Читать книгу Maintaining Mission Critical Systems in a 24/7 Environment - Peter M. Curtis - Страница 47

Mission critical facilities cannot be susceptible at any time to an outage, including during maintenance of the subsystems. Therefore, careful consideration must be given in evaluating and implementing redundancy in systems design. Examples of redundancy are classified as (N+1) and (N+2) configurations and are normally applied to the systems below:

 Utilities service

 Power distribution

 UPS

 Emergency generator

 Fuel system supplying emergency generator

 Mechanical Systems

 HVAC Distribution including piping, AHU’s and CRAH units

 Fire Protection Systems

A standard system (N+1) is a combination of two basic schemes that meet the criteria of furnishing an essential component plus one additional component for backup. This design provides the best of both worlds at a steep price, with no economies of scale considered. A standard system protects critical equipment and provides long‐term protection to critical operations. In a true (N+1) design, each subsystem is configured in a parallel redundant arrangement such that full load may be served even if one system is off‐line due to scheduled maintenance or a system failure. The alternate (+1) component is also fed from a different power source.

The next level of reliability is a premium system. The premium system meets the criteria of an (N+2) design by providing the essential component plus two components for backup. It also utilizes diverse electric services from different power distribution boards and two different utility substations. Under this configuration, any one of the backup components can be taken off‐line for maintenance and still retain (N+1) reliability.

Another level resiliency can be gained by having building electric services installed underground as opposed to aerial. Aerial services are susceptible to lightning strikes, fallen trees, or damaged utility poles.

Facilities can also be classified into different Tiers based on their required level of reliability and maintainability. Tiers for mission critical facilities range from I to IV, with Tier IV being the most redundant, reliable, and maintainable.

Table 3.3 Uptime Tiers

(Reference: Uptime Institute)

Tier I – Basic Non‐Redundant	Tier II – Basic Redundant	Tier III – Concurrently Maintainable	Tier IV – Fault and Failure Tolerant
No redundancySusceptible to interruptions from planned and unplanned activitiesEquipment configurations minimum required for equipment to operateOperation errors or failures will cause an interruption in service	Limited backup and redundancy reportSusceptible to disruptions from planned and unplanned activitiesMay contain limited criticality functions that can be shut down properly without adverse effects on businessUPS and/or generator backup may be installed for parts of the buildingFailures may cause a disruption in facility service	Full single system backup and redundancy (N+1)Planned preventative and programmable maintenance activities, repairs, testing, etc. can be conducted without interruption of serviceErrors in operation or spontaneous failures of infrastructure may cause disruption of power to the load	Facility functions cannot tolerate any downtimeNo single points of failure, and multiple system backup with automated recovery (2N)Capable of withstanding one or more component failures, errors, or other events without disrupting power to the loadFull load can be supported on one path without disruption while maintenance/testing is performed on the other

Load Classifications

 Critical Load: Requires 100% uptime. Must have uninterrupted power input to safeguard against facility damage or losses, prevent danger and injury to personnel, or keep critical business functions on line.

 Essential Load: Supports routine site operations. Able to tolerate power failures without data loss or affecting overall business continuity. Critical cooling can be classified as essential load when critical spaces have a “ride through time,” allowing chillers, cooling towers, and pumps to restart automatically in a limited period.

 Discretionary Load: Load that indirectly supports the operation of the facility, such as administrative, cafeteria/pantries/fitness centers/retail spaces, and office functions. Can be “shed” without affecting overall business continuity in order to keep critical loads on line.