Page 461 - Industrial Power Engineering and Applications Handbook

Testing of metal-enclosed switchgear assemblies 141435
14.3.11 Verification of mechanical operation (LT Insulation resistance method
and HT)
The insulation resistance can be checked with the help
This test is conducted to establish the satisfactory of an appropriate megger (MQ-meter), as recommended
functioning of mechanical parts, such as switching devices by IS 101 18-3, and shown in Table 14.7. According to
and their interlocks, shutter assembly, draw-out mecha- this, for an LT system, an insulation resistance of 1 MR
nisms and interchangeability between identical draw- with a 1 kV megger for a completed switchboard,
out modules. A brief procedure to test these features is as irrespective of the number or outgoing circuits, is
follows. considered to be safe. With this insulation resistance, the
switchboard can be put to an HV test or actual use. The
Switching devices values of insulation resistances for higher system voltages
and the recornmended rating of megger are indicated in
These should be operated SO times and removable parts Table 14.7.
inserted and withdrawn 25 times each. IEC 60439- 1 recommends the minimum insulation
resistance for an LT system to be 1 kR/V per circuit
Mechanical interlocks rderred to the rated voltage to the ground.
The interlocks should be set in the intended position to Leakage current method
prevent operation of the switching device and insertion
or withdrawal of the removable parts. Fifty attempts must The leakage current is measured as in IEC 60298. This
be made to operate the switching device and removable method is generally applicable to an HT system by
parts inserted and withdrawn 25 times each. applying the full rated voltage between the insulating
The test may be considered successful if the operations surface, say, between a phase and the ground. The leakage
of the interrupting mechanism, the interlocks and current thus measured Fhould not exceed 0.5 mA.
other mechanical features after the test are found satis-
factory. Illustration
Referring to Table 14.7, the recommended insulation value
will ensure the following leakage current for an LT system:
14.4 Procedure for routine tests System voltage - say, 415 V
Insulation resistance = 1 MR = lo6 (2
The tests against step numbers 1, 2 and 3 in Scction 41 5
14.2.2 are of a general nature and no test procedure is :. Maximum leakage current = -A
10
prescribed. The rest are similar to the tests covered under
type tests. The procedures of tests and requirements of = 0.415 mA
test results will remain the same as discusscd carlicr. If the system had been 660 V, this current would exceed
0.5 mA, which is permissible for an LT system.
For an 11 kV system, the recommended insulation resistance
according to the same table = 100 MR = 100 x lo6 !J
14.5 Procedure for field tests
:. Maximum leakage current = ii xi03 A
The tests against step numbers 1, 2 and 3 are of general 100 x 106
nature (see Section 14.2.4) and no test procedure is laid = 0.11 mA
down for them and for 33 kV = 0.33 mA
4. Verification of insulation resistance or Table 14.7 Insulation resistance for different voltage systems
measurement of the leakage current
Swtem iolrage Minimum inrrr/utroii Tipe of mrgger
The purpose of this test is to check for proper insulation re,ic.tance
of all the insulated live parts and components and to MR -~ kV(dc J
ensure protection of a human body against electrical shocks 1 Auxiliary and Lontrol I for one or inore Mdnudl. 0 5 ~
while the equipment is energized and is in operation. circuit\ (dll secondary circuits
The test detects weak insulation, if any, and this must be wiring circuits)
rectified before putting the equipment into service. 2 For a completed 1 Mdnud. IO
This test should be conducted both before and after switchboard of up to
the HV test if this test is to be camed out. The test before 1000 V with a number
the HV test ensure the quality of insulation and the test of outgoing circuits
~ ~~~ ~__~
after the HV test checks that this has not deteriorated 'i Above 1000 V and I 00 Motorired.
after the HV test. If the insulation resistance is found to up to and including min 25
be lower each circuit and component must be checked 33 kV
separately to identify the weak area and corrective steps 4 Above 33 kV IO00 Motorized.
taken to improve the resistance to the required level. The ~- ~ min 25 ~~~
test can be conducted in two ways. A? in IS I01 18-3