The test instruments and test leads used by the electrician for testing an electrical installation must meet all the requirements of the relevant regulations. The HSE has published Guidance Notes GS 38 (download below) for test equipment used by electricians. The IEE Regulations (BS 7671) also specify the test voltage or current required to carry out particular tests satisfactorily.
All test equipment must be chosen to comply with the relevant parts of BS EN 61557. All testing must, therefore, be carried out using an ‘approved’ test instrument if the test results are to be valid.
The test instrument must also carry a calibration certificate, otherwise the recorded results may be void. Calibration certificates usually last for a year.
Test instruments must, therefore, be tested and recalibrated each year by an approved supplier. This will maintain the accuracy of the instrument to an acceptable level, usually within 2% of the true value.
Modern digital test instruments are reasonably robust, but to maintain them in good working order they must be treated with care. An approved test instrument costs equally as much as a good-quality camera. It should, therefore, receive the same care and consideration.
Let us now look at the requirements of four often used test meters //
To measure accurately the resistance of the conductors in an electrical installation we must use an instrument which is capable of producing an open circuit voltage of between 4 and 24V AC or DC, and deliver a short-circuit current of not less than 200 mA (Regulation 612.2.1).
The functions of continuity testing and insulation resistance testing are usually combined in one test instrument.
The test instrument must be capable of detecting insulation leakage between live conductors and between live conductors and earth.
To do this and comply with Regulation 612.3 the test instrument must be capable of producing a test voltage of 250, 500 or 1000 V and deliver an output current of not less than 1 mA at its normal voltage.
The test instrument must be capable of delivering fault currents as high as 25A for up to 40ms using the supply voltage. During the test, the instrument does an Ohm’s law calculation and displays the test result as a resistance reading.
Where circuits are protected by an RCD (residual current device) we must carry out a test to ensure that the device will operate very quickly under fault conditions and within the time limits set by the IEE Regulations. The instrument must, therefore, simulate a fault and measure the time taken for the RCD to operate.
The instrument is, therefore, calibrated to give a reading measured in milliseconds to an in-service accuracy of 10%.
If you purchase good-quality ‘approved’ test instruments and leads from specialist manufacturers they will meet all the Regulations and Standards and therefore give valid test results.
However, to carry out all the tests required by the IEE Regulations will require a number of test instruments and this will represent a major capital investment in the region of USD 1000.
Historically, many electrical contractors have had test instruments calibrated on an annual basis. The instrument would be sent away to a calibration house and it would arrive back, some time later, with a certificate stating the date of calibration, the time period for which the certificate would be valid along with the findings of the assessment in the form of a table of results.
What can go wrong with test instruments?
Problems with test instruments are not always immediately obvious so it is important that any irregularities are discovered as early as possible.
Damage to the instrument
Consider the scenario // A test instrument was calibrated by a calibration house six months ago and is still within the stated 12-month calibration period. It appears to be working well but, unbeknownst to the contractor, five months ago it had a collision with a conduit bender in the back of the firm’s van and is now out of calibration.
Therefore, it is extremely important that the instrument is regularly assessed.
Prospective fault current test (VIDEO)
- Advanced Electrical Installation Work by Trevor Linsley (Purchase paperback)
- Ongoing accuracy of test instruments by Mark Coles