Electrical Safety Testing

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A practical overview of the Periodic Maintenance and Inspection of Electrical Equipment under the Electricity at Work Regulations 1989.

Periodic maintenance of all electrical equipment is necessary under Reg. 4 of the Electricity at Work Regulations 1989 to prevent risk of injury. It is for dutyholders to decide how this is to be organised locally.

Ideally, electrical equipment should have an inventory compiled. From this, identify that equipment which requires maintenance, decide on maintenance regime in each case and set up a scheme for record keeping.

After an initial physical evaluation, electrical tests are carried out to ensure the continued safety of each appliance. The test results are logged. Equipment which fails a safety test is withdrawn from use, the cause investigated and it is either repaired or replaced.

Legal Obligations

The Electricity at Work Regulations 1989 came into force on 1st April 1990. Like much modern safety legislation, the Regulations describe aims to be achieved rather than particular schemes of work. It is for an employer to lay down the safety rules relevant to his particular undertaking and for each employee to cooperate by observing these rules. Although we all use electricity as a source of power, relatively few study it as an academic pursuit or have more than basic knowledge of it's safety problems. This guide is therefore aimed at the majority of organisations who do not have ready access to an electrician to allow them to assess their needs as regards the periodic maintenance and inspection of electrical equipment and plan accordingly.

The law on the use of electrical apparatus at work places obligations on those who design, manufacture and sell electrical apparatus, those who buy it for use at work by others (the employer) and those who actually use it (the employee).

Under Section 2(2)(a) of the Health and Safety at Work etc. Act 1974, electrical equipment used at work must be maintained in a safe condition, so far as is reasonably practicable. The Electricity at Work Regulations 1989 amplify this (Regulation 4) by describing the maintenance of electrical systems and saying that regular inspection and testing is necessary for all electrical installations and all equipment connected to the installation where this is necessary to prevent danger. Detailed up to date records of maintenance programmes are also required.

The named dutyholder is responsible for the provision of safe electrical supplies within the fabric of the buildings up to and including the wall sockets. Fixed installations need to comply with the IEE Wiring Regulations.

Where reference is made to portable electrical equipment, this should not be taken to be restricted to equipment which is simply capable of being carried in the hand by one individual. It should be taken to mean all equipment which can be attached to the electrical supply by a 13A plug. This definition thus includes portable hand tools, the wide variety of electrically powered laboratory equipment, office equipment such as electric typewriters, computers, photocopiers etc, and electrical apparatus which although battery powered could under fault conditions deliver enough electrical energy to an individual to cause injury. Also included is equipment which is not portable in the sense of being easily manhandled and in fact is never physically moved from its place of installation throughout its working like (e.g. a mainframe computer or mass spectrometer). If any part of such an installation contains enough electrical energy to cause a risk of injury then a value judgment has to be made on the magnitude of this risk and the appropriate maintenance instituted.

Each item of portable or transportable equipment should carry its own unique identification marker. If the supply cable is permanently attached to the apparatus then it will automatically be tested with the equipment. If a detachable supply cable is used then it may be either identified with the number of its associated appliance or if the cable is one of a central pool it should bear its own identification number. Whichever system is used it is important that supply cables can be identified or are traceable to a particular appliance and can thus be regularly tested.

The test procedures are designed to find out if equipment has deteriorated from its original design specification. In many cases this specification and the appropriate test method are set out in British Standards. It will be safe to test equipment to those standards by the test method specified in the relevant standard. In other cases the manufacturers advice should be sought.

Implementing a Safety Test Programme

Who should do the Testing?

The person carrying out the electrical testing has to have certain technical knowledge and information available to him to enable valid prospective judgements to be made. In legal parlance the individual has to be competent for the job. A person who is not professionally or academically qualified in a scientific subject qualified may also carry out testing provided they fulfil the necessary criteria. These criteria will include:

What type of Equipment Should be Tested?

The equipment to be tested will normally be constructed in one of 3 basic classes designated Class I, II or III under British Standard 2745. (Class O equipment which has no provision for the earthing of metalwork is of foreign origin and safe only in earth-free zones.)

Class I Apparatus is provided with basic insulation. In addition the metalwork is earthed so that it cannot become live in the event of an insulation failure.

Class I Apparatus is the most common type; its safety depends on the integrity of its one layer of insulation and the earth bonding of its metalwork. If there is an insulation fault, the metalwork is entirely dependent on the integrity of the earth bonding if it is not to attain a hazardous voltage. Inspection of the earthing conductors and connections is very important where this type of apparatus is used.

