Glossary

Absorption current, also called polarization current, is the component of the current drawn by insulation under direct voltage that decays slowly as the molecules of the dielectric align with the applied field. It is the reason an insulation resistance reading on sound insulation keeps rising for minutes after the voltage is applied.

Accuracy is the closeness of agreement between a value measured by an instrument and the true or reference value of the quantity. On a test instrument it is stated as a specification that bounds the largest error permitted under stated operating conditions, usually as a fraction of the reading plus a fixed term.

Apparent charge is the quantity by which a partial discharge is measured in conventional testing: the charge which, if injected in a very short time across the terminals of the test object, would produce the same change in terminal voltage as the discharge pulse itself. It is not the charge actually transferred at the discharge site, which cannot be measured directly.

As-found data are the readings an instrument gives, compared with the reference standard, when it arrives for calibration and before any adjustment, and as-left data are the readings after any adjustment or repair. The pair shows how far the instrument had drifted during the previous interval and the state in which it was returned to service.

Breakdown voltage is the voltage at which an insulating material or an insulation system loses its insulating property and a conductive path forms through it, marked by a sudden rise in current and a collapse of the applied voltage. A withstand test is set below it, and a ramp or breakdown test raises the voltage until it is reached.

Burden is the load connected to the secondary of an instrument transformer, expressed as an impedance or as the apparent power drawn at rated secondary current, within which the transformer holds its accuracy class. The related term burden voltage names the voltage drop across the internal shunt of a multimeter or ammeter when it measures current, a drop that reduces the current in the circuit under test.

Calibration is the comparison, under stated conditions, of the readings of an instrument against a reference standard of known value and uncertainty, producing a documented relationship between the two. Adjustment of the instrument to bring its readings within tolerance is a separate step that may follow the comparison.

A calibration interval is the planned period between successive calibrations of an instrument, chosen so that the instrument has a high probability of remaining within its specification throughout the period. It is set from the maker's published interval, the instrument's own calibration history, how often it is used and the conditions it is used in.

Capacitive charging current is the initial surge of current that flows when a direct test voltage is applied to insulation, as the capacitance formed by the conductors and the dielectric charges to the applied voltage. It is highest at the moment of application and falls away quickly, before the absorption current has decayed.

Contact resistance is the resistance across a closed pair of contacts, a joint or a bolted connection, measured by passing a direct current through the connection and reading the voltage drop across it; a rising value indicates oxidation, loosening, erosion or contamination of the conducting surfaces.

Contact travel is the displacement of the moving contact of a circuit breaker during an operation, recorded against time by a transducer coupled to the operating mechanism; the travel curve yields stroke, overtravel, rebound, contact wipe and velocity, which reveal the condition of the mechanism and its damping.

Crest factor is the ratio of the peak value of a waveform to its root-mean-square value; as an instrument rating it states the highest peak-to-RMS ratio the converter can measure without clipping, and for a measured signal it indicates how far the waveform departs from a sine wave.

On an insulation resistance tester with timed modes the two time points for the dielectric absorption ratio are user-selectable and the instrument reports the ratio itself. The test is quick, is regarded as largely independent of temperature and shows up damp or contaminated insulation, but it is not suited to insulating oils.

Dielectric frequency response is the measurement of the capacitance and the dielectric loss of an insulation system over a range of frequencies that extends from far below the power frequency to above it. The shape of the resulting curve depends on the insulation geometry, its moisture content, the conductivity of the insulating liquid and the temperature, and it is fitted to a model to estimate the moisture in the solid insulation.

Dielectric loss is the energy an insulating material dissipates as heat when it is subjected to an alternating electric field. It arises from the resistive component of the current through the insulation, the part that flows in phase with the applied voltage rather than in quadrature with it as a capacitive current does.

Drift is a slow change in an instrument's indication for a constant input over time, arising from the aging of components, temperature cycling and mechanical wear. It is the principal reason calibration intervals exist, and the as-found data from successive calibrations reveal its size and direction.

Fall of potential is the ground electrode resistance method in which a test current is driven between the electrode under test and a remote current stake while the potential between the electrode and a movable probe stake is read; the resistance is the ratio of potential to current at probe positions outside the zone of influence of the electrode, where successive readings level off.

A four-terminal measurement, also called a Kelvin measurement, passes the test current through one pair of leads and reads the voltage drop through a separate pair connected directly across the item under test, so that the resistance of the current leads and their contacts does not enter the reading. It is the method used by low-resistance ohmmeters, micro-ohmmeters and winding resistance meters.

A guard terminal is a third connection on an insulation resistance tester that provides a return path for surface leakage current which bypasses the measuring circuit, so that the reading reflects the current through the insulating material alone. It is connected to a conductive band or bare wire wrapped around the insulation between the two measuring connections.

Lead-resistance compensation is the subtraction of the resistance of the test leads from a two-terminal resistance or continuity reading, obtained by shorting the leads together and storing the resulting value as a zero before measuring. It corrects for the leads but not for the contact resistance at the probe tips, which only a four-terminal measurement removes.

Leakage current is the small, steady current that flows through or over the surface of insulation under a sustained direct voltage after the charging and absorption currents have died away. It is the component that an insulation resistance reading ultimately represents, and a leakage current that rises from one test to the next indicates deterioration.

A live-dead-live check is the sequence in which a voltage tester is verified on a known live source, used to confirm the absence of voltage on the circuit under test, and verified again on the known source; the two verifications establish that the tester was working when it read the circuit as dead.

