CURRENT RATINGS AND VOLTAGE RATINGS

What Would be Normal Current Ratings to Design an Electrical Energy Transmission Underground Lines?

Current ratings in accordance with IEC 59 shall be adopted, unless otherwise agreed with the Engineer.

Every current carrying part of the equipment shall be capable of carrying its site rated current continuously under the Site conditions as specified, and in no case shall the permitted temperature rise be exceeded.

The current ratings specified are the continuous current ratings required at the Site, under the specified maximum temperature and solar gain conditions.

What Would be Short-Time Current Ratings of Underground Cable Transmission Lines?

Electrical equipment shall be adequately supported and braced to withstand the forces associated with the maximum short-circuit currents specified or pertaining, whichever is the greater, and assuming that the inception of the short-circuit is at such a time that gives maximum peak currents, No provision for current decrement shall be made unless specifically permitted by the appropriate Standard, or elsewhere in this specification.

Equipment shall be so constructed as to withstand the specified maximum short-circuit currents for a duration of 3 seconds, (with the exception of cables which shall be 1 second rated) without the temperature exceeding the specified maximum short-time temperature or value stated in the relevant standard, under these conditions. The equipment shall be considered as being operated at the maximum permitted continuous temperature prior to inception of the short-circuit.

The final temperature attained as a result of the passage of short-circuit current shall not cause permanent damage, or deterioration sufficient to reduce the normal operating characteristics below the specified or most onerous operating requirements whichever is the highest (unless otherwise agreed with the Engineer).

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What Would be Normal Voltage Ratings for Underground Energy Transmission Cables Lines?

Unless otherwise specifically stated, any reference to voltage rating shall be deemed to refer to the Nominal rated voltage or voltages of electrical equipment. Standard voltage levels in accordance with IEC 38 shall be adopted, unless otherwise specified or agreed with the Engineer. Transformer rated voltages shall comply with IEC 76.

All electrical equipment shall, except where otherwise specified, be capable of continuous operation at a voltage level in the range of 90 per cent to 110 per cent of the nominal voltage without deterioration.

Alternating current rotating electrical machines shall be capable of providing rated output within the voltage range 95 per cent to 105 per cent of normal rated voltage.

The temperature rise of electrical equipment continuously operating at the specified minimum or maximum voltage shall not exceed the temperature rise when operating at nominal voltage by more than 5 degrees Celsius.

All D.C. equipment shall be capable of satisfactory continuous operation in the conditions specified when subjected to the maximum boost voltage output of the associated battery chargers unless dual battery & charger systems adequately controlled and interlocked are provided in which case they shall be capable of continuous operation at the maximum possible float charge voltage.

In addition all D.C equipment shall perform satisfactorily at D.C. voltage down to 85% of the specified nominal voltage. Tripping relays and coils etc shall operate down to 80% of the specified nominal voltage.

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What Would be Short-Time Voltage Ratings of Underground Power Transmission Lines?

All electrical equipment shall be so designed such as to withstand abnormal system voltages as required by the appropriate Standard (unless otherwise specified).