In 2018 an earth fault occurred internal to 33 kV switchgear at a main substation in New Zealand. The 33 kV switchgear was located in a Switchroom Building located on its own earth grid. Prior to the fault it had been assumed the Switchroom Building earth grid was strongly bonded to the main site earth grid. Unfortunately, this was not the case, and some interesting equipment damage occurred, due to 1) the loosely bonded earth grids, and 2) the high fault level.
Based on this, a review was undertaken to identify sites with split site earthing (that is, sites with separate distinct earth grids that are bonded together via a few earth conductors) and high fault current that may circulate between earth grids. Once the sites were identified, testing (known as Internal Site testing or Split Site testing) was conducted to determine the effects the circulating current would have on the site safety, and equipment safety.
This paper and presentation cover:
1. A summary of the initial site fault event and the consequences.
2. The details of the fault current circuits, including fault current flowing from internal and external sources, and how these interact.
3. The testing methodology for the Internal/Split Site testing.
4. Case studies from selected sites that were tested. The results from these tests can, and are, being used to inform site primary and secondary design.