In the NEM, there are six Contingency FCAS markets, designed to ensure there is enough frequency response in the system to deal with a single credible contingency, which is typically the loss of a large generating unit or major industrial load.
The amount of Contingency FCAS procured is equal to the size of the largest credible contingency minus assumed load relief. Load relief is an assumed change in load that occurs when power system frequency changes. It relates to how particular types of load (particularly traditional motors, pumps, and fans) draw less power when frequency is low, and more power when frequency is high. Since the NEM commenced, mainland load relief has been assumed to be 1.5%; this means that for a 1% change in frequency (0.5 Hertz), the total mainland demand is assumed to change by 1.5%.
AEMO is acting on a recent review of load relief in the National Electricity Market (NEM). Accordingly, from September 2019, AEMO will slowly reduce assumed mainland load relief from 1.5% to 0.5%, with a review point at 1%.