8.7.1 - Basic requirements

Guidance Note 3 - Inspection and Testing - makes it clear that instrument accuracy is required to be at least that shown later in this sub-section for the various types of instrument. Regular recalibration using standards traceable to National Standards is now required, together with checking after any incident which has involved mechanical mishandling. Many electrical installers will not be used to sending their instruments regularly for recalibration, but must now do so. Guidance Note 3 is not specific on the time intervals at which recalibration must be carried out, but it would seem sensible for occasionally used instruments to receive attention every two years, whilst those used frequently are likely to need recalibration annually.

If installations are to be tested to show that they comply with BS 7671, the following instruments will be necessary. After the name of the instrument are brief notes which may be helpful in choosing a new instrument or in deciding if one already to hand will be satisfactory. The first four instruments listed are absolutely essential for all tests, although the low resistance tester and the insulation resistance tester may be combined in a single instrument. An instrument which can test continuity, insulation resistance, earth-fault loop impedance and the operation of RCDs is available from many manufacturers, and will save a great deal of time in changing instruments and test leads - even more time can be saved with an instrument which will save the test results for downloading to a computer. The last two instruments will not often be required on simple installations, since applied voltage and earth electrode resistance tests are often not needed. All instruments used should conform to the appropriate British Standard safety specification (BS EN 61010 for electronic types, and BS 5458 for electrical instruments).  Test leads, including prods and clips, must be in good order and have no cracked or broken insulation. Fused test leads are recommended to reduce the risk of arcing under fault conditions.

The basic accuracy of +5% quoted below is for digital instruments. Analogue instruments should have an accuracy of +2% at full scale deflection, which will give the required accuracy of +5% over the useful part of their scales.

Low resistance ohmmeter
Instruments to BS EN 61557-4 will comply.
Basic instrument accuracy required is +/- 5%
Test voltage ac or dc, between 4 V and 24 V
Test current not less than 200 mA
Able to measure to within 0.01 Ohms (resolution of 0.01)
May be the continuity range of an insulation resistance tester.

Insulation resistance tester
Direct test voltage depends on the circuit under test, but will be:-
250 V for extra-low voltage circuits
500 V for other circuits supplied at up to 500 V
1000 V for circuits rated between 500 V and 1000 V.
Must be capable of delivering a current of 1 mA at the minimum allowable resistance level, which is:-

250 kOhms for the 250 V tester
500 kOhms for the 500 V tester
1 MOhms for the 1,000 V tester
Basic instrument accuracy required is +/-5%
Must have a facility to discharge capacitance up to 5 microF which has become charged during the test
May be combined with the low resistance ohmmeter

Earth-fault loop impedance tester
Instruments to BS EN 61557-2 will comply
Must provide 20 to 25 A for up to two cycles or four half-cycles
Basic instrument accuracy required is +/-5%
Able to measure to within 0.01 Ohms (resolution of 0.01)
Instruments to BS EN 61557-3 will comply

Residual current device (RCD) tester
Must perform the required range of tests (see (8.6.3))
Suitable for standard RCD ratings of 6.10, 30, 100, 300 and 500 mA
Must NOT apply full rated test current for more than 2 s
Currents applied must be accurate to within +/-10%
Able to measure time to within 1 ms (resolution of 1)
Must measure opening time with an accuracy of +/-5%

Applied voltage tests
Must apply a steadily increasing voltage measured with an accuracy of +/-5%
Must have means of indicating when breakdown has occurred
Must be able to maintain the test voltage for at least one minute
Maximum output current must not exceed 5 mA
Maximum output voltage required is 4,000 V

Earth electrode resistance tester
Basic instrument accuracy required is +/-5%
Must include facility to check that the resistance to earth of temporary test spikes are within limits
Able to measure to within 0.01 Ohms (resolution of 0.01)