Page 145 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

To facilitate this the DC produced by the the hand generator is changed to AC through a current reverser. After
the alternating current passes through the electrodes, the measurement should be done by an ohmmeter which
requires DC supply.

To change the alternating voltage drop outside the instrument to direct voltage drop inside, a synchronous rotary
rectifier is used (Fig 2)
Sometimes the meter needle vibrates during measurement due to the fact that strong alternating currents of the
same frequency as the generated frequency enters the measuring circuit.
In such cases the handle rotating speed of the instrument may be either increased or decreased. In general these
instruments are designed such that the readings are not affected by strong currents or by electrolytic emfs.
Method of earth resistance measurement:To measure the earth electrode resistance, the earth electrode is
preferably disconnected from the installation. Then two spikes (the current and pressure spikes) are to be driven
into the ground at a straight line at a distance of 25 metres and 12.5 metres respectively from the main electrode
under test. The pressure and current spikes and the main electrode need to be connected to the instrument
( Fig 1)
The earth tester has to be placed horizontally and is rotated at a rated speed (normally 160 r.p.m.). The resistance
of the electrode under test is directly read on the calibrated dial. To ensure correct measurement, the spikes
are placed at a different position around the electrode under test, keeping the distance the same as in the first
reading. The average of these readings is the earth resistance of the electrode.

I.e. rules pertaining to earthing
Earthing shall generally be carried out in accordance with the requirements of Indian Electricity Rules 1956, as
amended from time to time, and the relevant regulations of the electricity supply authority concerned. The following
Indian Electricity Rules are particularly applicable to both system and equipment earthing: 32,51,61,62,67,69,88(2)
and 90.

Extracts from Indian Electricity Rules, 1956
Rule no. 32: Identification of earthed and earthed neutral conductors and position of switches and
cut-outs therein.

Where the conductors include an earthed conductor of a two-wire system or an earthed neutral conductor of a
multi-wire system or a conductor which is to be connected thereto, the following conditions shall be compiled with.
1 An indication of a permanent nature shall be provided by the owner of the earthed or earthed neutral conductor,
or the conductor which is to be connected thereto, to enable such a conductor to be distinguished from any live
conductor. Such indication shall be provided:
a where the earthed or earthed neutral conductor is the property of the supplier, at or near the point of
commencement of supply
b where a conductor forming part of a consumer’s system is to be connected to the supplier’s earthed
or earthed neutral conductor at the point where such connection is to be made.
2 No cut-out, link or switch other than a linked-switch arranged to operate simultaneously on the earthed or
earthed neutral conductor and live conductors shall be inserted or remain inserted in any earthed or earthed
neutral conductor of a two-wire system or in any earthed or earthed neutral conductor of a multi-wire system
or in any conductor connected thereto with the following exceptions:
a a link for testing purposes or

b a switch for use in controlling a generator or transformer.
Rule no.51: Provisions applicable to medium, high or extra high voltage installations
All metal work enclosing, supporting or associated with the installation, other than that designed to serve as ta
conductor, shall, if considered necessary by the Inspector, be connected with earth.

132

CITS : Power - Electrician & Wireman - Lesson 20-25