Page 465 - IRSEM_Main Book

(a) Test Procedure: At the selected test site, four electrodes are driven into the
earth along a straight line in a chosen direction at equal intervals 'S'.

(b) The depth of the electrodes in the ground shall be of the order of 10 to 15 cm.
The megger is placed on a steady and approximately level base, the link
between terminals P1 and C1 opened and the four electrodes connected to
the instrument terminals as shown in the fig. An approximate range on the
instrument is then selected to obtain clear readings, avoiding the two ends
of the scale, as far as possible. The readings are taken while turning the
crank at about 135 rev/min (in case of Magneto Generator Type) or pressing
concerned button in some Other Type of Meggers. Pl see equipment
Manufacturers manual and follow as stated there in.

(c) Resistivity is calculated by substituting the values of 'R' in the equation P =
2πSR.

(d) The earth resistance shall be measured using suitable method. For
measurement of small earthing system like that of single electrode or ring
earth, ‘Fall of Potential’ method can be used.

The typical diagram and measurement method is given in Annexure: 19-A3.

19.11.8 Maintenance of outdoor earths

(a) Watering:

Conventional Earths shall be regularly watered. Earth enhancement
material should be periodically added to Maintenance free earth to improve
the earth resistance. Earth pit to be regularly cleaned.

(b) Earth Connections:

All Earth connections shall be carefully examined and kept intact and joints
soldered. The wire between each earth and the connected equipment shall
be electrically isolated. The exothermic welding termination on maintenance
free earth rod shall be checked and cleaned.

19.11.9 Regular Checks/Upkeep of Earths
(a) Block earths and their connections shall be examined at intervals of not
more than one month by JE (Signals) and not more than three months by
Sectional SSE(Signal)/Incharge.

(b) Block earths shall be tested for resistance at intervals of not more than 12
months by Sectional JE/SSE (Signal). Where the resistance exceeds 10
ohms, action shall be taken to reduce the resistance by providing additional
earths in parallel.

(c) If routine testing indicates that existing earth electrode system is not
satisfactory, a new earth electrode system (or part of a system to supplement
the existing system) shall be provided.

Chapter 19: Installation, Testing & Maintenance of Signalling Equipment Page 413 of 535