BỘ LUYỆN NGHE TIẾNG ANH 8 GLOBAL SUCCESS CẢ NĂM (GỒM 12 UNITS, MỖI UNIT GỒM 3...
17th Edition Part 4 4
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8. Required minimum impulse withstand voltage Refer to Table 44.4 for examples of various impulse categories of equipment Nominal voltage of the installation V Required minimum impulse withstand voltage kV Category IV (equipment with very high impulse voltage) Category III (equipment with high impulse voltage) Category II (equipment with normal impulse voltage) Category I (equipment with reduced impulse voltage) 230/240 or 277/480 6 4 2.5 1.5 400/690 8 6 4 2.5 1000 Values to be determined by the system engineer or, in the absence of information, the values for 400/690 V can be chosen.
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10. 444 – Measures against electromagnetic influences Reserved for future use
Section 442 includes some difficult concepts for many students and covers requirements/situations many students are unlikely to get involved with. Section 442 gives rules for the designer and installer of the substation. It is suggested, therefore, that the best approach to this subject matter is to provide an overview of the general requirements It would be wise to mention that the requirements of Regulations 442.2.1 and 442.2.2 are deemed to be fulfilled for installations receiving a supply at low voltage from a system for distribution of electricity to the public In addition, refer to Regulation 442.2.3 for the need NOT to perform the calculation U 1 , U 2 and U f (see Fig 44.1) as it mainly falls into the responsibility of the substation installer/owner/operator to fulfil requirement provided by IEC 61936-1.
This section deals with protection of electrical installations against transient overvoltages of atmospheric origin transmitted by the supply distribution system and against switching overvoltages generated by the equipment within the installation. The term ‘overvoltages of atmospheric origin’ relates to lightning, although direct lightning strikes on the LV lines of supply network or on electrical installations are not taken into account . Often, overvoltages of atmospheric origin are induced into overhead lines by nearby lighting strikes or the close proximity and movement of charged clouds . Two levels of protection against overvoltage are identified in Section 443: Equipment should have an adequate impulse withstand voltage for its intended use, or surge protection devices are used, which is not required in the majority of installations in the United Kingdom.
The rated impulse withstand voltage of an item of equipment is the peak value of an impulse voltage of prescribed form and polarity that the equipment can withstand under specified test conditions without failure. Surge protection devices is an alternative to the AQ criteria. (443.2.4 refers.)
This is an empty Section at the moment. Information may be added when Amendment No. 1 is issued.
Suitable precautions need to be taken where a reduction or loss in voltage may occur to such a level so as to cause failure of equipment, or in some cases damage depending on the nature of the equipment connected. For example, a three-phase motor may be damaged by the motor drawing current from only two phases. Where unexpected restarting of a motor might cause danger it should be fitted with means to prevent automatic restarting after a stoppage due to a drop in voltage or failure of supply
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