Basic impulse level (IEC) 1

[mastergate]

I have a question regarding rated impulse test voltage (BIL) selection for an MV switchgear (IEC):
-operatin voltage 10kv 50hz
-rated insulation: 17.5kv
-rated Power-Frequency Withstand voltage (1min)38kv.

Per IEC for this case, both 75kv and 95kv are possible for rated Impulse Test Voltage (BIL).

please advise how to decide about sufficieny of 75kv, and if there is any calculation, example..
thanks.

 

[7anoter4]

I think you are referring on IEC 60694 Table 1a
For explanation see IEC 600071-2 art.5.1.2 Standard lightning impulse withstand :
a)for equipment protected by close surge arresters, the low values of lighting impulse withstand level are applicable.....
b)for equipment not protected by surge arresters (or not effectively protected), only the
higher values of lightning impulse withstand voltages shall be used.....

 

[mastergate]

thanks for your reply.
Can I understand that if I have surge arrestors on transformer side and on MV switchgear (on main incomer), I do not need to use the higher value 95kv

 

[7anoter4]

In my opinion, the cable length from surge arrester mounted on transformer and the switchgear is important for effectively protection of equipment. See:

http://blog.schneider-electric.com/...ical-parameter-in-installing-surge-arresters/

 

[krisys]

For the cable distribution system, the lower value is generally used, as the system is inherently protected from the lightning strokes.
For the system connected with lot of exposed conductors, higher LI voltage is preferred.