Why is switchgear compartment de-gassing for cable termination required only for 72.5 kV upwards?

[electrical429]

Gas insulated equipment containing high pressure compartments, e.g. 145kV and 72.5kV gas insulated switchgear is normally provided with “plug in” cable connectors.

The normal operating practice is that no work should be carried out on gas insulated High Voltage systems on one side of a gas barrier, with the other side at full pressure.

This also applies to the cable connection system. When plugging in or removing cable connectors, or making the terminations when “plug in” is not provided, the gas pressure in the compartment on the other side of the work shall be reduced to about 0.5 bar.

These precautions are necessary to prevent danger to the operation staff in the event that the barrier fails during the work being carried out.

However I have never heard the same being applicable to 11 kV or 33 kV GIS switchgear. Is there a reason for this?

 

[cranky108]

Why would you need gas insulated gear below 35kV? Air works just fine.
Maybe that is the reason. Gas insulation below 35kV is not very common.
Then again our standard voltages are 15kV class and 35kV class.

 

[RRaghunath]

In MV GIS, the air itself can provide adequate insulation for rated voltage, in case the gas leaks out.
This cannot be said with 72.5kV and above rated GIS. Hence, it is safe to reduce the pressure in a controlled way and keel the gas seal safe.

 

[electrical429]

Thanks!