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Littelfuse Glossary of Terms

Littelfuse 10/9/2018
Littelfuse Glossary of Terms

Active Power
Measured in kW. In a diesel generator application, it is the power produced by the engine.

Alarm Level
A setting on a protection relay at which an LED or output contact operates.

Alarm Relay Contact
An output of a relay that acts as a switch and is typically connected to a visual or audible alarm.

Analog Output
A discrete, continually variable 0-1 mA, 4-20 mA, or 0-5 Vdc signal from a protection relay used to pass information to a device or controller.

Apparent Power
The vector sum of the active and reactive power.

Arc Flash Hazard
A dangerous condition associated with the possible release of energy caused by an electric arc.

Arc Flash Risk Assessment
A study investigating a worker’s potential exposure to arc flash energy, conducted for the purpose of injury prevention and to determine safe work practices, arc flash boundary, and the necessary types of personal protective equipment (PPE).

Arc Flash Suit
A complete arc-rated clothing and equipment system covering the entire body, except for hands and feet.

Arc Flash Boundary
When an arc flash hazard exists, the boundary is an approach limit at a distance from a prospective arc source within which a person could receive a second degree burn if an electrical arc flash were to occur.

Arc Rating
The value attributed to materials that describes their performance to exposure to an electrical arc discharge. The arc rating is expressed in cal/cm2 and is derived from the determined value of the arc thermal performance value (ATPV) or energy of break open threshold (EBT) (Should a material system exhibit a break open response below the ATPV value). Arc rating is reported as either ATPV or EBT, whichever is the lower value.

Asynchronous Motor
A motor in which the speed of the rotor is not the same as the connected system frequency.

Charging Current
System charging current is the current that will flow into the grounding connection when one phase of an ungrounded system is faulted to ground. Although not physically connected to ground, electrical conductors and the windings of all components are capacitively connected to ground. Consequently, a small current will flow to ground from each phase. This current does not occur at any particular location; rather, it is distributed throughout the system just as the capacitance to ground is distributed throughout the system.

Conformal Coating
A Silicone coating used to protect circuit boards from pollutants, corrosion, mildew, etc.

Core-Balance Current Transformer
See Earth-Fault Current Transformer.

Current Transformer (CT)
A transformer that produces a current in its secondary circuit in a known proportion to current in its primary circuit.

CT Verification
A continuous check of CT continuity to verify connection.

CT Saturation
A condition that occurs when a CT cannot maintain a secondary current proportional to a relatively large primary current.

CT Local Saturation
A condition where the magnetic flux is not evenly distributed throughout the CT. A resulting secondary current could be induced when no ground fault is present; it may lead to the false operation of a protective relay. This could occur if conductors are not centered in a CT window.

CT Saturation Compensation
A feature in which a protective relay can recognize that a CT is saturated and compensate for the condition in order to maintain service.

Data Logging
Collecting and storing information in a format that can be reviewed for trending, troubleshooting, and reporting.

Free from any electrical connection to a source of potential difference and from electrical charge; not having a potential different from that of the earth.

See Discrete Fourier Transform.

Differential Module
An accessory for the MPU-32 Motor Protection Relay and MPS Motor Protection System to add phase- differential protection.

Digital Harmonic Filter
The use of digital signal-processing techniques such as a discrete Fourier Transform to eliminate the measurement of harmonic components. With regard to ground- fault detection, this allows for a setting below the background noise level.

Discrete Fourier Transform
A mathematical algorithm used to extract a single frequency, such as the fundamental frequency, from a signal.

Earth Leakage
See Leakage Current.

Earth-Fault Current Transformer
A current transformer used to measure low-level ground-fault current.

Electrical Hazard
A dangerous condition such that contact or equipment failure can result in electric shock, arc flash burn, thermal burn, or blast.

Electrical Safety
Recognizing hazards associated with the use of electrical energy and taking precautions so hazards do not cause injury or death.

Electrically Safe Work Condition
An electrical conductor or circuit part has been disconnected from energized parts, locked/tagged in accordance with established standards, tested to ensure the absence of voltage, and grounded if determined necessary.

Fail-Safe Mode (also known as Undervoltage or UV)
An output relay is energized during normal (not tripped) operation. If the protection relay loses supply voltage, the system will trip or alarm. (Also see Non-Fail-Safe.)

Fault Current
A current that flows when a phase conductor is faulted to another phase or ground.