Class IIA (All-insulated) Apparatus has two layers (or equivalent) of insulation, one of which covers or comprises the outer casing so that metalwork cannot be touched. Class IIB (Double-insulated) Apparatus has all exposed metalwork separated from the conductors by two layers of insulation so that the metalwork cannot become live. There is no earth connection and the operator's safety depends on the integrity of the two layers of insulation. The first layer is basic or functional insulation and the second layer is supplementary or protective insulation. In some cases a single insulation layer is allowed if it is mechanically and electrically equivalent to double insulation. Class II equipment is marked with a special symbol.

Class III Apparatus operates on safety extra low voltage (selv) i.e. at a voltage not exceeding 50V AC between conductors or to earth. The apparatus has basic insulation only and unearthed metalwork.

Class III motor operated tools are not widely used in the UK as they are difficult to obtain. They are larger than comparable mains voltage tools and larger cables have to be used to avoid volt drop problems. 50V or 25V handlamps, soldering irons and special purpose heaters are commonly used.

While the isolating transformer provides the essential electrical separation from the mains supply, some users also choose to earth the centre tap of the low voltage winding of the transformer and also earth the exposed metalwork of the apparatus. This is strictly speaking a Class I extra low voltage installation with a line-earth voltage of half the system voltage.

Class III equipment should not be fitted with plugs or connectors of a type standardised for operation at mains voltage.

Priority Equipment and Equipment with Special needs

When instituting a test programme, priority should be given to certain types of equipment whose usage and operational environment mean that they are more likely to give rise to electrical danger. Examples of these include the following:

Extension leads should be treated in a similar fashion to removable mains leads. They should have unique serial numbers and be tested at regular intervals. The following tests are recommended:

As the length of the extension cable will have an effect on earth resistance, compare the test value with the resistance expected for the particular conductor size. Extension leads are likely to be exposed to a high level of abuse. Visual inspection of the insulation is therefore extremely important as is examination of the terminations to the plug and socket.

Modern low voltage transformers are manufactured to British Standard 3535 which allows either Class I or Class II construction. The transformer should be treated as any other piece or portable equipment. An additional test may be required to ensure that the primary voltage does not appear on the secondary winding. A flash test conducted on a Class I transformer will have additionally proved the insulation between the primary winding and the earthed screen thus ensuring primary/secondary isolation. The flash test conducted on a Class II transformer will not have tested the integrity of the insulating barrier between primary and secondary winding. A high voltage insulation test will therefore be required between the input and the output connections.

Portable Appliance Testers are in themselves items of portable equipment and will therefore require safety testing in accordance with the criteria described elsewhere. In addition, the equipment should be subjected to a full certified calibration procedure on an annual basis.

Frequency of Testing

There are no absolute rules regarding how often an item of electrical equipment should be tested. The Health and Safety Executive Guidance Notes on the safety of electrical apparatus advise "regular testing" and this is generally interpreted as a requirement for annual testing. However, circumstances and conditions of use will vary and it is therefore up to the dutyholder to assess the conditions of use of each piece of apparatus to determine, using the following guidelines, the test regime which is most appropriate.

The criteria which should be used to assess the need for testing the equipment includes the following:

Based on these criteria some suggested test periods are as follows:

Within this general framework, most equipment in laboratories or workshops should be tested at intervals of between 12 and 24 months, depending on the 7 criteria listed above. Most equipment in offices, libraries and similar accommodation should be tested every 12-24 months.

Record keeping and Initial Evaluation

The first step in the testing process is to produce a test results sheet for each appliance. This is a form for recording the relevant details of an appliance together with results of all tests carried out on it.

Each appliance currently in use should be logged. There should be no exceptions to this and the best time to do it is when the applicance if first purchased and before it is issued to the user.

This initial evaluation is most important. The peculiarities of each appliance should be noted and a decision made as to which tests should and which should not be carried out and at what frequency. This is a process of balancing the greater safety likely to result from testing frequently against the cost and effort involved. Such judgments are allowed under Regulations 4(2) where the standard is that of reasonable practicability. Having decided on which tests are appropriate, they provide benchmarks against which later test results may be judged. When the initial tests are carried out, the equipment itself must also be labelled to show the date tested and when the retest is due.

The electrical tests planned should reflect the British Standard Institutions Type Approval Test applicable to the type of appliance being tested. Routine safety tests should show that the unit's original design safety features are still operating, and are of such integrity that the equipment is likely to remain safe at least until the next scheduled test date. The safety tests required by the various standards are essentially those of earth continuity and high voltage leakage tests. If the equipment is double insulated then the earth test is unnecessary. When the equipment under test contains certain integrated circuits, then high voltage testing may damage these components. The test regime thus varies according to the type of equipment being tested.

Test Equipment Needed

To perform the actual tests, a purpose designed Portable Appliance Tester is recommended. For the essential tests a simple device such as the Seaward PAC500 or Megger PAT3 or Metrohm PAT is suitable. For the optional tests a more sophisticated appliance tester is necessary and devices such as the Megger PAT2 or PAT101 or the Seaward PAT1000 (capable of recalling many sets of test results for downloading to a computer) would be more suitable.

