The increasing complexity of electrical installations places extra responsibility on electrical test personnel charged with conforming to stringent international standards.
It is imperative to use suitable test tools for performing the stringent tests imposed by the International Electrotechnical Commission (IEC).
IEC 60364 and its national equivalent standards specify the requirements for fixed electrical installations in buildings. Section 6.61 of this standard describes the requirements for the verification of compliance with IEC 60364.
Basic IEC requirements
IEC 60364.6.61 and its national equivalents state that verification of the installation shall start with visual inspection and followed by testing of:
IEC 60364.6.61 refers to IEC/EN 61557 to test the protective measures as described here.
The basic requirements of IEC/EN 61557
The European Norm EN 61557 addresses the requirements for test equipment used in installation testing. It consists of general requirements for test equipment (Part 1), specific requirements for combined measuring equipment (Part 10) and covers the specific requirements for measuring/testing:
Testing an electrical installation
The visual inspection is carried out first, to confirm that permanently-wired electrical equipment is compliant with the safety requirements and not visibly damaged, and that fire barriers, protective, monitoring, isolating and switching devices and the relevant documentation are present. Electrical testing may commence after this inspection.
Note that the test methods described are given as reference methods in IEC 60364.6.61.
Fig. 1: It is essential to use the right tools for testing to IEC standards.
Other methods are not precluded, provided that they give equally valid results. Only with the appropriate experience and training, safe clothing and the right test tools is a person considered competent to test installations to IEC 60364.6.61. When testing is undertaken it should be ensured that adequate precautions are taken to avoid damage or injury to people, equipment or property, and ensured that unauthorised persons are kept away from danger.
Continuity
Continuity testing of protective conductors is normally carried out with an instrument able to generate a no-load voltage in the range 4 to 24 V (DC or AC), with a minimum current of 0,2 A. The most common continuity test is to measure the resistance of protective conductors, which involves first confirming the continuity of all protective conductors in the installation, and then testing the main and supplementary equipotential bonding conductors.
All circuit conductors in the final circuit are also tested and the resistance of the test leads must be compensated for as continuity testing measures very low resistances.
Insulation resistance
Insulation integrity is critical to prevent electric shock. It is generally measured between live conductors and between each live conductor and earth. The installation must be switched off, all lamps removed and all equipment disconnected to measure the insulation resistance between live conductors and earth. All fuses must be left in, and circuit breakers and final circuit switches closed.
With direct current, measurements are taken with instruments able to supply test voltages of 1000, 500 or 250 V, depending on the nominal circuit voltage. On single-phase supply systems, insulation testing is normally done using a 500 V test voltage.
Before testing, disconnect equipment and take measures to prevent the test voltage damaging voltage-sensitive devices such as dimmer switches, delay timers and electronic starters for fluorescent lighting.
The resistance values, according to IEC 60364.6.61, should be greater than 1 MΩ for 1000 V test voltage; 0,5 MΩ for 500 V and 0,25 M MΩ for 250 V.
Separation of circuits
The separation of the live parts from those of other circuits and from earth should be verified by a measurement of the insulation resistance. The resistance values obtained should be identical to the values mentioned previously, with all appliances connected (as far as possible).
Floor and wall resistance
If applicable, at least three floor and wall resistance measurements should be made per location, one being approximately 1 m from any accessible extraneous-conductive part in the location, with the remaining two measurements taken at greater distances. The series of measurements is repeated for each relevant surface of the location.
Automatic supply disconnection
Verification of the effectiveness of the protection measures against indirect contact by automatic disconnection of supply depends on the type of system. In summary, it is as follows:
Earth electrode resistance
Measurement of the resistance of an earth electrode is done, for example, by means of two auxiliary earth electrodes or “spikes”. The earthing rod must be disconnected from the installation’s main earthing terminal before testing. As a consequence, the installation will have no earth protection and must therefore be de-energised completely prior to testing. Earth resistance testing must not be done on live systems.
One auxiliary electrode is placed at a set distance from the earth electrode and the other at 62% of the distance between the two in a straight line. The test measures the earth resistance and detects the voltage between the auxiliary electrodes and, if this exceeds 10 V, the test is inhibited.
Fault-loop impedance
Fault-loop impedance is measured by using the nominal frequency of the circuit (50 Hz). The earth-loop impedance test measures the resistance of the path a fault current would take between line and protective earth, which must be low enough to allow sufficient current to flow to trip a circuit protection device such as a miniature circuit breaker.
Determining the prospective fault current (PFC) is important to ensure that the ability of fuses and over-current circuit breakers is not exceeded. Measuring loop impedance can actually trip current-operated devices (RCDs) in the under test, preventing further measurement.
Testing RCDs
RCDs are often fitted for additional protection, where they detect currents flowing to earth that are too small to trigger over-current operated protective devices or to blow fuses, but would still be sufficient to cause a dangerous shock or to generate enough heat to ignite. Basic RCD testing involves determining the tripping time (in milliseconds) by introducing a fault current in the circuit.
The test is done for both 0 and 180° phase settings. The longest time is recorded because some RCDs are more sensitive in some half-cycles than in others.
Polarity test
Where local regulations forbid the installation of single-pole switching devices in the neutral conductor, a polarity test must be done to verify that all such devices are connected in the phase only. Incorrect polarity results in parts of an installation remaining connected to live phase conductors even when a single-pole switch is off, or an over-current protection device has tripped.
Functional test
All assemblies such as switchgear and control gear assemblies, drives, controls and interlocks should be functionally tested to show that they are mounted properly, adjusted and installed in accordance with the relevant requirements of the standard. Protective devices must be tested to check whether they are properly installed and adjusted.
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