All circuit conductors between the service equipment or other power-supply source and the load or branch-circuit overcurrent device.

Feeder Protection
Overcurrent or overvoltage devices installed on a feeder circuit to interrupt the supply in the event of a fault.

Flux Conditioner
A ring of magnetically permeable material inserted in an earth-fault current transformer window; used to reduce local saturation.

Fundamental Frequency
In an alternating-current power system, the frequency of the generated voltage. In North America this is typically 60 Hz (60 cycles per second).

Ground Check Conductor
An insulated conductor in a trailing cable used to assist in monitoring continuity of the ground conductor. Typically designed to be the smallest conductor, it is the first to break connection when cable couplers are disconnected.

Ground-Check Loop
A circuit that includes a ground-check conductor, a ground-check termination device, and a ground conductor.

Ground-Check Termination
A device installed at the load end of a ground-check loop.

Ground-Continuity Monitor
A protection relay that continuously monitors a ground-check loop and trips if the loop opens or shorts.

Ground Fault
An unintentional contact between a phase conductor and ground or equipment frame. The words “ground” and “earth” are used interchangeably.

Ground-Fault Current
A current that returns to the supply neutral through a ground-fault and ground-return path.

Ground-Fault Current Transformer
See Earth-Fault Current Transformer.

Ground-Fault Relay
A protection relay designed to detect a phase-to-ground fault on a system and trip or alarm when the condition exceeds its pickup setting for longer than its time delay.

Ground-Fault Protection
The use of a ground-fault relay or indication system in order to interrupt the supply or alarm personnel in the event of a ground fault.

Ground Reference Module
A resistor network that limits ground-fault current and provides a system reference for a DC ground-fault relay.

Harmonic Filter
A device or method to remove or ignore non-fundamental frequency components of a signal.

Harmonic Frequency
Harmonic-frequency components (voltage and current) are multiples of the fundamental frequency and, in a power system, can be considered noise. Harmonic- frequency components are often present with the use of adjustable-speed drives.

High-Resistance Grounding
Using a neutral-grounding resistor to limit the current to a low level. Typically, High- Resistance Grounding limits ground-fault current to 25 A or lower. (Also see Low-Resistance Grounding.)

High Tension Coupler
An accessory used to isolate system voltage from a protective relay.

I^2t (I squared t)
Thermal capacity, or used thermal capacity. With regard to motor protection, thermal capacity is used to measure and describe motor heating in terms of current (I). This method is more accurate than temperature sensing because of temperature-sensor placement and the time delay inherent in temperature measurement.

IEEE Device Numbers
The devices in switching equipment are referred to by numbers, according to the functions they perform. These numbers are based on a system which has been adopted as standard for automatic switchgear by the IEEE. This numbering system is used on connection diagrams, in instruction literature, and in specifications.

Incident Energy
The amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. One of the units used to measure incident energy is calories per centimeter squared (cal/cm2).

Incident Energy Analysis
Used to predict the incident energy of an arc flash for a specified set of conditions.

Insulation Monitoring
Monitoring the resistance from phase to ground to detect insulation breakdown on a system.

Insulation Resistance
A measurement of the ability of an insulator, such as a cable jacket, to prevent current flow when a voltage is applied; typically measured in megaohms (M omega). Insulation resistance change can be monitored to predict failure.

Inverse-Time Overcurrent Protection
A method by which time-to-trip of a protective device, such as an overcurrent or ground-fault relay, decreases as the magnitude of the fault increases.

Leakage Current
Low-level ground-fault current, typically measured in milliamperes (mA).

Low-Resistance Grounding
A Resistance-Grounding System that allows high currents to flow during a ground fault. Typically, 100 A and higher is considered Low-Resistance Grounding. (Also see High-Resistance Grounding.)

LSIG Protection
An acronym for Long-time, Short-time, Instantaneous overcurrent, and Ground-fault protection; a term often used to describe protection required for a power-distribution feeder, or a protection relay with these functions.

Motor Protection
Technology designed to ensure that a motor operates within its rated thermal capacity in order to maximize its service life.

Neutral-Grounding Resistor (NGR)
A current-limiting resistor connecting the power-system neutral to ground.

N.C. Contact (Normally Closed Contact)
A relay contact that is closed when the relay is not energized.

N.O. Contact (Normally Open Contact)
A relay contact 15 that is open when the relay is not energized.