Loop impedance, in full earth fault loop impedance, is the impedance of the path a fault current would follow from the supply source through the line conductor to the point of the fault and back through the protective conductor and the source winding; its value determines the fault current available to operate the protective device.

A measurement category is the classification, used in instrument safety standards, of the part of a low-voltage installation on which a measuring instrument and its accessories are designed to be used, graded by the energy of the transient overvoltages that can occur there. The categories rise from receptacle-connected loads, through the fixed distribution installation, to the utility service connection and outdoor conductors, and the voltage rating that accompanies a category ranks instruments only within that category.

Measurement uncertainty is a parameter attached to a measurement result that characterizes the spread of values that could reasonably be attributed to the measured quantity. It is evaluated from the statistical analysis of repeated observations and from other knowledge such as calibration certificates, instrument specifications and the behavior of the reference standards used.

A partial discharge is a localized electrical discharge that bridges only part of the insulation between conductors, typically inside a gas-filled void, along a surface or at a sharp edge. Repeated discharges erode the insulation and precede failure, which is why their detection and measurement is a diagnostic test for high-voltage apparatus.

The partial discharge inception voltage is the lowest applied voltage at which partial discharges of a specified magnitude first appear as the voltage is raised gradually, and the extinction voltage is the voltage at which they cease as the voltage is then lowered. The extinction voltage is normally the lower of the two, because discharges once started persist below the voltage needed to start them.

Phase displacement is the angle by which the secondary current or voltage of an instrument transformer leads or lags the primary quantity, taken as positive when the secondary leads; it is the angular component of transformer accuracy and complements the ratio error.

Phase sequence is the order in which the voltages of a three-phase supply reach their positive peaks, labeled positive or negative rotation; it sets the direction of rotation of a connected motor and must agree before two supplies are paralleled.

Insulation resistance testers with a timed test mode compute the polarization index automatically from the resistance readings they log at the two set times, and report the ratio alongside the final resistance at the end of the timed test.

Prospective fault current is the current that would flow if a fault of negligible impedance occurred at a given point in an installation; the ground fault value follows from the supply voltage and the loop impedance between the line and protective conductors, and the short-circuit value from the line-to-neutral impedance.

Ratio error is the amount by which the actual ratio of an instrument transformer departs from its rated ratio, expressed as a fraction of the actual ratio; it describes how far the magnitude of the secondary quantity differs from the value the nameplate ratio predicts.

A reference standard is a measuring instrument, material measure or reference material of known value and uncertainty against which other instruments are calibrated. It is itself calibrated against a higher-level standard, forming one link in a traceability chain that ends at a national metrology institute.

Repeatability is the closeness of agreement among successive measurements of the same quantity made under the same conditions, with the same instrument, operator, procedure and location within a short time. It is the short-term component of measurement precision, as distinct from reproducibility, which admits changes of instrument, operator or time between the measurements.

Residual current is the vector sum of the currents in the live conductors of a circuit, which is zero in a healthy circuit and departs from zero when current leaks to ground; a residual current device trips when this sum exceeds its rated residual operating current.

Resolution is the smallest change in a measured quantity that an instrument can detect and show on its display. It is a property of the display and the converter behind it rather than a statement of accuracy, since an instrument can resolve finer steps than it measures correctly.

Soil resistivity is the resistance that a unit volume of soil offers to the passage of electric current, a property that depends on soil composition, moisture content and temperature; it is measured with stakes driven in a line so that a grounding system can be designed before its electrodes are installed.

A step voltage test is an insulation resistance test in which the direct test voltage is raised in successive equal steps, each held for the same period, with the resistance or the leakage current recorded at every step. A marked fall in resistance as the voltage rises points to a weakness, such as a crack or a pinhole, that a lower voltage does not reveal.

Tan delta is the tangent of the loss angle of an insulation system, the angle by which the current through the insulation under alternating voltage departs from a purely capacitive current. It is the dissipation factor expressed as a ratio, and it rises as the insulation takes up moisture, ages or develops conductive paths.

A timing test is a circuit breaker test that records the instant at which each main, resistor and auxiliary contact changes state after a trip or close command, yielding the operating time of each pole and the synchronism between poles and between the contacts of one pole.

Total harmonic distortion is the ratio of the root-mean-square value of all the harmonic components of a voltage or current to the root-mean-square value of its fundamental component, expressed as a percentage; it summarizes in one figure how far a waveform departs from a pure sine wave.

Metrological traceability is the property of a measurement result by which the result can be related to a stated reference, usually a national realization of an SI unit, through a documented and unbroken chain of calibrations, each of which contributes to the measurement uncertainty. A calibration certificate for a test instrument names the reference standards used so that the chain can be followed.

True RMS describes a measurement that computes the root-mean-square value of a waveform from its actual shape rather than by scaling a rectified average, so that the reading represents the heating value of the signal whatever its distortion.

Verification of test equipment is a check, made between calibrations, that an instrument still reads correctly, typically by measuring a stable reference or a known value and confirming that the reading falls within tolerance. Unlike a calibration it produces no new documented relationship and no adjustment, only evidence that the instrument remains fit for use.

Very low frequency, in high-voltage testing, denotes an alternating test voltage at a frequency far below the power frequency. It is used so that a compact source can energize the large capacitance of a long cable or a large machine winding with alternating stress rather than with direct voltage.

Withstand voltage is a specified test voltage, above the normal operating voltage, that insulation is required to sustain for a set time without breakdown or excessive leakage current. A withstand test is a pass or fail proof test rather than a measurement of insulation quality.