"Spare parts and servicing equipment are usually carried as inventory and recognised in profit or loss as consumed. However, major spare parts, stand-by equipment and servicing equipment qualify as property, plant and equipment when an entity expects to use them during more than one period."
This would mean that if it is a stand-by equipment, that is kept as a back-up can be capitalised. The function of a back up asset is itself to be in use when the time arises/emergency comes. Though the direct economic benefit does not arise from the stand by equipment, it is made available to safeguard the continuous flow of benefits from the production activities.