Page 174 - Power Quality in Electrical Systems

156 Chapter Eleven

2. Apply faults to the network lines and buses in accordance with aver-
age industry fault data.
3. Compare the resulting voltages at the connections to equipment with
the known sensitivities of the equipment.
4. Calculate the number and duration of the interruptions per year of
each piece of critical equipment.
5. Make decisions on correction measures.

Method 2
1. Review all critical load at a facility, ranging from personal comput-
ers to complete industrial processes or data processing systems.
2. Estimate the cost of an interruption to each piece of equipment.
3. Carry out prevention and correction measures at a cost commensu-
rate with that of an interruption.

In the real world, the design of an electrical system to supply critical
load is based on many factors: required availability, cost, space, record
of utility service, and others. The limit in system design occurs when the
complexity of the correction measure overrides the predicted reliability
and availability of the system.

Personal Computers

A personal computer (PC) is a general-purpose computing device
designed to be operated by one person at a time. PCs can exert real-
time control of external devices, operate online control of communica-
tions, or be part of process-control applications. The malfunction of PCs
incorporated in a real-time system because of voltage disturbances
produces bigger consequences than the malfunction of the PC used
offline [11.2].
A first measure to match the capabilities of PCs to line-voltage dis-
turbances was the CBEMA curve shown in Figure 11.1, from IEEE Std
446-1987 [11.3 and 11.4]. The curve defines the tolerance level of “auto-
matic data processing” equipment to voltage sags, swells, and short
interruptions. The curve was updated in 1995 to the ITIC curve for
“information technology equipment” for 120-V/60-Hz single-phase serv-
ice. A similar curve, SEMI F47, was proposed for “semi-conductor pro-
cessing” equipment. The three curves are shown in Figure 11.2 [11.2].
An example of measured voltage disturbances at a customer’s site is
shown in Figure 11.3 [11.5]. Here, the concern was damage to the PC
from voltage surges. PCs can withstand surges of up to 3 kV.