Basic electrical installation testing

Concern for public safety and the increasing complexity of fixed electrical installations place extra responsibility on electrical test engineers.

It is therefore important to have suitable test tools for carrying out the stringent tests imposed by the International Electrotechnical Commission (IEC) and the European Committee for Electrotechnical Standardisation (CENELEC).

IEC 60364 and its various associated national equivalent standards (see Table 1), specify the requirements for fixed electrical installations in buildings. Section 6.61 of this standard describes the requirements for the verification of the compliance of the installation with IEC 60364.

Basic requirements of IEC 60364.6.61

Electrical contractors are already familiar with IEC 60364.6.61 or its national equivalents. It states that verification of the installation shall be carried out by visual inspection, followed by testing of:

• Continuity of protective conductors.
• Insulation resistance.
• Protection by separation of circuits.
• Floor and wall resistance.
• Automatic disconnection of supply.
• Polarity.
• Functional performance.

In addition, electric strength test and the voltage drop test are under consideration. IEC 60364.6.61 refers to IEC/EN 61557 to test these protective measures.

The basic requirements of IEC/EN 61557

EN 61557 addresses the requirements for test equipment used in installation testing. It consists of general requirements for test equipment (Part 1) and specific requirements for combined measuring equipment (Part 10), and covers the specific requirements for measuring or testing of:

• Insulation resistance (Part 2).
• Loop impedance (Part 3).
• Resistance of the earth connection (Part 4).
• Resistance to earth (Part 5).
• RCD performance in TT and TN systems (Part 6).
• Phase sequence (Part 7).
• Insulation monitoring devices for IT systems (Part 8).

Testing an electrical installation

The visual inspection is performed first to confirm that permanently wired electrical equipment complies with the safety requirements and is not visibly damaged, and that fire barriers, protective, monitoring, isolating and switching devices, as well as all 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.

Other methods are not precluded provided 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.

Continuity

Testing the continuity of protective conductors is normally done with an instrument able to generate a no-load voltage in the range 4 – 24 V DC/AC with a minimum current of 0,2 A. The most common continuity test is measuring 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.

As continuity testing measures very low resistances, the resistance of the test leads must be compensated for. The Fluke 1650 Series multifunction installation tester has an auto-null feature which measures and stores the test lead resistance even after the instrument has been switched off.

Insulation resistance of electrical installation

Insulation integrity is critical to prevent electric shock. It is generally measured between live conductors and between each live conductor and earth. The complete installation must be switched off, all lamps removed and all equipment disconnected before measuring the insulation resistance between live conductors and earth. All fuses must be left in and circuit breakers and final circuit switches closed.

Measurements are done with direct current using an instrument capable of supplying a test voltage of 1000, 500 or 250 V, depending on the nominal circuit voltage. On single phase supply systems, insulation testing is normally undertaken using a test voltage of 500 V. 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.

According to IEC 60364.6.61, the resistance values should be greater than 1 MΩ for 1000 V test voltage, 0,5 MΩ for 500 V, and 0,25 MΩ for 250 V.

Protection by 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 values obtained should be identical to the values mentioned previously, with all appliances connected.

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.

Verifying protection by automatic supply disconnection

Verification of the effectiveness of the measures for protection against indirect contact by automatic disconnection of supply depends on the type of system. In summary, it is as follows:

• For TN systems: Measurement of the fault loop impedance and verification of the characteristics of the associated protective device.
• For TT systems: Measurement of the earth electrode resistance for exposed conductive parts of the installation and verification of the characteristics of the associated protective device.
• For IT systems: Calculation or measurement of the fault current.

Measurement of the earth electrode resistance

Before testing, the earthing rod must be disconnected from the installation’s main earthing terminal. In doing this, the installation will consequently have no earth protection and must therefore be completely de-energised prior to testing. Earth resistance testing must not be done on a live system.

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.

Measurement of fault loop impedance

Measurement of the fault loop impedance is done using the same frequency as the nominal frequency of the circuit (50 Hz). The earth-loop impedance test measures the resistance of the path that a fault current would take between line and protective earth. This must be low enough to allow sufficient current to flow to trip a circuit protection device such as an MCB.

Determining the prospective fault current (PFC) ensures that the capability of fuses and over-current circuit breakers is not exceeded. The Fluke 1650 Series instruments can also measure the earth resistance component of the total loop resistance, and line impedance (source impedance between line and neutral, or the line-to-line impedance in 3-phase systems) and calculate the prospective short circuit (PSC) current which could flow when there is a short circuit between line and neutral.

Testing RCDs

Residual current operated devices (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 generate enough heat to start a fire. Basic testing of RCDs 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 because some RCDs are more sensitive in one half cycle than the other. The longest time is recorded.

Polarity test

Where local regulations forbid the installation of single-pole switching devices in the neutral conductor, a test of polarity 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 a live phase conductor 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 properly mounted, adjusted and installed in accordance with the relevant requirements of the standard. Protective devices must be functionally tested to check whether they are installed and adjusted properly.

The 1650 Series multifunction testers

The 1650 Series multifunction testers measure up to 500 V AC, and the instruments simultaneously display line voltage level (primary display) and frequency (secondary display). They are easy to set up for making measurements, with a clearly marked rotary control for setting the range, and a straightforward user interface with simple menus for setting test conditions. The display’s wide viewing angle also contributes to user convenience. The control panel markings are available in five languages (English, French, German, Italian and Spanish), and with universally recognised graphical symbols.

Caution!

This article is not intended to replace or supercede the recognised standards in IEC 60364 (or its national equivalents), but to provide a summary of the general requirements. Note that not all tests are mentioned.

Contact Comtest, Tel 010 595-1821, sales@comtest.co.za

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