Electrical safety testing

Last updated

In electrical engineering, electrical safety testing is essential to make sure electrical products and installations are safe. To meet this goal, governments and various technical bodies have developed electrical safety standards. All countries have their own electrical safety standards that must be complied with. To meet to these standards, electrical products and installations must pass electrical safety tests.

Contents

Some types of electrical safety tests include:

Electrical safety tests are described in various national and international standards.

Electrical safety tests

Dielectric voltage withstand test

A dielectric voltage withstand test (also known as a hipot test) is done by applying a voltage higher than operating voltage to the device or installation under test. In this test, the electric insulation of a product or installation is put under a voltage stress much higher than its normal operating voltage. If the hipot test is done for type testing purposes, a much larger voltage may be applied.

Insulation resistance test (IR test)

An insulation resistance test (IR test) measures the electrical resistance of insulation by applying a voltage between two locations, and measuring the resultant current flow. Proper safety precautions must be taken when doing this test, such as exclusion zones, making sure no wires are exposed, and personal protective equipment is worn. To do this test, you need to have an insulation resistance tester, which may be a small hand held battery powered device, up to a larger luggage sized tool able to achieve higher test voltages. Variables to choose before starting an electrical resistance test is the test voltage, test duration, and test points. Acceptable values of insulation resistance vary depending on circuit purpose, industry, and country. Typically, standards will dictate acceptable values. An insulation resistance test can be used for simple go/no-go testing, or for periodic condition assessment, where routine insulation resistance tests are performed, with the results tracked over time. By tracking the insulation resistance of a circuit over time, preventative maintenance may be able to be planned before equipment failure. Insulation resistance is dependent on temperature and humidity variations, therefore the temperature and humidity should be recorded when testing and compensation applied to the measured value given. [1] Many standards and safety agencies have specified the insulation resistance test as a required test for electrical installations and electrical products. This test may be referred to as "meggering', after the company Megger was one of the first companies to commercialize an insulation resistance test meter. These days, many other brands are widely available.

Earth continuity test

An earth continuity test checks if a metal part is connected to earth. The resistance of the connection to earth can also be measured.

Leakage current test

A leakage current test detects current that does not flow back through the circuit. This test may be used as part of testing GFCI or RCD operation.

Electrical safety test categories

Visual inspection

A survey without touching anything, and looking at the electrical installation or product before starting any electrical testing. A visual inspection seeks to identify damaged insulation, disconnected wires, incorrect installation, and any other visual issues.

Dead testing

Continuity test: a test to check the wiring is correct

Earth continuity test: this test is to make sure the earthing system is properly connected

Live testing

Earth fault loop impedance testing: this test is to check that if a fault did occur, that the system meets requirements to cause a disconnection of the supply within the time limit specified by standards

Insulation resistance testing

Polarity test: a test to check that the connections are connected in the right sequence

RCD testing: on modern electrical systems RCD's and RCBO's are regularly fitted, these devices react to electricity leaking from the circuit. Leakage may happen when a person touches live parts, which can cause electricity to passes through their body instead of the normal circuit return path.

Means Of Protection (MOP)

No one means of protection should be totally necessary to the device's safety, so that the failure of one should not make the device immediately dangerous. [2]

MOP practices are further divided into MOOP and MOPP. [3]

Means Of Operator Protection (MOOP)

A concept introduced in the standard for medical electrical equipment IEC 60601-1. [4] [2]

Means of Patient Protection (MOPP)

Electrical safety standard set forward by standards organizations across the globe such as the American National Standards Institute (ANSI), [5] Canadian Standards Association, [6] and European Commission in IEC60601-1. MOPP safety standards aim to set basic safety requirements for medical electrical equipment. "With hazardous voltages present in a system a robust and reliable approach to isolation is needed such that multiple and un-related insulation system failures would need to occur before an operator or patient is put at risk. To achieve this, two independent measures of protection are required or a single measure of physical isolation such as creepage/clearance or solid insulation deemed equivalent." [7] In addition to the insulation of the device being powered the additional main points the organizations have set forward with the implementation of the MOPP standards are: [8]

Example

ClassificationsIsolationCreepage/

Clearance

Insulation
1 x MOOP1500 Vac2.5mm / 2mmBasic
2 x MOOP3000 Vac5mm / 4mmDouble
1 x MOPP1500 Vac4mm / 2.5mmBasic
2 x MOPP4000 Vac8mm / 5mmDouble

Example showing the differences between the isolation and creepage requirements for MOOP and MOPP. [9]

See also

Related Research Articles

<span class="mw-page-title-main">Ground (electricity)</span> Reference point in an electrical circuit from which voltages are measured

In electrical engineering, ground or earth may be a reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth.

