Stevenson switchyard crushed rock aggregate complies with Transpower Specification TP PS 52.04, and is paramount to safety.

The purpose of using Stevenson crushed rock aggregate in insulating surface layers in substation switchyards is to reduce the body currents that could flow through personnel if they are in contact with metal objects such as structures, or equipment tanks etc. when system earth faults occur.

The surface layer of aggregate must provide high electrical resistivity, and a free draining surface that is low maintenance, and can be both driven and walked on without difficulty.

The electrical resistivity of wet crushed aggregate as a whole is also very important for safety reasons during power system earth faults. The greater the resistivity, the lower the shock current that may arise during earth fault.

Crushed aggregate is an integral part of safety within, and in some cases outside High Voltage switchyards. The $20 million Drury Switchyard project (visible from State Highway 1 and commisioned in mid 2010) involved connecting two 220 kV transmission circuits between Huntly and Otahuhu that pass over the substation site . This allows better load sharing between the existing circuits and improved security of electricity supply to the Auckland and Northland regions in the event of the loss of one or more transmission circuits.