Voltage Class and Insulation Level

Rated insulation levels for various highest system voltages from 0.66 kV to 765 kV have been detailed in I.S. 2705/1992 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.

Nominal	Highest	Power	Lightning
System	System	Frequency	Impulse
Voltage	Voltage	Withstand	Withstand
kV (rms)	kV (rms)	VoltagekV (rms)	Voltage kV (peak)
			List 1	List 2
Upto 0.60	0.66	3	-	-
3.3	3.6	10	20	40
6.6	7.2	20	40	60
11	12.0	28	60	75
15	17.5	38	75	95
22	24.0	50	95	125
33	36.0	70	145	170
45	52.0	95	250	250
66	72.5	140	325	325

                                                                       

Table 1B Rated insulation levels for   Highest System Voltages upto and includ Upto and including 245 kV

Nominal	Highest	Power	Lightning
System	System	Frequency	Impulse
Voltage	Voltage	Withstand	Withstand
kV (rms)	kV (rms)	VoltagekV (rms)	Voltage kV (peak)
110	123	185	450
		230	550
132	145	230	5650
		275	6850
220	245	360	850
		395	950
		460	1050

 

Table 1C Rated insulation levels for Highest System Voltage of 420 kV and above.

Nominal	Highest	Power	Switching
System	System	Frequency	Impulse
Voltage	Voltage	Withstand	Withstand
kV (rms)	kV (rms)	VoltagekV (rms)	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 indicated in I.S.2705 (Part I)/1992 (Note appended to clause 8 page 6) that the H.S.V., Power frequency voltage withstand test level and the Lightning impulse voltage withstand test level be indicated as H.S.V./ Power frequency Withstand Voltage/Lightning Impulse Withstand Voltage e.g. for C.T. suitable for H.S.V. of 145 kV the insulation level may be indicated as 145/275/650 kV.