Contents
Insulated Level

Rated insulation levels for various highest system voltages from 0.66 volts to 765 kV have been detailed in I.S.3156/1992 Part I in tables 1A, 1B and 1C. These tables are reproduced below for ease of reference.

    Table 1A
           Rated insulation levels for


Highest System Voltages upto and including 72.5 kV.

Table 1B
Rated insulation levels for Highest System Voltages of 123 kV and Above
Up to and Including 245 kV

Nominal System Voltage kV(rms)

Highest System Voltage kV(rms)

Power Frequency Withstand Voltage kV(rms)

Lightning Impulse Withstand Voltage kV(peak)

110

123

185

450

 

 

230

550

132

145

230

550

 

 

275

650

220

245

360

850

 

 

395

950

 

 

460

1050

 

Table 1B
Rated insulation levels for Highest System Voltages of 123 kV and Above
Up to and Including 245 kV

Nominal System Voltage kV(rms)

Highest System Voltage kV(rms)

Power Frequency Withstand Voltage kV(rms)

Lightning Impulse Withstand Voltage kV(peak)

110

123

185

450

 

 

230

550

132

145

230

550

 

 

275

650

220

245

360

850

 

 

395

950

 

 

460

1050

Table 1C
Rated insulation level for
Highest System Voltage of 420 kV and Above

Nominal System Voltage kV(rms)

Highest System Voltage kV(rms)

Power Frequency Withstand Voltage kV(rms)

Lightning Impulse Withstands Voltage kV(peak)

400

420

1175

950

 

 

1300

1050

 

 

1425

1050

525

525

1425

1050

 

 

1550

1175

765

765

1800

1300

 

 

2100

1425

 

 

2400

1550

It is a practice to indicate insulation level as

Highest System Voltage/Power Frequency Withstand Voltage/Lightning Impulse Withstand Voltage e.g. for an 11 kV P.T. the insulation level shall be indicated as 12/28/75 kV.

Power frequency withstand test on primary winding cannot be carried out on earthed potential transformers. In such cases, therefore, Induced high voltage test accordance with clause 9.3.1.2 of I.S.3156 (Part I)/1992 is carried out and unless the other end of the H.T. winding is directly earthed, power frequency high voltage test at 3 kV is carried out as per clause 9.3.2.1 of this standard.

In certain cases the test level for induced high voltage is much in excess of five times the rated primary voltage. In such cases the level of test voltage ids to be limited to five times the rated primary voltage. This is in accordance with the note appended to clause 9.3.1.2 of I.S.2705 (Part I)/1992.