Cable Voltage Rating

Ask the manufacturer.

It doesn't need to be; you can use 300v rated cable (which I think is the next standard step above 230v).

due to reasons like cable over loads (thermal characteristics), its safer to use the high rated cable than the exact or step above, thought it can be expensive

But over loads will be taken care by the insulation provided to the cables.

This is nothing but the cable designation as per British standards. As per BS-6346,

The cables shall be designated by the rated voltages U0 and U, expressed in the form U₀/U. The rated voltages recognized for the purposes of this standard are 600/1000 V.

Rated Voltage U0 : The nominal power-frequency voltage between conductor(s) and armour or earth for which the cable is suitable.

Rated Voltage U : The nominal power-frequency voltage between phase conductors for which the cable is suitable.

Vinu_Answers Sure_Answers

It is an old Standard that you have quoted from. As per revised IEC such practice is not advised.

Although the cable is operated at 230 or 415 volts,it is designed to withstand a higher voltage of 600/1100 volts at power frequency.

In similar lines you may find from manufacturer's catalogues/data sheets that LT ACBs/MCCBs/Switches etc meant to be operated at 415 volts are designed to withstand 600/1100 volts at power frequency.

The idea is to make sure that the devices & cables etc do not fail under the eventuality of voltage surges encountered during their operating lives.

I beg to differ. 600/1100V is at Power Frequency and hence it cannot be the surge withstand rating. Surge ratings are still higher.

You are right.I stand corrected."voltage surges" mentioned in my post shall be read as "higher voltages".

Thanks a lot EE65 for your observation.

Standardisation of manufacturing

Why only 600/1100 V selected for standardisation ? Why not any other value ,say 750/1250 V ? Can any expert give the basis for the selected voltage rating please ?

Because if it were 750/1250, then you would be asking why not 600/1100.

Are you ready for reading a lengthy paper on the topic?

The Magic of the multiples of 11

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

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(Published in the September 2008 issue of "HYDEL" - the Technical Journal of the Kerala State Electricity Board Engineers' Association)

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Foreword:

The author has been coming across the following question repeatedly from the participants in his training programmes. Many other electrical professionals and electrical engineering students would also be having the same question in their minds, but only hesitant to ask anybody. An attempt is made here by the author to answer this question.

Question: Why Transmission, distribution and utilization voltages are in multiples of 11, as in 110V, 220V, 440V, 1.1kV, 3.3kV, 6.6kV, 11kV, 22kV, 33kV, 66kV, 110kV, 132kV & 220kV?

Answer: It is true only with respect to AC power systems. The first known man-made source of electricity is a cell, which is DC in nature. But, after realizing the disadvantages of the DC electric equipments, AC Electricity generating machines were invented. When these AC Machines were developed, the power of these machines was to be compared with the already available DC electricity, as it is human tendency to compare anything new with the existing ones. As in the case of steam engines.

When steam engines were invented, the power of the steam engines was compared with that of the horses, which were the power sources before the invention of the steam engine. (Remember the story on horsepower, published by the author in the June 2008 issue of HYDEL?)

So, a value called RMS Value for AC Electricity was derived which compared the effectiveness of the AC Electricity with that of the DC Electricity. This value is the Effective Value of AC Electricity. As we were more interested in knowing the effect of AC electricity, all measuring instruments were and are designed to measure only the RMS value of AC electricity - may it be Voltage, Current, Power, etc.

But, for the designer sitting in the design lab, more than the effective value, the average value over a period on one sinusoidal cycle of AC Electricity was important. So, he designed an AC electric Generator, which would produce, on an average, a voltage over a period of one cycle, of say, 10kV (10 is a round figure, you know).

But, when this machine was built to the design and put to operation and when the output voltage was measured, it was found to be 11kV, as the meter was measuring NOT the average value but the effective or the RMS value. This relation existed for any voltage. So, a factor was arrived at - relating the RMS value and the average value, called Form Factor, which is the ratio of RMS value to the Average value, which for a sinusoidal wave form was about 1.1. Then, when the voltage was to be transformed, it was easy to have a whole number for the turns ratio of the transformer and hence all subsequent AC voltages became multiples of 11.

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And, that's how one got 1100V; and, the phase voltage for a line voltage of 1100V is 635V, perhaps rounded off to 650V.

Hope it is clear.

Thank you so much ! That's nice & revealing history which many of us might not have come across.

A minimum thickness of insulation is required to offer a reasonable level of resistance to mechanical damage. If only enough insulation was used to insulate to 240 volts, the covering would be too easily damaged.