Field Quality Control Method for Electrical Earthing or Grounding System Installation



Earth electrode resistance, Insulation resistance of factory-built materials installed at the site and Insulation resistance of site-built materials installed at site to be considered individually for field quality control check. It is further to be monitored earth resistance of individual earth unit and combined resistance of earth units.



Field Quality Control Method of Earth Electrode Resistance

To measure the resistance of an earth electrode, the test method shall be as described in BS 7430: 1991, Section 3, Part 16. Where it is not practicable to drive auxiliary earth test electrodes into the general earth mass then any known good earth may be utilized subject to agreement with the Consultant.

Combined resistance of earth electrodes is to be measured during dry season and checked against specified resistance.

Field Quality Control Method of Excessive Ground Resistance

If resistance to ground exceeds specified values, notify the Engineer promptly and include recommendations to reduce ground resistance. Electrical continuity of all earthing and protective conductors including main and supplementary equi-potential bonding conductors is to be checked.

Field Quality Control Method of Insulation Resistance

Every test described in BS 7671 (lEE Regulations) Section 713-04 shall be made and proven satisfactory before the installation is permanently connected to the supply.

A DC voltage not less than twice the normal voltage of the circuit concerned (RMS value for an AC supply) shall be applied for the measurement of insulation resistance, provided that the test voltage need not exceed 500 V DC for installations rated up to 500 V or 1000 V DC for installations rated above 500 V up to 1000 V (All in accordance with BS 7671 lEE Regulation 713-04-04).

When measured between all the conductors connected to any one phase or pole of the supply and, in turn all conductors connected to each other phase or pole, the insulation resistance shall be not less than 1 megohm. Wherever practicable, so that all parts of the wiring may be tested, all lamps shall be removed and all current using equipment shall be disconnected and all local switches controlling lamps or other equipment shall be closed.

Where the removal of lamps and / or the disconnection of current using equipment is impracticable the local switches controlling such lamps and / or equipment shall be open. Particular attention shall be given to the presence of electronic devices connected in the installation and such devices shall be isolated so that they are not damaged by the test voltage, (in accordance with BS 7671 lEE Regulation 713-04-02).

Where equipment is disconnected as prescribed in the BS 7671 lEE Regulations and the equipment has exposed conductive parts, the equipment shall be tested in accordance with BS 7671 lEE Regulation 713-04-06.

Field Quality Control Method of Insulation of Site-Built Assemblies

The protection against direct contact is intended to be afforded by insulation applied to live parts during erection in accordance with Regulation 412-02, it shall be verified that the insulation is capable of withstanding, without  break-down or flash-over, an applied voltage test equivalent to that specified in the British Standard for similar factory-built equipment (as required by BS 7671 lEE Regulation 713-05-01 ).

Where protection against indirect contact is provided by supplementary insulation applied to equipment during erection in accordance with BS7671 (lEE Regulation) 413- 03 it shall be verified by test (in accordance with lEE Regulation 713-05-02).

Field Quality Control Method of Earth Fault Loop Impedance (ZS)

Earth fault loop impedance of all circuits is to be measured and checked against calculated impedance figures. Use a 5 A 50 Hz test current. Measure and record source impedance (ZE). If alternative LV supply arrangements are available, measure ZS when using supply with highest impedance.

Measure ZS with main equi-potential bonding conductors connected. Do not summate values of several parts of each loop.

Field Quality Control Method of Earth Bonding of Conductive Parts

Verify all necessary bonding of exposed and extraneous metal work has been carried in accordance with BS 7671. Test conductive parts simultaneously accessible with exposed conductive parts of extraneous conductive parts. Establish that they are either not an extraneous conductive part, or that they are reliably connected by metal to main equi-potential bonding.

Confirm conductive parts, which are not extraneous conductive parts are separated from earth by an impedance greater than 50,000 ohms.

Confirm other conductive parts are bonded to equi-potential zone earth bar by an impedance not exceeding 0.1 ohms.

Additional Instructions for Earthing Field Quality Control Method

Field tests shall include all tests as per NETA.

All test records shall be numbered and certified.