting With this function it is possible to program two different sets of parameters LSIG and, through an external command, to switch from one set to the other. As we can see in the picture the protection conductor or the equipotential conductor have to be installed outside the eventual external toroid. Besides, they can guarantee an additional protection against the risk of fire deriving from the evolution of small fault or leakage currents which are not detected by the standard protections against overload.
Residual current devices having a rated residual current not exceeding 30 mA are also used as a means for additional protection against direct contact in case of failure of the relevant protective means. Their logic is based on the detection of the vectorial sum of the line currents through an internal or external toroid. The operating principle of the residual current release makes it suitable for the distribution systems TT, IT even if paying particular attention to the latter and TN-S, but not in the systems TN-C.
In fact, in these systems, the neutral is used also as protective conductor and therefore the detection of the residual current would not be possible if the neutral passes through the toroid, since the vectorial sum of the currents would always be equal to zero.
ABB Protection and control devices 45 1 Protection and control devices In presence of electrical apparatuses with electronic components computers, photocopiers, fax etc. In these cases it is necessary to use a residual current release classified as type A. In presence of rectifying circuits i. In this case it is necessary to use a residual current release classified as type B. This represents the sensitivity of the release.
According to their sensitivity to the fault current, the residual current circuit- breakers are classified as: - type AC: a residual current device for which tripping is ensured in case of residual sinusoidal alternating current, in the absence of a dc component whether suddenly applied or slowly rising; - type A: a residual current device for which tripping is ensured for residual sinusoidal alternating currents in the presence of specified residual pulsating direct currents, whether suddenly applied or slowly rising.
Such residual current relays are of the type with indirect action: the opening command given by the relay must cause the tripping of the circuit-breaker through a shunt opening release to be provided by the user. ABB Protection and control devices 51 2 General characteristics 2.
Operations] [No. Hourly Operations] 2. Hourly operations ] Tmax T moulded-case circuit-breakers ABB Protection and control devices 55 2 General characteristics 2. ABB Protection and control devices 57 2 General characteristics 25 25 25 25 60 60 60 60 15 15 12 10 15 15 10 8 30 30 30 30 E2 B-N-S 20 20 60 60 4 3 3 2 20 20 12 12 12 12 60 60 60 60 5 4 3 2 5 4 2 1. ABB Protection and control devices 59 2 General characteristics 2. The algorithms for the verifica- tion of the cable protection are described in the international standards.
The protection function against short-circuit is represented in the time-current curve by a vertical line, corresponding to the rated value of the trip threshold I3. In accordance with the Standard IEC , the real value of this threshold is within the range 0. The trip time of this protection varies according to the electrical characteristics of the fault and the presence of other devices: it is not possible to represent the envelope of all the possible situations in a sufficiently clear way in this curve; therefore it is better to use a single straight line, parallel to the current axis.
All the information relevant to this trip area and useful for the sizing and coor- dination of the plant are represented in the limitation curve and in the curves for the specific let-through energy of the circuit-breaker under short-circuit conditions.
The following pages show some examples reporting the settings of thermo- magnetic releases. To simplify the reading of these examples, the tolerance of the protection func- tions has not been considered. For a proper setting it is necessary to consider the tolerances referred to the type of thermomagnetic release used; for these information please refer to the technical catalogues.
ABB Protection and control devices 63 2 General characteristics Short-circuit instantaneous protection I To set the magnetic function of the release is necessary to know the minimum value of the short-circuit current that we can have in the plant. Note that, according to the conditions under which the overload occurs, that is either with the circuit-breaker at full working temperature or not, the trip of the thermal release varies considerably. For example, for an overload current of A, the trip time is between 6 and 30 s for hot trip, and between 30 and s for cold trip.
Return to Book Page. Seip Contributor. The Third Edition of this classic reference is designed to provide authoritative guidance for engineers and technicians who have responsibility for planning, designing, building and operating electrical installation systems.
Handbook of Electrical Installation Practice
The extensively revised scope includes a comprehensive overview of conventional and state-of-the-art installation equipment and its current usage. S The Third Edition of this classic reference is designed to provide authoritative guidance for engineers and technicians who have responsibility for planning, designing, building and operating electrical installation systems. Special emphasis is placed on equipment with communication capability and the way in which this equipment is networked to the instabus EIB bus system for a wide range of applications in residential and commercial buildings.
The construction, dimensioning and protection of electrical distribution systems are treated taking into account the latest developments in systems engineering. In view of the electricity market deregulation and globalization and the associated standardization initiatives that are underway, reference has been made, where appropriate, to international, European and German norms, regulations and standards.
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