Electrical Testing at Work

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If you run or manage a workplace where electrical testing is carried out or are a person who does the actual testing, then this guidance is for you.

Electrical testing may be carried out for a number of reasons, for example:

  1. quality assurance tests on electrical components;
  2. diagnostic testing;
  3. fault-finding on electrical plant;
  4. routine safety checks.

Some of the main ways in which this can be done include:

  1. following safe systems of work
  2. using test equipment that is suitable for the job;
  3. making sure that people doing the work are suitably trained and experienced.

What types of testing are covered?

This guidance covers electrical testing in situations (mostly low voltage, ie not exceeding 1000 V ac or 1500 V dc) where equipment like domestic appliances is being tested.

Some of the test voltages applied to equipment during testing may be above the low voltage limits.

What is the risk of injury?

The likelihood of touching live parts is increased during electrical testing and fault-finding, when conductors at dangerous voltages are often exposed. This risk can be minimised if testing is done while the equipment is isolated from any dangerous source of supply, although this cannot always be done, and care must also be taken to prevent contact with any hazardous internally produced voltages.

Carrying out a risk assessment

Some questions to ask when carrying out the risk assessment are:

  1. Can the work be done with the equipment dead or energised at a safe voltage or current?
  2. Is it absolutely necessary for someone to be working on or near to equipment that is live at dangerous voltages or current levels?
  3. What is the maximum voltage on conductors that will be exposed during the work activity?
  4. Are the testers competent? Are they adequately trained and knowledgeable to do the particular work and ensure that others are not put at risk?
  5. If testers are not considered fully competent, are they adequately supervised?
  6. What physical safeguards should be applied to the equipment under test to prevent injury, eg the use of temporary or permanent screens?
  7. Is the test instrumentation of safe design? Has it been properly maintained?
  8. Is it necessary to set up a permanent test area separate from the rest of the workplace, where equipment can be taken for testing? Is it necessary to set up a temporary test area around the equipment?
  9. Are the testers able to supervise the working area sufficiently and at all times to prevent danger to others?
  10. Where testing is part of an "after sales service" how much must be done at customers" premises? If testing is being done in a customer"s home, what special precautions are required to protect the tester and others?
  11. To what extent should the testers be supervised or accompanied?
  12. If the testers design, manufacture or use any special test equipment, does it meet BS EN 61010-1?
  13. How big is the unit under test and how much space is required around it to undertake the testing in a safe and unconfined manner?
  14. Are all the other workshop employees competent to avoid danger if there is a need for them to approach the equipment? If not, how can you make sure that they do not do so?
  15. Will the equipment be left unattended while live, for example while being "soak tested"?
  16. Does the workbench or separate area require a warning, eg a light, to show that testing is in progress?
  17. Is there a need for additional emergency switching devices for use by other employees to reduce the degree of injury to testers? Can residual current devices (RCDs) be used to provide supplementary protection?
  18. Is it possible to reduce the number of available paths to earth to reduce the likelihood of a phase-to-earth shock, eg by the use of barriers, screens and insulating mats?
  19. Is it possible to use unreferenced supplies, eg isolating transformers/batteries to reduce the likelihood of a phase-to-earth shock?

Managing electrical testing

You must provide a safe working environment and establish a safe system of work for your employees.

Personnel

All personnel must:

  1. understand that the risk of electric shock injury will still remain during the testing process, even with the use of earth-free test areas and/or isolating transformers and/or RCDs;
  2. fully understand the scenarios in which these electric shock injury risks can arise in the particular workplace(s);
  3. be given adequate first-aid training, including cardiac pulmonary resuscitation (CPR) skills.

Permanent test areas

These areas must:

  1. be under the control of a responsible person;
  2. be in an area set apart by barriers to prevent entry;
  3. have suitable warnings provided at the entrance;
  4. be accessible during testing only to authorised staff or people working under their direct supervision;
  5. have suitable warning lights indicating that testing is in progress and other warning lights to indicate when it is safe to enter the area;
  6. have emergency-stop push buttons or equally effective means to cut all test supplies in the event of emergency. These emergency controls should be prominently identified.
  7. display an electric shock poster, eg Electric shock: first-aid procedures, at prominent locations, showing emergency arrangements, especially telephone numbers;
  8. have good housekeeping arrangements, including adequate clear working space.

Temporary test areas

If live testing needs to be carried out, a temporary test area should be set up around the equipment. Make a test area as earth free as possible, in conjunction with the use of isolated supplies. Each item of equipment under test should be provided with its own test supply.

Risk assessment for test equipment and electricity supplies

If the hardware precautions resulting from the risk assessment include the provision of isolating transformers for the source of supply to mains-powered test equipment, then such provision should normally be made and the isolation transformer should be separate from the equipment under test.