It is possible to use equipment other than a purpose built Portable Appliance Tester to carry out an earth bond test but it must be capable of supplying 25A. A Continuity Tester is not suitable. Test equipment which does not supply 25A may give an invalid result and a false sense of security.

Test Precautions

The safety test programme consists of each item of apparatus receiving a visual inspection, essential tests (earth bond and insulation) and in some cases, additional tests such as flash, operational and earth leakage tests. Of the essential tests, the insulation test is always carried out but the earth bond test in only performed on class 1 appliances. The optional tests should be performed at the discretion of the competent person in charge, perhaps because there is some reason to think the appliance may become unsafe. Some optional tests should not be carried out more than is necessary as they may weaken the insulation of the appliance.

The following precautions are necessary during the testing of the apparatus:

1. Precautions for Essential Tests

2. Precautions for Optional Tests

All the precautions above apply to the optional tests plus those give below:

Testing Class I Equipment

The tests include:

A detailed visual inspection should be performed first and any faults corrected. Over 75% of all equipment defects can be found at this first stage. Hazards such as loose cable grips, plugs incorrectly fitted, unsafe cable joints, damaged cable, wrong value fuses or illicit fuses should all be discovered and corrected at this physical inspection stage before electrical tests are commenced.

Before performing the electrical tests, it is important to establish whether the appliance is a light current type. Assume the appliance to be light current if it has a 3 or 5A fuse fitted, if it has a power rating less than 1kW or if the appliance cable is thinner than the normal mains lead. If these criteria apply then the appliance tester will be operated in a different manner. The manufacturers instruction booklet should be referred to.

The first essential test for class 1 appliances is the earth bond test. This test is intended to ensure that the exposed metalwork on the appliance is securely connected to the safe earth potential. During the test a high current (up to 25A) is passed through a circuit comprising the appliance earth conductor of the supply lead, the exposed metal on the appliance and the temporary test lead from the appliance tester. If the exposed metal work is securely connected electrically to the conductor, the resistance of the above circuit will be low. To be acceptable it must be between 0 and 0.1 Ohms (0.5 Ohms for low current appliances). This test must be carried out on all exposed metal work. If the resistance is too high, the protection afforded by earthing will be limited.

If the reading on the appliance tester indicates a high resistance or the fail indicator lights, then this must be investigated. First check the logged value for the equipment. It may be a low current appliance. If the reading is the same as the previous recorded value then this may be accepted (the resistance of thin mains leads may be appreciable e.g. 2.5 metres of 0.5 sq mm cable is itself around 0.1 Ohms). A particularly long lead or extension cable may also have an appreciable resistance. If this is the case get advice as to whether a circuit breaker should be fitted.

Check the security of the connections in the mains plug although this should already have been done during the physical inspection. Do not progress with further tests until this fault has been cleared.

The second essential test for class 1 appliances is the insulation test. This is used to ensure that a breakdown of the insulation cannot occur between any live parts within the appliance and parts of the casing that the user may touch. The appliance tester must be able to deliver a 500V DC test voltage for all loads of 2 Mohms upwards. The test may be performed as many times as you like without causing undue stress to the appliance under test. Note that the test procedure is slightly different for class 1 and class 2 appliances.

The appliance under test is switched on. If the appliance is earthed then the insulation test is carried out between the earth pin and the combined live and neutral pins on the plug. This relies on a satisfactory result for the earth bond test. If the earthing is faulty this test should not be performed as the results would be totally invalid.

Although the essential tests must be carried out in the order described the optional tests may be carried out in any sequence, but the flash test should only be carried by fully trained personnel preferably under workshop conditions. If an appliance has been recently repaired the essential earth bond and insulation test must be repeated first then the optional test can be carried out. The optional tests, although straightforward, can be difficult to interpret and should therefore only be authorised, carried out and interpreted by highly skilled and preferably electrically qualified personnel.

Testing Class II Equipment

The test includes:

The first test conducted must be the detailed physical examination. Any faults found should be corrected and defective items replaced.

The first electrical test performed will be the insulation resistance test. If the appliance is double-insulated then the insulation test is carried out between the combined live and neutral pins on the plug and a probe that is applied to the outside of the appliance case. Make sure that the appliance under test is turned on. The resistance reading should be at least 2 Mohms. If the resistance reading is less than the value you must investigate why. Do not use the appliance until the fault has been cleared.

Testing Class III Equipment

Class III equipment is designed to be supplied by a reduced voltage which is normally 110V. Portable Appliance Testers are usually rated to operate at 240V and manufacturers usually provide each model in two versions to be able to test class III apparatus separately.

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