Abstract The total clearing time or
operating time of an overcurrent circuit protector is defined as the time
duration from overcurrent initiation to complete cessation of current flow.
This total operating time is divided into two principal sub-periods: the
detection time period and the interruption time period. The detection time
period is engineered to be commensurately shorter as levels of overcurrent are
higher. It is this period that determines the ability of circuit protectors to
coordinate with, or back up, one another. The interruption time period is the
"action" period in which the device forcibly opens the path of
overcurrent flow.Overload Detection Our discussion of the
physics of current interruption in the previous article (1.2 The Physics of
Circuit Interruption) did not address the question of detection of an
over-current state. Before the interruption process is initiated—that is, when
the contacts start to open or the injection of mobile carriers into a
semiconductor switch is restricted—the circuit protector must first make a
trip/no-trip decision. The period of time between the initiation of an
overcurrent condition within a circuit and the initiation of interruptive
action by the circuit protector is termed the detection period. The different
types of circuit protectors detect overcurrents in different ways. Thus, they
can have different detection periods for the same overcurrent conditions. The detection mechanism in a
fuse is the melting and the vaporization of a fusible link. In a thermal
circuit breaker, dissimilar metals, bonded together along a single surface,
expand differently under the direct or indirect resistive heating of the overcurrent.
This forces a lateral mechanical movement, perpendicular to the bonded surface,
which releases a latched contact separation mechanism. In some types of thermal
circuit breakers, the contact mechanism can be formed using the bi-metal
material itself. In these devices, the bi-metal arms/contacts snap open when
they absorb sufficient energy from the circuit overcurrent. Another form of
thermal circuit breaker utilizes the longitudinal expansion of a hot wire,
which carries the overcurrent, to release a contact latch. The detection portion of a
magnetic circuit breaker is comprised of an electromagnet driven by the circuit
current. An overcurrent will
develop, within the electromagnet, enough magnetic pull to trip a spring restrained
latch that, as in the thermal circuit breaker, allows the spring-loaded
contacts to separate. A solid-state switch detects
overcurrents electronically, in many cases by simply monitoring the voltage
drop across a low-value resistance that carries the circuit current. Obviously, the faster a
circuit protector can detect an overcurrent the shorter the detection period.
But, in the majority of cases, the fastest possible detection speed is not
desirable. The speed of detection must be controllable and inversely matched to
the severity of the overcurrent. Trip Time As noted in Article 1.1
(Overcurrent Protection and Overcurrent Protectors), series-connected circuit
protectors must be coordinated. For a given level of overcurrent, the device
nearest to, and upstream from, the cause of the over-current must have the
fastest response. Devices that are further upstream must have a delayed
response, such that the minimum circuit removal principle is adhered to. When we speak of response,
we are referring to the total response time, or total clearing time, of the
circuit protector, from the time of the overcurrent initiation to the final
current-zero at which interruption is completed. Since it is far easier to engineer the extent of the
detection period for a given level of overcurrent than it is to control the
extent of the actual current interruption process, the total response time of
any circuit protector is, by design, determined principally by the size of, and
the time required to detect the overcurrent state. The interruption period is
defined as the length of time between the start of interruptive action—for
example, when the contacts start to part—and the final current-zero. The sum of
the detection period and the interruption period is then the total clearing
time, or total trip time, of the circuit protector. These different time
periods are shown in Figure 1.4.
Figure 1.4 Time Periods in the Clearing Cycle of an
Overcurrent Protection Device In contrast to the detection
period, the interruption period cannot be engineered to decrease as the
intensity of an overcurrent increases. The interruption period is, however,
almost always designed to be as short as possible, since during this period the
circuit protector is absorbing energy, due to the over-current flowing through
the voltage drop across the contacts (or terminals in the case of a solid-state
device). If circuit protectors,
other than fuses, do not clear the overcurrents fast enough during this period,
they can be destroyed due to their own power dissipation. Of course, fuses by
design are always destroyed when they interrupt a circuit.
In AC circuits, the
interruption period will last to either the first forced current-zero or the
first natural current-zero at which the switching medium (arc or solid-state material)
can reach its non-conducting blocking state. In DC circuits, the current-zero state is always a result of
a forcing action by the circuit protector. There are additional time
periods of interest during the current interruption process, such as contact
travel time, arc restrike voltage transient time, thermal recovery time, and
charge storage time (for solid-state devices). These times are discussed in
other articles detailing the actions of particular interruption devices. >> Next: Article 1.4 Physical Characteristics
| 
Call: 
Circuit BreakersMP Series 752 
