N  represents the arrangement, size, and number of components necessary to serve the load of the computer center

(cooling, power, etc.). This may be a quantity of switchgear of a certain size, a quantity of pumps, etc.

N+1  represents the addition of one more component or system to serve the load in any category of

equipment.

This addition permits the failure of one random piece of equipment with no loss of service.

N+2  represents the addition of still one more component to serve the load in any category of equipment.

This addition permits any piece of equipment to be removed from load for scheduled preventive maintenance

while retaining the ability to tolerate the failure of a random piece of equipment with no loss of service.

2N  represents the arrangement and number of systems necessary to serve the load of the computer center

(cooling, power, etc.) while also providing the ability to remove a system from service or repair with no loss of service.

2 (N+1)  represents dual systems, each system having redundant components.

3N/2 represents an arrangement of systems wherein multiple systems are available, a system can be taken off-line

without processing risk, and yet total plant capacity is reduced.  This approach is more complex than 2N systems,

but has the advantage of improved performance of the UPS Modules due to their higher loading.