Brentb, Perhaps the use of the words "spinning reserve" is...

Brentb, Perhaps the use of the words "spinning reserve" is causing the confusion here. They suggest it is an unusual system.
There is no doubt however, that the power supply at Eureka is standby only, because it is connected to the unreliable local grid.

From the Himonsa website description of the project, where the words are used;
HIMOINSA Southern Africa were selected as the turnkey supplier to provide power to the mine after submitting a thorough technical response to a complex technical solution which guaranteed reliable power security. The scope includes the integration of the utility supply as well as back-up power diesel generators and on-site operational services. In conjunction with Delta, HIMOINSA Southern Africa developed an advanced diesel “spinning reserve” solution. In the event of multiple and consecutive grid outages, the diesel spinning reserve solution offers a greater level of reliability of supply to maintain the necessary power levels at the mine, without disruption. A SCADA system was installed and integrated to communicate with the grid and the HIMOINSA diesel generators. To avoid any downtime during periods of grid unreliability, the generators run continually at a low load factor, in conjunction with the grid power. Should the grid fail, the SCADA system shuts down the power supply to all non-critical parts of mine and the diesel generators pick up the full load of the mines critical process power requirements. Sufficient spinning reserve power is always available to provide the mine's critical power requirements instantaneously.If there is a prolonged outage, more generators will automatically start so that all the mine's operational power is generated and the HIMOINSA units seamlessly power the mine's full production capabilities. In total HIMOINSA Southern Africa provide 12MW power to the mine to ensure N+1 redundancy.It is important that the gensets run at their optimum fuel efficiency level and have sufficient spare capacity for peak demand and step-load spikes.When the utility restarts again, the gensets automatically return to running at the low load spinning reserve. Spinning reserve diesel generation in conjunction with the grid is quite unique. As Health and Safety is a critical value for HIMOINSA Southern Africa and Delta, the HIMOINSA solution encompasses aspects that are not typical for standard applications. For example, when generating power in parallel with the grid it is imperative that there is no chance of back-feed down the line. The SCADA system, plus additional protections, are in place to protect individuals, the mine and the utility from any injury or damage to equipment. A 33kV switchgear has been also provided to integrating the utility supply, the mines transformers and HRYW 1275 D5/6 HIMOINSA generators. Due to the size and potential scope of the mine's operations, HIMOINSA Southern Africa have made sure to futureproof the project by allowing sufficient spare incomers on the switchgear. As the mines power demand grows there is built-in spare capacity which will enable HIMOINSA Southern Africa to easily install additional power supply.