Concurrently maintainable and fault tolerant components of data center include telecommunications,
storage systems, power supplies, fire controls, air conditioning, security devices, electrical and electronic devices, and can continue to operate during planned or unplanned outages.
According to Uptime Institute,
Tier III data centers are concurrently maintainable site infrastructures with N+1 redundancy and network uptime of 99.982 percent. Tier III data centers facilitate scheduled activities such as preventive maintenance without causing any disruption of computer hardware operations. It should be noted that Tier III data center is not immune to unplanned activities.
Tier IV data centers are fault tolerant facilities with 2N+1 redundancy. These can sustain at least one worst case scenario without any serious load impact and enable planned activity without any interruption in hardware operations. These are provided with multiple paths for power and cooling and a continuous availability of at least a single active path. The network uptime guarantee of Tier IV data center is 99.995 percent.
Concurrently maintainable attributes of Tier III and Tier IV data centers facilitate planned maintenance without affecting data center operations. Every critical system in these
data centers is designed for N+1 or 2N redundancy. Fault tolerance against any unplanned failures is achieved with help of two exclusively independent plant and power management facilities to sustain the critical load in Tier IV data centers.