Glossary of Terms

Adjustable Alarm Level – A setting on a protection relay at which an LED or an output contact operates to activate a visual or audible alarm.

Adjustable Time Delay – A setting on a protection relay that determines the time between the fault detection and relay operation.

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

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

Asynchronous Motor – A motor in which the speed of the rotor is not proportional to the frequency of the system to which it is connected.

Conformal Coating – Coating used to protect circuit boards from pollutants, corrosion, and mildew.

Continuous Output– On an PGT-0400, continuous output is defined as current that does not automatically stop.

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

CT Loop – The electrical circuit between a current transformer and a protection relay or monitoring device.

CT Loop Monitoring– Continuous check of CT loop continuity to verify connection.

CT Saturation – A condition that occurs when a current transformer (CT) cannot maintain a secondary current proportional to a large primary current.

CT Local Saturation – Poorly installed phase conductors and large currents can cause an output from a zero-sequence (ground-fault) CT even though there is no fault.

CT Saturation Compensation– The fundamental current amplitude is compared to the peak-to-peak value. If the peak-to-peak value is much higher than the DFT value then the relay assumes the CT is into saturation and uses the peak-to-peak value.

Current-Based Protection – Protection parameters (trip-levels/data collection etc.) derived from current levels in a circuit.

Current-Trip Setting– Selectable level of current at which a relay will operate.

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

DFT (Discrete Fourier Transform) Harmonic Filter – An algorithm used to measure the fundamental component of current and voltage and reject harmonics. This allows lower trip settings and eliminates nuisance trips due to harmonics.

Differential Module – An accessory for the PGR-6200 and PGR-6300 motor protection relays to add phase-differential protection.

Digital Harmonic Filter – The use of digital signal-processing (DSP) techniques such as the DFT to eliminate the measurement of harmonic components. In terms of ground-fault detection, use of a harmonic filter allows a setting below the background noise level.

Discrete Fourier Transform (DFT) – A mathematical algorithm used in a DSP to extract a single frequency, such as the fundamental frequency, from a signal.

Earth Leakage – See Leakage Current.

EFCT (Earth Fault Current Transformer) –A current transformer engineered to accurately detect low level ground-fault current. The words earth fault and ground fault are used interchangeably.

Fail-Safe Mode (also known as Undervoltage or UV)– 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– The current that flows when a phase conductor is faulted to another phase or ground.

Feeder– All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.

Feeder Protection– Overcurrent or overvoltage devices such as the PGR-7200 installed on a feeder circuit to supplement, compliment or replace downstream protective devices.

Flux Conditioner – A ring of magnetically conductive material inserted in a current-transformer window, used to reduce local saturation which can cause nuisance tripping.

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 when cable is mechanically stressed.

Ground Check Loop – The circuit that includes the ground-check conductor, ground-check termination device, ground-continuity monitor, and ground conductor.

Ground Check Termination – A device such as the PGA-0TA6 or PGA-0TA7 installed at the end of the ground-check loop to terminate the measuring signal from the ground continuity monitor.

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

Ground-Fault – Unintentional contact between a phase conductor and ground or equipment frame. The words “ground” and “earth” are used interchangeably when it comes to electrical applications.

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

Ground Fault Current Transformer– See Earth Fault Current Transformer (EFCT)

Ground-Fault Relay– A protection relay designed to detect a phase-to-ground-fault on a system and trip when current exceeds the trip time setting.

Ground-Fault Protection – A system that protects equipment from damaging ground-fault current by operating a disconnecting means to open all ungrounded conductors of a faulted circuit. This protection is at current levels less than those required to operate a supply circuit overcurrent device.

Ground Reference Module – A resistor network that limits ground-fault current to 25 mA and provides a signal to 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 because of the use of adjustable-speed motor drives.

High-Resistance Grounding– Using a neutral-ground resistor (NGR) to limit the current to a low level. Typically high-resistance grounding is 25 A and lower. (Also see: Low-Resistance Grounding).

High Tension Coupler– An accessory such as the PGH-5000 and PGH-6000 used to step down line voltage to a usable level for a relay.

I2t (I squared t) – Thermal capacity, or used thermal capacity. In 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 IEEE. This system is used on connection diagrams, in instruction books and in specifications.

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 under the stress of a voltage; typically measured in mega-ohms. Insulation-resistance change can be monitored to predict impending failure.

Insulation Warning – A warning alarm triggered by a decrease in insulation resistance below a pre-determined value.

Integrated Motor Starter– A device, such as a motor-protection relay, with the built-in ability and a user interface to start and stop a motor.

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

Leakage Current– Very low level ground-fault current, typically measured in milliamperes (mA, thousandths of amperes).

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

LSIG Protection – An acronym for Long-time, Short-time, and 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 Lockout– A condition where for safety reasons, the operator is prevented from starting the motor.

Motor Protection – Overload protection designed to protect the windings of a motor from high current levels. Modern motor protection relays add many additional features, such as metering, data logging and communications.

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

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

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

Non-Fail-Safe (also known as Shunt Trip or SH) – 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)

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

Nuisance Trip – An undesired change in relay output due to misinterpreted readings.

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. For motor and 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 winding-to-winding failures and winding-to-ground failures in an ac motor.

Phase Loss – Loss of power on a single phase.

Phase Voltage – The voltage measured between a phase conductor and ground.

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. Under ideal conditions 500 A of primary current flow through the CT will produce 5 A of current out the secondary terminals.

Pulsing – 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). Another version of pulsing is imposing a higher frequency signal on power lines and using a wand detector to locate the point of fault on a conductor.

Pulsing Circuit – See Pulsing.

Online/Offline Monitoring – Insulation monitoring when the system is energized and de-energized.

Open CT Hazard – An open-circuited CT secondary can develop a dangerously high voltage when the primary is energized.

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

Relay Operating Mode – Fail-safe or non-fail-safe methods of operation used for Undervoltage or Shunt trip breakers.

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).

Ride-Through Time – The amount of time a protection relay can maintain operation during a control-power dip.

RTD – Resistive Temperature Detector. A material that experiences a linear change in resistance with a change in temperature. Used to provide temperature metering. Common RTDs are 100 Ω platinum, 100 Ω nickel, 120 Ω nickel and 10 Ω copper.

Sensitive Ground-Fault Protection – Protection designed to accurately detect extremely low ground-fault current levels without nuisance tripping.

Solidly Grounded System – An electrical system in which the neutral point of a wye connected supply transformer is connected directly to ground.

Trailing Cables – Power cables used to supply electrical power to mobile equipment. They typically contain 3 phase conductors, 2 ground conductors and a pilot wire (or ground-check conductor).

Trip Level Settings (current) – Selectable current levels to define when a relay should operate.

Trip Time Settings– The time a fault is required to be present before trip action is taken.

Trip State – The state of the output contacts after a relay trip.

True RMS – “Root-Mean-Square” calculation used to derive true current or voltage value in electrical measurement when the waveform is non-sinusoidal.

Ungrounded System – An electrical system in which no point in the system is intentionally grounded. This was most common in process industries where continuity of service during a single-phase-to-ground-fault was required.