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1.1. Low voltage circuit-breaker standards 6 1.2.United Kingdom Standards 7 2. Types of Circuit-Breaker 9 2.1. Miniature circuit-breakers 9 2.2.Moulded Case circuit-breakers 9 2.3. Air circuit-breakers 10 3. Low voltage circuit-breakers for use in household and similar installations11 3.1 History of the development of BS EN 60898 11
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- Choice of A Circuit-Breaker
- Choice of Rated Current in Terms of Ambient Temperature
- Uncompensated Thermal Magnetic Tripping Units
- Compensated Thermal-Magnetic Tripping Units
- Electronic Trip Units
- Selection of An Instantaneous, Or Short-Time-Delay, Tripping Threshold
- Selection of A Circuit Breaker According to The Presumed Short-Circuit Current
- Circuit Breakers Suitable For It Systems
- Selection of Circuit Breakers as Main Incomer and Feeders
- Selection of Feeder CBS and Final-Circuit CBS
The choice of a CB is made in terms of: 1. Electrical characteristics (AC or DC, Voltage...) of the installation for which the CB is intended 2. Its environment: ambient temperature, in a kiosk or switchboard enclosure, climatic conditions, etc. 3. Presumed short-circuit current at the point of installation 4. Characteristics of the protected cable...
The rated current of a circuit breaker is defined for operation at a given ambient temperature, in general: 1. 30°C for domestic-type CBs according to IEC 60898 series 2. 40°C by default for industrial-type CBs, according to IEC 60947 series. Different value may be proposed however. Performance of these CBs in a different ambient temperature depend...
Circuit-breakers with uncompensated thermal tripping elements have a tripping-current level that depends on the surrounding temperature. If the CB is installed in an enclosure, or in a hot location (boiler room, etc.), the current required to trip the CB on overload will be sensibly reduced. When the temperature in which the CB is located exceeds i...
These tripping units include a bi-metal compensating strip which allows the overload trip-current setting (Ir or Irth) to be adjusted, within a specified range, irrespective of the ambient temperature. For example: 1. In certain countries, the TT system is standard on LV distribution systems, and domestic (and similar) installations are protected a...
An important advantage with electronic tripping units is their stable performance in changing temperature conditions. However, the switchgear itself often imposes operational limits in elevated temperatures, so that manufacturers generally provide an operating chart relating the maximum values of permissible trip-current levels to the ambient tempe...
Figure H41below summarizes the main characteristics of the instantaneous or short-time delay trip units.
The installation of a circuit-breaker in a LV installation must fulfil one of the two following conditions: 1. Either have a rated short-circuit breaking capacity Icu (or Icn) which is equal to or exceeds the prospective short-circuit current calculated for its point of installation, or 2. If this is not the case, be associated with another device ...
In IT system, circuit breakers may have to face an unusual situation called double earth fault when a second fault to earth occurs in presence of a first fault on the opposite side of a circuit breaker (see Figure H42). In that case circuit breaker has to clear the fault with phase to phase voltage across a single pole instead of phase to neutral v...
Installation supplied by one transformer
If the transformer is located in a consumer’s substation, certain national standards require a LV circuit breaker in which the open contacts are clearly visible such aswithdrawable circuit breaker.
Example
(see Fig. H43) What type of circuit-breaker is suitable for the main circuit-breaker of an installation supplied through a 250 kVA MV/LV (400 V) 3-phase transformer in a consumer’s substation? In transformer = 360 A Isc (3-phase) = 9 kA A Compact NSX400N with an adjustable tripping-unit range of 160 A - 400 A and a short-circuit breaking capacity (Icu) of 50 kA would be a suitable choice for this duty.
Installation supplied by several transformers in parallel
(see Fig. H44) 1. The feeder circuit breakers CBP must each be capable of breaking the total fault current from all transformers connected to the busbars: Isc1 + Isc2 + Isc3 2. The main incomer circuit breakers CBM, must be capable of dealing with a maximum short-circuit current of (for example) Isc2 + Isc3 only, for a short-circuit located on the upstream side of CBM1. From these considerations, it will be seen that the circuit-breaker of the smallest transformer will be subjected to the hig...
Use of Table G42
From this table, the value of 3-phase short-circuit current can be determined rapidly for any point in the installation, knowing: 1. The value of short-circuit current at a point upstream of that intended for the CB concerned 2. The length, c.s.a., and the composition of the conductors between the two points A circuit breaker rated for a short-circuit breaking capacity exceeding the tabulated value may then be selected.
Detailed calculation of the short-circuit current level
In order to calculate more precisely the short-circuit current, notably, when the short-circuit current-breaking capacity of a CB is slightly less than that derived from the table, it is necessary to use the method indicated in Short-circuit current.
Two-pole circuit breakers (for phase and neutral) with one protected pole only
These CBs are generally provided with an overcurrent protective device on the phase pole only, and may be used in TT, TN-S and IT schemes. In an IT scheme, however, the following conditions must be respected: 1. Condition (B) of table in Figure G68for the protection of the neutral conductor against overcurrent in the case of a double fault 2. Short-circuit current-breaking rating: a 2-pole phase-neutral CB must be capable of breaking on one pole (at the phase-to-phase voltage) the current of...
Mar 25, 2021 · 1- High voltage circuit breaker n ormal and special service conditions. 2- High voltage circuit breaker terms and definitions. 3-High voltage circuit breaker rating. 4-High voltage circuit breaker d esign and construction. 5-High voltage circuit breaker type test. 6-High voltage circuit breaker routine test.
Co-ordination under short-circuit conditions between a circuit-breaker and another short-circuit protective device associated in the same circuit This annex gives guidance on and requirements for the co-ordination of circuit-breakers with other SCPDs (short-circuit protective devices) associated in the same circuit, as regards selectivity as well as back-up protection.
1.5. This standard covers circuit-breaker enclosures and accessory devices intended for use with the devices described in 1.1 and 1.2. 1.6. This standard does not cover low-voltage power circuit breakers covered in Annex B, Ref. No. 3 and Ref. No. 4 or supplementary protectors covered in Annex B, Ref. No. 5. 1.7.
Nov 14, 2022 · A self-contained portion of a circuit breaker that is interchangeable and replaceable in a circuit breaker frame by the user. It actuates the circuit breaker release mechanism and it sets the RATED CURRENT (In) of the circuit breaker unless a rating plug is used (to see also definition 32 RATING PLUG).
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The latest Edition includes a new annex covering separate "modular residual current devices" (MRCDs) which provide an output to operate the tripping function of a separate circuit-breaker. Under these conditions it is important that the MRCD provides an output that is compatible with the circuit breaker's tripping mechanism and that the overall system is itself self-protecting.