Non-Fail-Safe Mode (also known as Shunt Trip or SH)
An output relay is energized and contacts change state when a trip occurs. If the protective device loses supply voltage, the system can continue to operate but will not be protected. (Also see Fail-Safe Mode.)

Non-Volatile Memory
Data is retained when power is removed.

Nuisance Trip
A false operation of a protective relay.

Phase Current
Current present in a phase conductor.

Phase-Current Transformer
A current transformer installed so that current from one phase conductor flows in its primary winding. With regard to motor protection, feeder protection, and metering in a three-phase system, three current transformers are typically used to measure phase currents.

Phase-Differential Protection
Protection designed to detect low-level winding-to-winding and winding-to-ground failures in an AC motor.

Phase Voltage
The voltage measured between a phase conductor and ground, or another phase.

Power factor (cosφ)
The relation between the active power [kW] and apparent power [kVA].

Primary Rating (for CTs)
The current rating of the primary side of a current transformer. For example, the first number in the ratio 500:5 is the primary rating. 500 A of primary current flowing through the CT will produce 5 A of current out of the secondary terminals.

Pulsing Ground-Fault Systems
Modulating the ground-fault current on a resistance-grounded system using a contactor to short out part of the NGR elements (or to open one of two NGRs connected in parallel). This technique is used to locate ground faults by tracing the pulsing ground-fault current to the source of the fault.

Online or Offline Monitoring
Monitoring system parameters such as insulation integrity when the system is energized or de-energized, respectively.

Open-CT Hazard
An open-circuited CT secondary which can develop a dangerously high voltage when primary current is present.

Reactive Power
Measured in kVAR. The power used for magnetization of asynchronous alternators, motors and transformers, coils etc. The amount of reactive power has no effect on the torque of the prime mover (e.g. diesel engine). Therefore the reactive power has no effect on the engine. It is however very important for the alternator, as the total load on he alternator is the vector sum of active and reactive load.

Relay (1)
An electrical switch that opens and closes a contact (or contacts) under the control of another circuit. Typically an electromagnet.

Relay (2)
A device that receives inputs, compares them to set points, and provides outputs based upon that comparison.

Relay Operating Mode
Method of operation used for undervoltage or shunt-trip breakers. (Also see Fail-Safe Mode, Non-Fail-Safe Mode.)

Resistance-Grounded System
An electrical system in which the transformer or generator neutral is connected to ground through a current-limiting resistor. (Also see Solidly Grounded System, Ungrounded System.)

Reverse Power
An active power [kW] fed into a generator that thus is working as an electric motor, turning the prime mover. As this would damage the prime mover (e.g. an internal combustion engine), reverse power relays are used in applications where generators run in parallel with each other or with the utility. These relays detect the amount and direction of the power, and in case of excessive reverse power, disconnect the generator breaker.

Ride-Through Time
The amount of time a protection relay can maintain operation during a supply voltage loss.

RTD (Resistance Temperature Detector)
A device that experiences a linear change in resistance with a change in temperature. It is used to provide temperature metering. Common RTDs are 100 O platinum, 100 O nickel, 120 O nickel, and 10 O copper.

Sensitive Ground-Fault Protection
Protection designed to accurately detect low-level ground-fault current.

Shock Hazard
A dangerous condition associated with possible release of energy caused by contact or approach to energized electrical conductors or circuit parts.

Solidly Grounded System
An electrical system in which the neutral point of a wye-connected supply transformer is connected directly to ground. (Also see Resistance-Grounded System, Ungrounded System.)

Switchgear, Arc-Resistant
Equipment designed to withstand the effects of an internal arcing fault and that directs the internally released energy away from the employee.
A setting on a protection relay that determines the time between the fault detection and relay operation.

Trailing Cable
A power cable used to supply electrical power to mobile equipment. They typically contain three phase conductors, two ground conductors, and a pilot wire (also known as a ground-check conductor).

Trip Level
A setting on a protection relay at which an LED or output contact operates.

Trip Relay Contact
An output of a relay that acts as a switch and is typically connected to an undervoltage-release or shunt-trip coil of a circuit breaker.

Trip State
The state of the output contact during a relay trip.

True RMS
“Root-Mean-Square” calculation used to derive an average current or voltage value in a waveform.

Ungrounded System
An electrical system in which no point of the system is intentionally grounded, such as a delta- connected supply transformer.

Zero-Sequence Current Transformer
See Earth-Fault Current Transformer.