<span class="mw-page-title-main">Residual-current device</span> Electrical safety device used in household wiring

A residual-current device (RCD), residual-current circuit breaker (RCCB) or ground fault circuit interrupter (GFCI) is an electrical safety device that interrupts an electrical circuit when the current passing through a conductor is not equal and opposite in both directions, therefore indicating leakage current to ground or current flowing to another powered conductor. The device's purpose is to reduce the severity of injury caused by an electric shock. This type of circuit interrupter cannot protect a person who touches both circuit conductors at the same time, since it then cannot distinguish normal current from that passing through a person.

<span class="mw-page-title-main">Earth-leakage circuit breaker</span> Electrical safety device

An earth-leakage circuit breaker (ELCB) is a safety device used in electrical installations with high Earth impedance to prevent shock. It detects small stray voltages on the metal enclosures of electrical equipment, and interrupts the circuit if a dangerous voltage is detected. Once widely used, more recent installations instead use residual-current devices which instead detect leakage current directly.

<span class="mw-page-title-main">Isolation transformer</span> Electrical component

An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety reasons or to reduce transients and harmonics. Isolation transformers provide galvanic isolation; no conductive path is present between source and load. This isolation is used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected. A transformer sold for isolation is often built with special insulation between primary and secondary, and is specified to withstand a high voltage between windings.

Appliance classes specify measures to prevent dangerous contact voltages on unenergized parts, such as the metallic casing, of an electronic device. In the electrical appliance manufacturing industry, the following appliance classes are defined in IEC 61140 and used to differentiate between the protective-earth connection requirements of devices.

Electrical wiring in the United Kingdom is commonly understood to be an electrical installation for operation by end users within domestic, commercial, industrial, and other buildings, and also in special installations and locations, such as marinas or caravan parks. It does not normally cover the transmission or distribution of electricity to them.

An earthing system or grounding system (US) connects specific parts of an electric power system with the ground, typically the equipments conductive surface, for safety and functional purposes. The choice of earthing system can affect the safety and electromagnetic compatibility of the installation. Regulations for earthing systems vary among countries, though most follow the recommendations of the International Electrotechnical Commission (IEC). Regulations may identify special cases for earthing in mines, in patient care areas, or in hazardous areas of industrial plants.

<span class="mw-page-title-main">Capacitor types</span> Manufacturing styles of an electronic device

Capacitors are manufactured in many styles, forms, dimensions, and from a large variety of materials. They all contain at least two electrical conductors, called plates, separated by an insulating layer (dielectric). Capacitors are widely used as parts of electrical circuits in many common electrical devices.

An insulation monitoring device monitors the ungrounded system between an active phase conductor and earth. It is intended to give an alert or disconnect the power supply when the resistance between the two conductors drops below a set value, usually 50 kΩ. The main advantage is that the ungrounded or floating system allows a continuous operation of important consumers such as medical, chemical, military, etc.

<span class="mw-page-title-main">Power entry module</span>

A power entry module (PEM) is an electromechanical component used in electrical appliances, integrating the appliance inlet with other components such as:

<span class="mw-page-title-main">Portable appliance testing</span> Procedure in which electrical appliances are routinely checked for safety

In electrical safety testing, portable appliance testing is a process by which electrical appliances are routinely checked for safety, commonly used in the United Kingdom, Ireland, New Zealand and Australia. The formal term for the process is "in-service inspection & testing of electrical equipment". Testing involves a visual inspection of the equipment and verification that power cables are in good condition. Additionally, other tests may be done when required, such as a verification of earthing (grounding) continuity, a test of the soundness of insulation between the current-carrying parts, and a check for any exposed metal that could be touched. The formal limits for a pass/fail of these electrical tests vary somewhat depending on the category of equipment being tested.

The Comparative Tracking Index (CTI) is used to measure the electrical breakdown (tracking) properties of an insulating material. Tracking is an electrical breakdown on the surface of an insulating material wherein an initial exposure to electrical arcing heat carbonizes the material. The carbonized areas are more conductive than the pristine insulator, increasing current flow, resulting in increased heat generation, and eventually the insulation becomes completely conductive.

