CTV-PRB004-EN
49
Electrical System—Ratings
“Normal” and “overload” conditions … “fault current” … “interrupt” versus “short-circuit”
ratings” … “current-limiting.” Knowing what these terms mean and applying them correctly are
fundamental to designing safe, reliable electrical distribution systems. This is especially true in
light of more stringent code enforcement and the current design trend to deliver energy savings
by selecting low-impedance transformers: How does this influence safety? Lower transformer
impedances result in higher short-circuit currents.
Simply choosing a circuit breaker with a high-interrupt rating won't assure adequate protection
under short-circuit conditions. An “ounce of prevention” helps avoid the code official’s “red tag”
at the next system startup.The following section reviews the meaning of terms, defines some of
the issues related specifically to HVAC motor starter applications, and identifies practical effective
solutions.
Normal operation
“Normal operation” describes the full-load (or rated) conditions of each system component. For
motors, it includes the amps initially drawn at startup, i.e. inrush current, as well as the full- or rated-
load amps drawn while running. The magnitude of inrush current for a particular application
depends on the motor, voltage, and type of starter used.
Normal operating conditions determine wire and transformer sizing. They are also used in
conjunction with “fault conditions” to select overcurrent protection devices such as circuit
breakers and fuses. Rating factors are applied, based on the type and number of connected loads,
to assure that the devices selected adequately protect the motor as it starts and while it is running.
The size of the interconnecting wires between the transformer and starter reflects the type and
rated amperage draw of the load, i.e. the chiller motor. Sizing the wires on this basis assures that
they can carry the inrush current at startup without overheating.
Available Fault Current (AFC)
Refer to
Figure 39, p. 52
, for the reference points or label locations of each rating. AFC is the
calculated potential short-circuit current at a point just upstream of the starter. It is calculated by
the electrical engineer and is a function of the electrical distribution system—including the
transformers.
Imagine a wrench inadvertently left in a starter after servicing. Touching two power phases, it
completes the circuit between them when the panel is energized. This results in a potentially
dangerous situation, or “fault condition,” caused by the low-impedance phase-to-phase or phase-
to-ground connection … a “short circuit”.
Fault current, also called “short-circuit current” (I
sc
), describes the amount of current flow during
a short. It passes through all components in the affected circuit. Fault current is generally very large
and, therefore, hazardous. Only the combined impedance of the object responsible for the short,
the wiring, and the transformer limits the fault current.
One objective of electrical distribution system design is to minimize the effect of a fault, i.e. its
extent and duration, on the uninterrupted part of the system. Coordinating the sizes of circuit
breakers and fuses assures that these devices isolate only the affected circuits. Put simply, it
prevents a short at one location from shutting down power to the entire building.
Calculating the magnitude of short-circuit current is a prerequisite to selecting the appropriate
breakers and fuses. If the distance between the transformer and starter is short, the calculation can
be simplified by ignoring the impedance of the interconnecting wiring… a simplification that errs
on the side of safety. One can also assume that the source of the fault has zero impedance, i.e. a
“bolted” short. Given these assumptions, only the transformer impedance remains. (Impedance
upstream of the transformer is usually negligible.)
Suppose a 1,500-kVA, 480-volt transformer has impedance of 5.75 percent. With this value, use the
equation below to determine how much fault current a short circuit will produce.The resulting I
sc
shows that a short would force the wiring to carry more than 30,000 amps when it was designed
to handle only 400 amps!
CTV-PRB004.book Page 49 Sunday, December 18, 2011 6:39 PM