Some of the risks associated with the use of test equipment can be reduced, but not eliminated by placing all test equipment on an insulated shelf immediately above the test bench. This will reduce the chance of simultaneous contact between the test equipment and the equipment under test.

When the hardware precautions resulting from the risk assessment do not include the provision of isolated supplies to the mains-powered test equipment, all supplies to test equipment should be protected by 30 mA RCDs. NB: For supplies in excess of 16 A to equipment under test or where the equipment under test has a high leakage current it may not be practicable to use an RCD because of nuisance tripping.

Setting up safe test areas

It is important to make sure that anyone not involved with electrical testing work is kept free from risk. This can be done by confining testing work to a designated test area.

Otherwise, unauthorised people should be prevented from approaching the equipment under test by physical barriers or other demarcation methods which form part of the test area boundary.

Protecting the people doing the testing

Methods of reducing the risk of a shock from simultaneous contact with conductors include:

  1. testing at reduced, non-hazardous voltages and currents;
  2. using interlocked test enclosures in which the unit under test is contained;
  3. using temporary insulation;
  4. replacing covers which need not be removed for the purposes of the test, for example once supply connections have been made;
  5. creating an area which is as earth free as practicable;
  6. using isolating transformers connected to the mains supply;
  7. using 30 mA RCDs.

Safe voltages and currents

It may be possible to test the equipment by energising it with non-hazardous voltages and current levels.

Interlocked enclosures

If it is not possible to earth the equipment automatically, it should be manually earthed by the use of a suitably insulated earthing tool.

Temporary insulation

Where there is risk from simultaneous contact with hazardous conductors, do not assume that employees will be able to avoid accidental contact. Consider using temporary insulation which may be in the form of purpose-made screens or insulating sheets or shrouding.

Earth-free areas

Where it is not possible to make an area earth free because it is necessary for the test equipment and/or the equipment under test to be earthed, it is still possible to reduce the extent of the accessible earthed metalwork.

Isolating transformers

Isolating transformers connected in the test supply will prevent the risk of electric shock if a person touches a single live conductor of the isolated supply while in contact with an earthed conductor.

The integrity of the isolation from earth should be tested regularly, or earth fault monitoring devices should be installed, to ensure that dangerous earth faults are detected.

Residual current devices (RCDs)

These are supplementary protection devices which do not prevent an electric shock, but are able to limit the duration of some shocks by being able to cause rapid disconnection of the electricity supply in the event of a relatively low current flowing to earth, such as may occur during an electric shock. They are, therefore, able to provide a much higher level of protection against the risk of a severe electric shock than could be provided from an unprotected source.

Test equipment

Where possible, test equipment should be of a proprietary design. And where applicable, test equipment should be manufactured to BS EN 61010. Purpose-built test equipment must be designed and constructed to the same standards of safety as proprietary equipment.

Insulation test instruments can generate high voltages at their output and some have an option to limit the output current to a safe level. The generally accepted safe limited current is 5 mA (traditionally 5 mA ac has been used, but since May 2001 new equipment should be limited to 3 mA ac). If accidental contact with the output conductors occurs, the risk of injury will be minimised if these current levels are not exceeded. The majority of insulation testing can be done within the safe current limits specified above.

The connecting leads of test equipment must be of a design that provides adequate protection from electric shock.

Test instruments

The hazardous voltages may arise from the voltages being measured on the equipment under test, or in some cases, from the breakdown of the insulation of the oscilloscope itself.

In a Class I oscilloscope these parts are connected to the supply protective conductor. One method is to supply the oscilloscope via an isolation transformer, so removing the earth reference in the supply and allowing floating measurements.

A second method is to use a proprietary isolation monitor. This allows the oscilloscope to operate with its protective conductor disconnected but the monitor continuously checks the voltage between the oscilloscope"s enclosure and the supply earth. If the oscilloscope"s enclosure reaches a hazardous voltage, the monitor removes the mains supply to the oscilloscope and usually reconnects the supply protective conductor.

Safe systems of work

Where testing is confined to diagnostic testing on electrical distribution systems and equipment (eg switchgear), by electrically competent persons, the contents of the written documents should cover the essential safe working practices.

Test personnel who work in customers" premises might have to work under different rules, as compared to working at a factory-based test facility.

Training

All personnel involved in testing should be given specific safety training relevant to the work they are doing. Appropriate training or instruction must also be given to anyone who may attempt to enter test areas and approach test benches.

Competence of testers

Specialist competent advice may be needed to ensure that the testing procedure is safe.

A safety review should be carried out when changes are made that may affect testing work, such as changes in production methods, supply arrangements, test methods and instruments, or when personnel changes are made.

Legal requirements

The Electricity at Work Regulations 1989 are the principal legislation relating to electrical testing activities.

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