SOP Earthing

PROCEDURE FOR INSULATION RESISTANCE MEASUREMENT 1. Purpose: The purpose of this document is to establish a uniform procedure for Earth RESISTANCE MEASUREMENT. 2. Scope: This instruction applies to all the staff and officers who are involved in INSPECTION AND MAINTENANCE of Electrical equipment. 3. Responsibility: i) It is the responsibility of the ELECTRICAL INCHARGE to ensure that these instructions are followed. ii) No changes will be made to these instructions without the authorization of the Installation Manager. 4. Process: 4.1 Equipment Detail : Earthing Electrodes. 4.2 Procedure 6.3.2 Earthing & Bonding The following shall be General Inspection items: i)

Visual checks for external damages to earth conductors, earth electrodes and earthing connections.

M

ii)

Operation of earth (Simulation Tests)

HY

iii)

Corrosion effects and tightness of connections

HY

iv)

Identification of earth electrodes in the field with permanent markers

HY

v)

Earth resistance on individual wet weather

HY

vi)

Continuity of earthing conductors

Y

vii)

Bonding and earthing of all metallic structures, pipe racks, armour of cables, and electrical equipment enclosures

Y

leakage annunciation and trip circuits

electrodes once in dry and one in

viii) Whether buried earthing conductors are exposed at ix)

Earth loop impedance (measurement)

any location TA

The following shall be the Special items Inspection applicable to different categories of installation: i)

Neutral earthing of the electrical system

HY

ii)

Body earthing of the electrical equipment

HY

iii)

Tankage (bonding of floating roof, ladder rollers, rails of staricase HY and lubrication on rollers) Truck loading HY Process equipment in the plant area (such as heat exchanger, Y vessel, column, piping etc.) whether grounded.

iv) v)

LEGEND: D : Daily W : Weekly F : Fortnightly M : Monthly

HY : Half yearly nY : Once in n years TA : Turn around Q: Quarterly

ii) MEASUREMENT OF EARTH- RESISTANCE a) Earth resistance can be directly read through an earth test Megger which has associated Test, auxiliary Current and Potential electrodes. This instrument which is a combination of ohmmeter and generator works on ‘fall of potential’ principle. Test voltage is derived from the generator of the Megger. MEASURING METHODS (Earth Tester model ST-1 5 2 0)

·

Connect green, yellow and red test leads to instrument terminals E, P and C with auxiliary earth bars P1, C1 driven into earth "IN A STRAIGHT LINE". (Fig. above)

· ·

·

Set the function switch to ACV position and make certain that the voltage reading is less than 10V AC, otherwise accurate earth resistance measurement may not be made. Set the Range Switch to suitable range and set the function switch to ohm position then press the PUSHON BUTTON and TIMER ON BUTTON at the same time and take the reading on the display. When none of E, P and C terminals connected with test leads, the display shows "1" at ohm function. Follow the proper connection such as Fig above , the LED(red) indicator will lit. This proves a correct current circulation is under its operation.

b) Allowable earth-Resistance Values The resistance value of an earthing system to general mass of the earth should not exceed. n 4 Ohms for electrical systems and metallic structures n 7 Ohms for storage tanks n 1 Ohm for main earth grid, and bonding connections between joints in pipelines and associated facilities. n 2 Ohms for each electrode to the general mass of the earth.