Glossary of Electrical Saftey Testing Terms



Alternating current, an electric current that has one polarity during part of the cycle and the opposing polarity during the other part of the cycle. Residential electricity is AC.

Applied Part

Part of the medical product that in normal use comes in contact with the patient. They are often referred to as patient connections. There are three types of applied parts; B, BF and CF. They are classified based upon usage: C being cardiac versus non-cardiac and F being floating/isolated from earth ground.


American National Standards Institute, an industry association that defines standards for data processing and communication.


Sparking or "flashing over" caused by a breakdown of electrical insulation.

Basic Insulation

Insulation providing basic protection against electrical shock.


Failure of electrical insulation to provide a dielectric barrier to current flow.


The ratio of charge on either plate of a capacitor to the potential difference (voltage) across the plates. When a voltage is applied, current flows immediately at a high rate and then decays exponentially toward zero as the charge builds up. If an ac voltage is applied, an ac current appears to flow continuously because the polarity of the voltage is reversed at the frequency of the applied voltage. The waveform of this current, however, is displaced in time from the applied voltage by 90°.

Charging Current

An insulated product exhibits the basic characteristics of a capacitor. Application of a voltage across the insulation causes a current to flow as the capacitor charges. This current instantaneously rises to a high value as voltage is applied then exponentially decays to zero as the DUT becomes fully charged. Charging current decays to zero much faster than dielectric absorption.

Class I Product

Product that use Basic Insulation in combination with an additional safety level of using Protective Earth. Class I products have a three prong power cord.

Class II Product

Product that relies not only on basic insulation but an additional safety level of supplemental Insulation or Reinforced insulation. Class 2 products typically have a two-prong power cord.


Clearance is the shortest distance between two conductors through air or insulating medium.


Creepage is the shortest path along the surface of an insulator or insulating medium that separates two conductors. The insulator or insulation medium cannot be air.


Canadian Standards Association.

Current Draw

The mains current consumed by the product or DUT.


Direct current, non-reversing polarity. The movement of charge is in one direction. Used to describe both current and voltage. Batteries supply direct current.

Delay Time

The amount of time an instrument waits before performing a task.

Dielectric Absorption

The physical phenomenon in which insulation appears to absorb and retain an electrical charge slowly over time. Apply a voltage to a capacitor for an extended period of time and then quickly discharge it to zero voltage. Leave the capacitor open circuited for a period of time then connect a voltmeter to it and measure the residual voltage. The residual voltage is caused by the dielectric absorption of the capacitor.

Dielectric Strength

The Dielectric Strength of a material is the ratio between the voltage at which breakdown of the insulating material occurs and the distance between the two points subject to the applied voltage.

Dielectric Withstand Test

A high voltage either AC or DC is applied to determine if a breakdown will occur in the insulation of the DUT. Also referred to as a HIPOT test or Dielectric Withstand test.


The act of draining off an electrical charge to ground. Devices that retain charge should be discharged after a DC hipot or IR test.

Double Insulated

A descriptive term indicating that a product is designed so that a single ground fault cannot cause a dangerous voltage to be applied to any exposed part of the product that a user might touch. Double Insulation Insulation comprising of both Basic Insulation and Supplemental Insulation.


Device Under Test - the product being tested. Dwell Time The amount of time the DUT is allowed to stabilize at the test voltage before measurements are performed.

Earth Leakage Current

The leakage current from all earthed parts of the product. The current flowing from the mains supply through or across insulation into the Protective Earth Conductor.

Electric Current

The flow of electrons (or electron "holes") through a conducting material, which may be a solid, liquid, or gas; the rate of flow of charge past a given point in an electric circuit. The magnitude of current flow through the conductor is proportional to the magnitude of voltage or electrical potential applied across the conductor and inversely proportional to the resistance (or impedance) of the conductor. Current is expressed in amperes or milliamperes (amperes/1000).

Fall Time

The amount of time it takes to gradually decrease the voltage to zero potential.


