Keith Wilson MIEE, an electrical engineer, gives his views on the topic of testing electrical installations in buildings:
Dear Editor...
Proper testing of electrical installations in buildings is essential to ensure their safety. However, if the tests are performed with inaccurate instruments, the result is, in some ways, worse than no testing at all - incorrect measurements may create a false sense of security by indicating that all is well when, in fact, there are serious deficiencies.
Because of this, electrical contractors are required to regularly check the accuracy of their instruments. To help them do so, a number of products have appeared on the market, often described as test boxes or check boxes.
The idea behind these boxes - to provide a fast and convenient way of checking instruments without the need to return them to the manufacturer for full calibration - is excellent. Unfortunately, the products currently available are incapable of doing what is claimed for them.
To give just one example of many: when testing insulation resistance of an installation, it's vital not only that the correct result is obtained, but also that the test is carried out at the correct voltage. Present-day test boxes, however, make no attempt at all to verify the test voltage.
This is a potentially dangerous problem. Tested at, say, 100V insulation may appear satisfactory, but a test at the correct voltage - usually 500V - imposes much greater stress and could reveal serious shortcomings.
Concerns of equal magnitude apply to the features provided by the boxes for verifying the performance of continuity test, loop test and RCD test instruments.
In summary, currently available test boxes cannot verify that instruments are suitable for meeting the legal requirement of carrying out tests in line with the 16th Edition of the IEE Wiring Regulations.
Until a test box appears which fully addresses these issues, contractors would be well advised to seek alternative methods of verifying the accuracy of their test equipment.
Such methods are likely to be less convenient and more time consuming than using a test box but they do, at least, eliminate the indisputable risks of relying on test results of uncertain accuracy.
Yours truly,
Keith Wilson MIEE, an electrical engineer
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