Page 1012 - Industrial Power Engineering and Applications Handbook

Testing a metal-enclosed bus system 32/957
Effect of solar radiation 32.3.7 Verification of protective circuits
The effect of solar radiation must be considered in the All the protective circuits will be checked for continuity
outdoor part of a bus enclosure exposed to solar heat. and operational requirements.
This effect may be observed for all LT and HT bus
enclosures, as all are influenced equally by such radiations, 32.3.8 Verification of clearance and creepage
particularly IPB enclosures, as discussed in Section 3 I .4.4. distances

32.3.5 Verification of short-circuit strength The clearance will be verified as in Table 28.4 while for
creepages Table 28.5 may be followed.
For straight lengths and tap-offs
32.3.9 Verification of degree of protection
The test procedure is generally the same as that discussed
for switchgear assemblies (Section 14.3.6). It is carried Enclosure test
out on a similar section to that used for the temperature
rise test and the power supply can be single-phase or The types of protection and their degree are generally
three-phase. If three-phase, the bus conductors are shorted the same as those defined for motors in Section 1.15.
at one end while the other ends are connected to the Tables I. 10 and I. 1 I. The test requirements and methods
power source. If a single-phase power is used, the circuit of conducting such tests are also almost the same as for
should be so arranged that the current flows through the those motors, discussed in Section 1 1 S.3.
two adjacent phase conductors. The force due to a three-
phase fault would be approximately 86.6% of this. Watertightness test
Therefore, for single-phase tests, the current would be
zix6.6 or approximately 93% of that prescribed in Table This test is applicable to all outdoor parts of any bus
13.10. The test results should show no sign of undue system and on both indoor and outdoor parts of an isolated
deformation. Slight deformation may be acceptable, phase bus (IPB) system. Each enclosure to be tested
provided that the clearance and the creepage distances (such as the enclosure of one phase in an isolated phase
are complied with and the degree of protection is not bus system) should be complete in all respects with all
impaired. The insulation and mounting supports, however, its fittings and mounts in place. The length of the testpiece
must show no signs of deterioration. must be the same as that considered for the temperature
rise test (Section 32.3.4).
As ANSI C-37120C and IS 8084 water under a head of
For the tap-08s in a power-generating station, 1 1 m through a hose of 25 mm diameter held at 3 m from
connecting a UAT through the main bus section the enclosure will be impinged at an angle of at least 45"
between the generator and the generator from the horizontal on all sides and the entire length of
transformer the enclosure for a period of 5 minutes. No water should
enter the enclosure.
As discussed in Section 13.4.1(5), these sections are under
the cumulative influence of two power sources and may
be tested for a higher short-time rating, which would be Air leakage test
the algebraic sum of the two fault levels, one of the This test is prescribed by ANSI C-37/20C for isolated
generator and the other of the generator transformer as phase bus systems after they have been assembled at
noted in Table 13.8. Also refer to Figures 3 1. I and 13.18 site. The test is conducted by filling the enclosure with
for more clarity. air up to a pressure of 15 cm of water (1 500 N/m'). After
the air supply is shut off, the pressure must not drop to
For the ground bus less than 7.5 cm of water in 15 seconds. All breathers
and drain holes must be sealed before the test.
In an isolated phase bus (IPB) system, the ground bus
must be capable of carrying the same short-time current
as for the main conductors for 2 seconds, for both 32.3.10 Measurement of resistance and
discontinuous and continuous grounding systems. reactance
The mean values of the resistance and reactance per phase
32.3.6 Verification of momentary peak or are determined on at least two trunking units. including
dynamic current the joints, from a total length of 6 m. If the total length
The test method and test results will generally be the of the bus section is less than this, the values may be
same as for a switchgear assembly. discussed in Section dctcrmined on the entire length of the bus system.
If Vdt, Vd2 and Vd, are the voltage drops in the three
14.3.7. phases, then the average voltage drop
For the tap-offs, connecting a UAT through the main
bus section between the generator and the generator 'dl
transformer. however, as discussed above, the momentary Vd = vd2 + vd3 volts per phase and
3
peak current will depend upon the short-time rating of I, +I, +I, Amps per phase
such tap-offs. The likely ratings are noted in Table 13.8. average current I,, =
3