The rate at which a current or voltage reverses polarity and then back again completing a full cycle, measured in Hertz (Hz) or cycles per second.


An acronym for Ground Fault Circuit Interrupter, a safety device that breaks a power circuit as soon as it detects current flow of a certain magnitude through the ground return of a power circuit. Also known as GFI.


The base reference from which voltages are measured, nominally the same potential as the earth. Also the side of a circuit that is at the same potential as the base reference.

Ground Bond Test

Verifies that all conductive parts of a product that are exposed to user contact are connected to the power line ground. The GB test verifies the integrity of the ground connection using a high current AC signal (as high as 30Amps). GB provides a better simulation of how a product will perform under an actual fault condition.

Ground Continuity Test

A test to verify that all conductive parts of a product that are exposed to user contact are connected to the power line ground. GC Test normally performed with a low current DC signal that checks to ensure the ground connection has a resistance of <1 W.


The unit of measure of frequency, equivalent to cycles per second.

High Limit

The upper value for a test to be considered a PASS. If the measured value is higher than the high limit the test is considered a FAIL. In hipot, leakage current and ground bond tests a high limit is required.

Hipot Tester

An instrument for testing dielectric strength using a high potential (voltage); hence, the term "hipot".


An acronym for Institute of Electrical and Electronic Engineers, a professional association of engineers.

IEEE 488

General Purpose Interface Bus (GPIB) - an industry standard definition of a parallel bus connection for the purpose of communicating data between devices.


A term used with alternating current circuits to describe the "ac resistance" to the flow of current through a circuit when an ac voltage is applied across the terminals of that circuit. Impedance is a complex quantity composed of real (in phase with voltage) and reactive (out of phase by 90°) components. Impedance is calculated as voltage divided by current.


The protection against unwanted flow of current through a path, as between a circuit of a product and the ground reference. Materials that prevent current flow are referred to as insulators or dielectrics.

Insulation Resistance

Characteristic of an insulating material that being subject to voltage, indicates a resistance such that the value of leakage current which flows through it stays within acceptable limits.


A device or arrangement by means of which the functioning of one part is controlled by the functioning of another, for safety purposes.

Kelvin Connection

A circuit configuration that automatically compensates for measurement errors caused by resistance of leads between a tester and the point of measurement on a DUT.

Leakage Current

The residual flow of current through insulation after a high voltage has been applied for a period of time.


The total resistance or impedance of all circuits and devices connected to a voltage source.

Low Limit

The lower value for a test to be considered a PASS. If the measured value is lower than the low limit the test is considered a FAIL. In insulation resistance mode a low limit is required.


An instrument designed to measure high values of resistance using a dc voltage usually greater than 50 V DC.


An instrument designed to measure low values of resistance using a dc current or voltage.


The test that is to be performed such as AC Hipot (WAC), DC Hipot (WDC), Insulation Resistance (IR), Ground Bond (GR) or Leakage Current (LC).


National Institute of Standards and Technology, an agency of the U.S. Government that sets standards for physical measurements and references, formerly called the National Bureau of Standards.


Acronym for Nationally Recognized Testing Laboratory, such as Underwriters Laboratories (UL), Factory Mutual (FM), or Canadian Standards Association (CSA).


An automatic zeroing function to correct for leakage currents or additional resistance due to test leads or fixtures. An offset is performed by making a measurement at the programmed test settings, calculating the difference between the leakage current or resistance measured and the ideal current or resistance and then subtracting this difference from all future measurements.

Ohm's Law

The fundamental law of electrical circuits that describes the relationship between voltage, current and impedance (or resistance). For DC circuits, Ohm's Law states that Current =Voltage/Resistance. For AC circuits, Current = Voltage/Impedance. Stated conversely, Voltage = Current x Resistance (DC) or Current x Impedance (AC). The difference between the dc resistance and ac impedance is that ac circuits must deal with phase and time relationships and dc circuits do not.

Ohms (Ω)

The unit of measure of resistance and impedance, derived from Ohm's Law.


