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1. The earthing truck with pre-sensing contacts
is inserted into the switchgear till it reaches
its test position.
2. For ensuring safety, the door is closed before
racking earthing truck into service position.
3. The truck is then racked-in from test to service
position.
4. If the pre-sensing contact which is connected
to PT or VDI senses a voltage at cable/bus side
when it get engaged with switchgear contact
pin, it will give an audible alarm as an
indication that the primary side is live. Under
such circumstances one should withdraw the
truck back to test position and investigate the
reason for the system being alive.
5. If no audible alarm is given by earthing truck,
continue the movement of the truck till it fully
reaches service position. Then rack-out the
earthing truck to test position. As safety is
now ensured, open the door, and rack-in the
earthing truck again to its service condition.
Now use the same racking handle rack-in the
secondary contacts to their service position to
ensure complete earthing.
8.4.1.1 Testing the off-circuit condition
In switchgears which are not equipped with
capacitive voltage indication checking the
off-circuit condition is carried out with a HV
tester on the contact pin, the corresponding
upper shutter or lower shutter opened, if the
switchgears are equipped with capacitive voltage
indication. Checking the off-circuit condition
can be carried out by means of this device. In this
case, proceed according to the manufacturer's
instructions for the indicators. In case of any
doubt about correct operation of capacitive
voltage indication, the off-circuit condition must
be checked using a HV tester.
CAUTION!
Checking the off-circuit condition must
always be carried out in compliance
with the relevant safety regulations and
local operating conditions
8.4.2 Earthing truck with making capacity
Earthing trucks with making capacity carry out
the same functions as the earthing switches with
'making capacity', and have the capacity to earth
live circuits.
These trucks consist of circuit breakers fitted
either with top terminals (for main busbar
earthing) or with bottom terminals
(for power cable earthing)
The set of contacts without terminals are short-
circuited by means of a copper bar and connected
to earth through the apparatus truck.
Earthing trucks with making capacity, in general,
retain the important characteristics of circuit
breakers such as full making capacity, and
breaking the live circuits under fault conditions -
ensuring extremely effective earthing even on
circuits stressed under fault.
They have spring actuator, which enables us to
carry out opening and closing operations, rapidly,
also through remote electric control means.
The use of these trucks foresees removal of the
circuit breaker from the switchgear, and its
replacement with the truck.
These trucks are available in two versions:
•
Main busbar earthing
•
Power cable earthing
During the rack-in process, the main busbar
earthing truck operates only the top shutter and
earths the busbar contacts with switchgear
earthing.
Whereas, the power cable earthing truck, during
the rack- in process operates only the bottom
shutter and earths the power cable contacts with
switchgear earthing. These trucks can also be
used in the bus tie units. Wherein, both sides of
the busbar system can be earthed by using both
the types of trucks one after another.
8.4.2.1 Operation of Earthing Trucks with making
capacity
1. Ensure that the incoming supply of the
busbars is turned OFF; by racking-out the
incoming feeder's circuit breaker and outgoing
feeder's circuit breaker, which are feeding the
busbars and /or cables identified for earthing,
as appropriate.
2. Remove circuit breaker from the switchgear
identified for busbar and/or cable earthing,
and replace the circuit breaker with earthing
truck and move it till Test position.
3. Lock the front door of the circuit breaker
compartment.
49
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