Medium Voltage Distribution
MCset sales argument
No. 7 - November 2000
Earthing truck (cmalt)
operation
MCset sales argument
Earthing truck (cmalt)
operation
cmalt.tif
Function
E72893.EPS
The earthing truck has the function of connecting the busbars in one
half-switchboard to earth. Naturally, the earthing operation must be carried out
when de-energised so as not to create a short-circuit.
Interlocking
In order to de-energise a half-busbar in complete safety, it is necessary to use
a key centre. This centre enables the operator to ensure that all of the circuit
breakers are racked out in the half-switchboard to be connected to earth.
The operation is carried out in the following manner (in order to earth busbar A,
cf. figure 1):
b rack out all of the circuit breakers on the half-switchboard, including that of
the bus-sectioning cubicle (circuit breakers in cubicles 1 - 2 - 3),
b lock these circuit breakers in the “racked out” position and remove the keys
which are released (keys D1, D2 and C),
b place these three keys in the corresponding key centre and remove the key that
is released (STA). This then locks the circuit breaker keys in place,
b unlock the earthing truck using the released key, which only enables the base
to be racked in the half-switchboard which has just been de-energised.
fig.1cmalt_en.eps
Figure 1: single line diagram example
AD type
feeder cubicle
AD type
incomer cubicle
Busbar A
D1
CL-GL type bussectioning cubicle
D2
1
C
2
STA
STB
AD type
feeder cubicle
D5
3
Earthing truck
1
AD type
incomer cubicle
Busbar B
D6
4
5
Key centre
C
D2 D1 STA
C
D6 D5 STB
Schneider Electric
MCset sales argument
Earthing truck (cmalt)
operation
Mechanism to disable racking of the earthing
truck in the energised half-switchboard
Cubicles in a switchboard equipped with an earthing truck include a mechanism
to avoid incorrect operation. This fool-proof system is different according to whether
the cubicle is part of the half-busbar A or B in the switchboard (cf. figure 1).
The earthing truck cannot be racked into a cubicle unless the key latch opposite
the fool-proof system is lowered using the right key.
The fool-proof system is mounted on the left in cubicles belonging to the halfswitchboard on the left hand side (cf. figure 2, safety no. 1), on the right in cubicles
belonging to the half-switchboard on the right hand side (cf. figure 2, safety no. 2).
Both of the fool-proof systems are mounted on the bus-sectioning cubicle to make it
impossible to rack in the earthing base (cf. figure 2, safety no. 3).
EclatéVerrou.tif
Safety device no. 1:
Insertion impossible
in the right hand
half-switchboard
fig.2cmalt_en.eps
Figure 2: locking mechanism, front view
Cubicle
Earthing truck
STB
STA
Safety device no. 2:
Insertion impossible
in the left hand halfswitchboard
Cubicle
Earthing truck
STA
STB
Safety device no. 3:
Bus-sectioning cubicle,
impossible to insert
the earthing truck
Cubicle
Earthing truck
STA
Schneider Electric
STB
2
MCset sales argument
Earthing truck (cmalt)
operation
synoptique.tif
Operation
Once the base is unlocked, the sequence of operations is as follows:
b insert the earthing base in one of the cubicles of the de-energised
half-switchboard,
b rack-in the device with the door open,
b manually charge the spring using the handle,
b close the contacts of the earthing truck (press on the closing button).
When the earthing truck is closed, a padlockable runner enables access to
the opening button to be disabled, and thus enables the earthing truck to be locked
in the closed position.
Moreover, an interlock disables the possibility of racking the earthing truck out when
it is closed (as for a circuit breaker). The earthing of busbars can therefore be made
secure using padlocks (three padlocks at most).
fermé.tif
ouvert.tif
When the earthing truck is open, this same runner also enables padlocked
disabling of access to the closing button. In this position, racking out is possible.
Closed
Open
These features should enable an earthing truck to be correctly installed and used
on conventional switchboards having only one bus-sectioning cubicle but with
an unlimited number of feeder and incomer cubicles.
In the case of large switchboards with several bus-sectioning cubicles
(i.e. switchboards in which it is not possible to talk in terms of half-switchboards)
a specific study is required to adapt the interlocks to the configuration in question.
Electrical characteristics
An earthing truck exists for each cubicle width (MCset 1- 2 - 3).
Un
insulation voltage
closing capacity
3
kV
U impulse
power frequency
kA
63
80
100
125
cmalt 1
7.2
60
20
b
b
12
75
28
b
cmalt 2
7.2
60
20
b
b
b
b
12
75
28
b
b
b
17.5
95
38
b
b
cmalt 3
7.2
60
20
b
b
b
b
12
75
28
b
b
b
b
17.5
95
38
b
b
Schneider Electric