Occupational Safety and Hazards Administration, an agency of the U.S. Government that regulates industrial safety.

Patient Auxiliary Current

Current flowing between patient connections and that is not intended to be there to produce an effect in the patient.

Patient Leakage Current

The current flowing from every individual part of the applied part back to earth or the current flowing from an unintended appearance of a voltage on the patient back to an F-Type Applied Part.


The time relationships between alternating voltages, currents, and impedances. Usually expressed as complex vectors with "real" (in-phase) and "reactive" (out of phase) components.


A term used to describe a "one way" limitation on the insertion of a plug into a receptacle for a corded product. A polarized plug can be inserted in only one orientation and cannot be reversed.


Electrical potential is a term equivalent to "voltage".


The prefixes for Multiple Scientific Engineering Symbols are:

1000000000000000 1015 Peta P
1000000000000 1012 Tera T
1000000000 109 Giga G
1000000 106 Mega M
1000 103 Kilo k
0.001 10-3 milli m
0.000001 10-6 micro µ
0.000000001 10-9 nano n
0.000000000001 10-12 pico p
0.000000000000001 10-15 femto f

Protective Earth

Conductor that connects between any protectively earthed parts of a Class I product and an external protective earth connection.


One millionth of a second.

Ramp Time

The gradual increase of voltage from zero potential over a period of time (step).


The component of an ac voltage, current, or impedance that is 90° out of phase with the "real" or in phase component. Reactive components are associated with capacitive or inductive circuits.


The component of an ac voltage, current, or impedance that is in phase with the "real" component. Real components are associated with purely resistive circuits.


When applied to electrical circuits, regulation refers to the variation in output voltage that occurs when the input voltage changes or when the connected load changes. When applied to test laboratories and agencies, refers to the control exercised by these entities over test specs and rules.

Reinforced Insulation

A single system of insulation that provides two levels of protection against electrical shock.


The electrical characteristic that impedes the flow of current through a circuit to which voltage has been applied. Resistance is calculated by Ohm's Law as voltage divid - ed by current (for DC circuits). For AC circuits, it is the inphase or "real" component of impedance. Units are expressed in ohms (Ω).


An industry standard definition for a serial line communication link or port.


A scanner is a device designed to switch or matrix sig - nals.

Signal Input/Output Part (SIP/SOP)

Part of the medical product that is not an Applied Part but is intended to send or receive signals from other external equipment.

Single Fault Condition

Condition in which a single means of electrical safety protection is defective or an abnormal condition is present. Examples of a single fault condition would be interruption of the ground conductor on a Class I product or opening of the neutral supply conductor to the product.


The Standards Council of Canada, an agency of the Canadian Government analogous to OSHA in the United States.


A large momentary deviation from a normal voltage or current waveform.

Stabilization Time

The time required for a transient disturbance to decay to a steady state value.


Most electrical safety testers can perform tests in a sequence. The step number indicates in which order the tests will be performed. For example if step 1 is a ground bond test, step 2 an AC hipot and step 3 an insulation resistance measurement then when a test is started the electrical safety tester will perform a ground bond test followed by an AC hipot then an insulation resistance measurement.

Supplemental Insulation

Independent insulation applied in addition to Basic Insulation in order to provide protection against electrical shock in the event of a failure of Basic Insulation.

Touch/Chassis Leakage (Enclosure)

Leakage Current from the enclosure or other parts, excluding applied parts that are not connected to a protective earth conductor.

Type Test

A one-time test intended to verify adequacy of the design of a product to meet a safety standard.


Underwriters Laboratories, Inc., an NRTL located in Illinois.


The electrical potential applied to a circuit.


The instantaneous value of a variable such as voltage or current plotted against time.

X (Reactance)

Reactance is the imaginary component of Impedance.

Y (Admittance)

Admittance is the reciprocal of Impedance. Y = 1/Z

Z (Impedance)

Impedance is the sum of alternating current oppositions (capacitive reactance, inductive reactance and resistance). Z = R + jX