<span class="mw-page-title-main">Megohmmeter</span>

A Megohmmeter or insulation resistance tester, is a special type of ohmmeter used to measure the electrical resistance of insulators. Insulating components, for example cable jackets, must be tested for their insulation strength at the time of commissioning and as part of maintenance of high voltage electrical equipment and installations.

<span class="mw-page-title-main">Megger Group Limited</span>

Megger Group Limited is a British manufacturing company that manufactures electronic test equipment and measuring instruments for electrical power applications.

Network isolators are installed as part of a wired Ethernet system as galvanic isolators to reduce the potential for electrical injury and limit the extent of damage due to lightning strikes.

<span class="mw-page-title-main">High-voltage cable</span> Cable used for electric power transmission at high voltage

A high-voltage cable, sometimes called a high-tension cable, is a cable used for electric power transmission at high voltage. A cable includes a conductor and insulation. Cables are considered to be fully insulated. This means that they have a fully rated insulation system that will consist of insulation, semi-con layers, and a metallic shield. This is in contrast to an overhead line, which may include insulation but not fully rated for operating voltage. High-voltage cables of differing types have a variety of applications in instruments, ignition systems, and alternating current (AC) and direct current (DC) power transmission. In all applications, the insulation of the cable must not deteriorate due to the high-voltage stress, ozone produced by electric discharges in air, or tracking. The cable system must prevent contact of the high-voltage conductor with other objects or persons, and must contain and control leakage current. Cable joints and terminals must be designed to control the high-voltage stress to prevent the breakdown of the insulation.

Dielectric absorption is the name given to the effect by which a capacitor, that has been charged for a long time, discharges only incompletely when briefly discharged. Although an ideal capacitor would remain at zero volts after being discharged, real capacitors will develop a small voltage from time-delayed dipole discharging, a phenomenon that is also called dielectric relaxation, "soakage", or "battery action". For some dielectrics, such as many polymer films, the resulting voltage may be less than 1–2% of the original voltage, but it can be as much as 15% for electrolytic capacitors. The voltage at the terminals generated by the dielectric absorption may possibly cause problems in the function of an electronic circuit or can be a safety risk to personnel. In order to prevent shocks, most very large capacitors are shipped with shorting wires that need to be removed before they are used and/or permanently connected bleeder resistors. When disconnected at one or both ends, DC high-voltage cables can also "recharge themselves" to dangerous voltages.

<span class="mw-page-title-main">Dielectric withstand test</span> Test to endure electrical insulators work under required conditions

In electrical engineering, a dielectric withstand test is an electrical safety test performed on a component or product to determine the effectiveness of its insulation. The test may be between mutually insulated sections of a part, or energized parts and ground. The test is a means to qualify a device's ability to operate safely during rated electrical conditions. If the current through a device under test is less than a specified limit at the required test potential and time duration, the device meets the dielectric withstand requirement. A dielectric withstand test may be done as a factory test on new equipment, or may be done on apparatus already in service as a routine maintenance test.

CEAR namely Central Electricity Authority Regulations, 2010 are regulations framed by Central Electricity Authority of India under the Indian Electricity Act, 2003, to regulate measures relating to safety and electric supply in India.

References

  1. "A guide to insulation resistance testing" (PDF). Biddle Megger. Archived from the original (PDF) on 2009-02-06.
  2. 1 2 "Electrical Safety Tests". EBME.co.uk. Retrieved 18 February 2016.
  3. Wilson, Richard. "Know your MOPPS from your MOOPS in medical power supply design". Electronics Weekly. Retrieved 18 February 2016.
  4. Wilson, Richard (15 November 2013). "Know your MOPPS from your MOOPS in medical power supply design". Electronics Weekly. Retrieved 18 February 2016.
  5. "AAMI Standards and Recommended Practices" (PDF). aami.org. American National Standards Institute, Inc. Archived from the original (PDF) on 2015-09-21.
  6. "CAN/CSA-C22.2 NO. 60601-1:08 (R2013)". scc.ca. Standards Council of Canada.
  7. Bayliss, Ann-Marie. "Understanding the 3rd edition medical safety standard and using DC-DC converters to provide isolation" (PDF). go.murata.com. Murata.
  8. 1 2 3 "Medical Approvals for Power Supplies" (PDF). epsma.org. EPSMA.[ dead link ]
  9. "MOPP and MOOP in IEC 60601-1 3rd" (PDF). Mouser.com.