Definitions:
Ground-fault protection: Ground-fault protection is used in solidly grounded neutral
distribution systems where ground faults may be limited by high impedances (arc
presence, incorrect connection between frames and ground). High impedance current
levels are not detected by either the overload protection devices or the short-circuit
protection devices (pickup level too high). The purpose of ground-fault protection is to
protect the electrical system from serious damage and risk of fire.
Zone-selective interlocking (ZSI, also called zone restraint): Zone-selective
interlocking is used to reduce the stress on electrical distribution equipment during fault
conditions while maintaining coordination. ZSI limits fault stress on the power
distribution system by reducing the time it takes to clear the fault, while maintaining
system coordination between overcurrent protective devices.
ZSI allows electronic trip devices to communicate with each other so that a short-circuit
or ground fault will be isolated and cleared by the nearest upstream circuit breaker with
no intentional time delay. Devices in all other areas of this system (including upstream)
remain closed to maintain service to unaffected loads.
Without ZSI, a coordinated system results in the circuit breaker closest to the fault
clearing the fault, but with an intentional delay. With ZSI, the device closest to the fault
will ignore its preset short-time and/or ground-fault delays and clears the fault with no
intentional delay.
Eliminating intentional delay, without sacrificing coordination, results in faster tripping
times. This limits fault stress by reducing the amount of let-through energy the system is
subjected to during an overcurrent. The reduction in let-through energy can be as much
as 80%.
Long Time Ampere Rating and Delay: This adjustable function simulates the effect of
a bimetal in a thermal-magnetic circuit breaker. It reacts to overload conditions and
determines how much current the circuit breaker will carry continuously. The nominal
pickup point where a circuit breaker trip unit detects an overload is a predetermined
amount (typically 1.1) times the selected ampere rating of the circuit breaker. After the
circuit breaker has picked up, it will not trip until the delay determined by the long time
delay adjustment has been achieved.
Short-Time Pickup and Delay: The short-time function allows the circuit breaker to
delay before tripping on high level overcurrents, resulting in maximum coordination.
A high level selective override is placed into all (???) circuit breakers to instantaneously
trip them in the event of an extremely high level short-circuit. On circuit breakers
without the instantaneous pickup adjustment, this particular function is referred to as
short-time withstand rating.
I2 t IN: results in an inverse time delay characteristic that coordinates best with thermalmagnetic and electronic trip circuit breakers.
I2 t OUT: results in a constant delay characteristic that coordinates best with thermalmagnetic and electronic trip circuit breakers.
Instantaneous Pickup: The instantaneous pickup function simulates the magnetic
characteristic of a thermal-magnetic circuit breaker. This function trips the circuit
breaker with no intentional time delay. In circuit breakers with both short-time and
instantaneous pickup, the instantaneous pickup will override the short-time pickup if the
instantaneous pickup is set at the same or lower setting than the short-time pickup.
Ground Fault Pickup and Delay: The ground fault function is divided into pickup and
delay components. The pickup portion determines at what point the circuit breaker will
begin detecting a ground fault. The delay adjustment determines how long the circuit
breaker will delay tripping after a ground fault has been detected.
Interrupting Rating: The highest current at rated voltage that a circuit breaker is
intended to interrupt under standard test conditions. The interruption rating of a circuit
breaker must be equal to or greater than the available short-circuit current at the point at
which the circuit breaker is applied on the system
Shorter definitions:
Continuous Current Rating: The designated amperes (rms ac or dc) that a device or
assembly will carry continuously in free air without tripping or exceeding temperature
limits.
Current Transformer (current sensor)(CT): An instrument transformer that converts
actual system current to a lower value of current for metering or control.
Ground-Fault: An unintentional current path, through ground, back to the source.
Ground-Fault Delay: The length of time the circuit breaker trip unit will delay before
initiating a trip signal to the circuit breaker after a ground fault has been detected.
Ground-Fault Pickup: The level of ground-fault current at which the trip system begins
timing.
Instantaneous Pickup: The current level at which the circuit breaker will trip with no
intentional time delay.
Inverse Time: A qualifying term indicating that there is a purposely introduced delayed
tripping in which the delay decreases as the magnitude of the current increases.
I2 t IN: An inverse time delay characteristic.
I2 t OUT: A constant time delay characteristic.
Long-time Delay: The length of time the circuit breaker will carry a sustained
overcurrent (greater than the long-time pickup value) before initiating a trip signal.
Long-time Pickup: The current at which the long-time delay function is initiated.
Overcurrent: Any current in excess of the rated continuous current of equipment or the
ampacity of a conductor.
Overload: A sustained low-level overcurrent.
Rating Plug: A component that plugs into the electronic trip unit, establishing the
maximum continuous current rating of the circuit breaker.
Sensor Ampere Rating: The size of the current transformer for rated output.
Short-Time Delay: The length of time the circuit breaker will carry a short-circuit
current (greater than the short-time pickup value) before initiating a trip signal.
Short-Time Pickup: The current at which the short-time delay function is initiated.
Zone-Selective Interlocking (ZSI): A communication capability between electronic trip
systems and ground-fault relays that permits a short circuit or ground fault to be isolated
and cleared by the nearest upstream device with no intentional time delay.
OR
Zone Selective Interlocking establishes a higher level of circuit protection by enabling
electronic trip circuit breakers and ground fault relays to communicate with each other.
The communication between relays allows the system to locate the short circuit or ground
fault and isolate it. It is then cleared by the nearest upstream device with no intentional
time delay.