Optimal Switchgear Testing Interval

When I first started in 1996 our answer for switchgear with no prior indication of PD was to spot test every 2 years and monitor for a week every 4. The intervals were developed at a time when a lot of switchgear had porcelain bushings and bitumen filled chambers. Internal voids were the common cause of PD and defects more likely to evolve slowly, i.e. with a long P-to-F interval.

As testing became widespread we found more examples of defects developing and failing in shorter periods. Also, several early designs of switchgear incorporating cast resin insulation started suffering from surface PD and tracking; in poor environments PD could accelerate to failure in a short space of time.

So today, 12-months is the most common recommended testing interval. However, it should be appreciated that we have several case studies with a P-F interval <6 months. As ever cost and consequence must be balanced so shorten for high consequence assets and consider lengthening for low consequence assets, accepting that more failures will occur.

For critical switchboards, look at the use of permanent monitoring as the reduction of risk that this provides will often make economic sense.