1. Field of the Invention
This invention is directed to an electrical switching apparatus and, more particularly, to a circuit interrupter, such as a circuit breaker, including interlocks with another circuit breaker.
2. Background Information
Electrical switching apparatus include, for example, circuit switching devices and circuit interrupters such as circuit breakers, contactors, motor starters, motor controllers and other load controllers. Circuit breakers are generally old and well known in the art. Examples of circuit breakers are disclosed in U.S. Pat. Nos. 4,751,606; and 5,341,191. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition.
Molded case circuit breakers include a pair of separable contacts per phase which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. Typically, such circuit breakers include an operating mechanism, which is designed to rapidly open and close the separable contacts, and a trip mechanism, which senses overcurrent conditions in an automatic mode of operation. Upon sensing an overcurrent condition, the trip mechanism trips the operating mechanism to a trip state which moves the separable contacts to their open position.
In coordinating the delay times and performance characteristics of the circuit interrupters associated with an electrical distribution system, a time-trip curve of the desired current response characteristics of the circuit interrupter over time may be employed. For example, in the time-trip curve, the current factor is shown on the horizontal axis and the time factor is shown on the vertical axis. Typically, the time-trip curve includes various types of overcurrent trip conditions, such as a long delay trip, a short delay trip, an instantaneous trip, or a ground fault trip. Modern trip mechanisms often employ a microprocessor to detect these overcurrent trip conditions.
The long delay protection feature generally follows an I.sup.2 t sloped portion of the log-log time-trip curve. The long delay feature is sometimes referred to as a thermal trip feature since it most closely resembles a thermal-type tripping operation typically offered by predecessor non-electronic circuit interrupters. This feature consists of both a selectable long delay current pickup factor (e.g., LDPU factor) and a long delay time factor (e.g., LDT factor). The LDPU factor selectively adjusts the time-trip curve along the horizontal or current axis and the LDT factor selectively adjusts the time-trip curve along the vertical or time axis. In this manner, the limits of the long delay protection feature provide a first trip-curve portion which is adjustable in both axes. Typically, the long delay protection feature provides an I.sup.2 t trip characteristic for currents exceeding the LDPU factor wherein, at higher levels of current in excess of the LDPU factor, a shorter LDT will result, although other trip characteristics such as I.sup.4 t may be employed.
At higher levels of current flowing through the electrical circuit protected by the circuit interrupter, it is necessary that the circuit interrupter provide a more rapid response than that provided by the long delay protection feature. This more rapid response is commonly referred to as a short delay protection feature and is characterized by a portion of the trip-curve designated as the short delay trip-curve portion. The short delay protection feature may be selectively configured in various manners such as a fixed time response or an I.sup.2 t response.
The current level at which a short delay trip condition is initiated is commonly referred to as a short delay pickup factor (e.g., SDPU factor). Under certain conditions, it is necessary that the short delay trip condition be initiated immediately upon sensing a current value in excess of the SDPU factor. Other conditions utilize a fixed time short delay trip-curve portion and still other conditions arise where it is necessary to impose an I.sup.2 t trip characteristic trip curve portion.
The next level of protection offered by the circuit interrupter is an instantaneous trip-curve portion which corresponds to an instantaneous protection feature. At very high levels of overcurrent through the electrical circuit, it is necessary that the circuit interrupter initiate a trip condition as rapidly as possible (e.g., within 20 milliseconds or less of sensing the faulted condition). This overcurrent level is selectively adjustable within the instantaneous trip-curve portion of the time-trip curve.
Another type of protection is the ground fault protection feature which provides the same types of protection as does the short delay protection feature, although the ground fault pick-up level is more sensitive than the short delay pick-up level. The ground fault protection feature provides that, should a certain level of current be flowing through a ground path associated with the electrical circuit in excess of a ground fault pickup factor (e.g., a GFPU factor), a ground fault trip condition is initiated. The GFPU factor is selectively adjustable in a ground fault trip-curve which typically employs a fixed time or I.sup.2 t ground fault protection. Under certain conditions, it is necessary to wait a selectively adjustable period of time, designated as the ground fault time factor (e.g., GFT factor), before initiating a ground fault trip condition.
In a typical electric power distribution system, a main bus provides power to a number of additional buses which, in turn, energize a plurality of distribution circuits. Often, power transformers step down the voltage at various points in the distribution system. Typically, overcurrent protection devices are provided in the main bus and in at least some, if not all, of the other branches of the distribution system. Each of the overcurrent protection devices has its own overcurrent/time trip characteristic for responding to faults in the distribution system. Typically, these overcurrent/time trip characteristics of the various overcurrent protection devices are coordinated through a hierarchical arrangement in order that only the closest protection device above the fault trips to minimize the interruption to service in the distribution system.
In some installations, zone interlocks between the overcurrent protection devices are employed. In such an arrangement, if a lower order overcurrent protective device of the hierarchy sees an overload current, it sends an interlock signal to the next higher order device to block generation of a trip signal by the latter and to give the former time to react. This permits adjacent overcurrent protective devices in the hierarchy to have their overcurrent/time trip characteristics set closer together, while assuring that the lower order device will trip first if conditions warrant.
U.S. Pat. No. 4,827,369 discloses the use of zone interlocks between upstream and downstream circuit interrupters to adjust the timing of short delay and ground fault protection in the upstream devices. If a short delay current threshold is exceeded in the downstream device, it sends a short delay zone interlock signal to the upstream device to indicate that a short delay fault condition has been identified. If the short delay current threshold is exceeded in the upstream device, and the short delay zone interlock signal is not received from the downstream device, then a short delay trip is rapidly initiated by the upstream device on the second consecutive recognition of that threshold being exceeded (e.g., to prevent the occurrence of a false initiation of the short delay trip condition resulting from a possible late signal from a downstream device due to possible asynchronous conditions). Otherwise, if the short delay current threshold is exceeded in the upstream device, and the short delay zone interlock signal is received, then a time delayed portion (e.g., either an I.sup.2 t or fixed time type) of the short delay routine is executed by the upstream device.
As further disclosed in U.S. Pat. No. 4,827,369, ground fault protection is implemented in a similar manner as the short delay protection, with each of the upstream and downstream devices inputting an input ground fault zone interlock signal and outputting an output ground fault zone interlock signal. For both the ground fault protection and the short delay protection, two separate input interlock signals indicate whether respective ground fault and short delay conditions were identified by a downstream circuit interrupter, and two separate output interlock signals indicate to an upstream circuit interrupter whether respective ground fault and short delay conditions were identified by the intermediate circuit interrupter.
It is also known to employ a separate input interlock signal to indicate whether a long delay condition was identified by a downstream circuit interrupter, and a separate output interlock signal to indicate to an upstream circuit interrupter whether a long delay condition was identified by the intermediate circuit interrupter.
However, there is room for improvement in zone interlocks for electrical switching apparatus.