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Measurement of Earth Electrode Resistance

Fall of Potential Method

In this method two auxiliary earth electrodes, besides the test electrode, are placed at

suitable distances from the test electrode are placed at suitable distances from the test

electrode. A measured current is passed between the electrode A to tested and an

auxiliary current electrode C and the potential difference between the electrode A

and the auxiliary potential electrode B is measured. The resistance of the test

electrode A is then gi en by!

V

R = ---------

I

"here

# $ #esistance of the test electrode in ohms,

% $ #eading of the olts, and

I $ #eading of the ammeter in amperes

If the test is made at power fre&uency, that is '( c)s, the resistance of the

oltmeter should be high compared to that of the auxiliary potential electrode B and

in no case should be less than *(,((( +hms. The source of current shall be isolated

from the supply by a double wound transformer. At the time of test, where possible,

the test electrode shall be separated from the earthing system. The auxiliary

electrodes usually consist of *.'mm dia mild steel rods dri en up to m into the

ground.

All the test electrodes and the current electrodes shall be so placed that they

are independent of the resistance area of each other. If the test electrode is in the formof rod, pipe or plate, the auxiliary current electrode -C shall be placed at least /(m

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away from it and the auxiliary potential electrode -B shall be placed midway

between them.

The test shall be repeated by increasing the distance between the electrodes

-A and -C up to '(m and each time placing the electrode -B mid way between

them until / consecuti e readings of test electrode resistance with different spacing of

electrode agree.

0. 1oil #esisti ity

1oil resisti ity is largely depends up on the type of soil as shown in the table

gi en below. 2arth conducti ity is affected by moisture content of the soil, chemical

composition and concentration of salts dissol ed in the water, closeness of pac3ing,

grain si4e, as they decide manner in which moisture is stored.

0. 5actors which 6etermine #esisti ity of 1oil

a7 Type of 1oil

b7 8oisture Content

c7 Chemical Composition of 1alt dissol ed in the contained water.

d7 Concentration of 1alts

e7 Temperature of 8aterials

f7 9rain si4e and distribution of grain si4e

g7 Closeness of pac3ing

Type of 1oil #esisti ity

In +hm:Cm

. ;oamy 9arden 1oil '(( < '(((

*. Clay =(( < '(((

/. Clay, 1and and 9ra el 8ix >((( < *'(((

>. 1and and 9ra el ?((( < (,(((

'. 1lates, slab 1and 1tone ((( < '(,(((

?. Crystalline #oc3s *(,((( < ,((,(((

"here there is any option, a site should be chosen in the order of preference shown

below.

a7 "et marshy ground

b7 Clay loamy soil, arable land, clayey soil, clayey soil or room mixed with small

&uantity of sand@

c7 Clay and loom mixed unit arying proposition of sand, growl and stores@d7 6amp and wet sand pit.