POLE MOUNTED LV FUSE CUTOUTS AND CARTRIDGE FUSE LINKS
1.POLE MOUNTED LV FUSE CUTOUTS AND CARTRIDGE FUSE LINKS
Pole mounted fuse cutout unit shall be used to protect distribution pole mounted transformers connected to overhead networks and feeding into an overhead LV network system.
In this specification, a pole mounted fuse cutout unit is defined as a shrouded apparatus designed for operation from ground level and for use as a fuse unit, a linking switch or a short circuiting device. The cutout shall be suitable for mounting on a wood pole
1.1 Ratings
The cutout bases and fuse carriers shall have the following current ratings at 230/400V:
· three phase and neutral for transformers up to 500kVA: 400A
Allitems of the LV fuse cutout shall be designed to meet the performance requirements of the various clauses of this specification. They shall be adequately rated for their intended application and retain this rating during their normal lifetime in an outdoor environment.
Thermal performance shall be in accordance with BS 88-1 and BS 88-5. All metal parts shall be non ferrous. To facilitate maintenance the design shall be such that when the fuse carrier iswithdrawn a minimum number of parts remain connected to the incoming conductors.
1.2 Housing
The outer housing of the units shall be comprised of grey insulating material of sufficient mechanical strength as to prevent it from becoming damaged due to vandalism. The material shall be resistant to ultra violet degradation and have flame retardant properties.
Grey, colour RAL 7035 code system is that of the DNA (German Standards Board, Frankfurt, Germany).
Lexan 943A unreinforced polycarbonate or any approved cast resin are considered to be suitablematerials.
The fuse carrier shall be required to take J.P. Type cartridge fuse links with 82mm centres, conforming to BS 88. Tightening screws shall have insulated tops to enable the fuse carrier to be removed without safety devices (rubber gloves, etc.).
1.3 Fuse carrier
The cartridge fuse link shall comprise insulating container, extinguishing medium, fuse element, end caps, enclosing the tube ends and anchoring the fuse element and end tags attached to the outside of the end caps and electrically connected to the element. The fuse links supplied shall be of a type or equivalent to the English Electric J.P. Type fuse with 82mm between centres.
Fuse links shall be required to give satisfactory performance for voltages up to 440V. The rated current of the fuse link shall be carried continuously without deterioration of the current carrying parts. Fuse links with the following current ratings are standard in the ZECO rural networks:
· 40 A;
· 80 A; and
· 160 A
· 200A
· 250A
· 315A
· 400A
The fuse link shall be suitable for use with the fuse cutout described in this specification, both from the dimensional and from heat dissipation points of view. The maximum values of power dissipation shall be in conformity with BS 88-5.
The rated breaking capacity of the fuse link shall be 80kA as per BS 88-5. The pre arcing characteristic for each fuse rating offered and the operating time-current characteristic for each fuse rating offered shall be given with the bid.
Figure 5 shows a typical outline of a low voltage J.P. type cartridge fuse link.
1.4 Terminals
The cutout shall facilitate cable entry from the bottom with the fuse carrier also being withdrawn downwards by opening the cover of the cutout.
The maximum cable size which will require to be accommodated is 100sq mm.
1.5 Tests
The cartridge fuse links shall be tested in accordance with BS 88-1 and BS 88-5 or equivalent. At least the following tests shall be conducted:
· verification of rated power dissipation of fuse links: these tests shall be carried out in accordance with BS 88-1 and the maximum values at the rated currents shall be in accordance with BS 88-5;
· verification of time-current characteristics: this test shall be performed in accordance with BS
88-1. The verified points on the plotted time-current characteristic shall not deviate by more than 10% in terms of current from the manufacturer's curves and shall lie within the standard zones as given in the relevant graphs in BS 88-5; and
· verification of conventional non fusing current and conventional fusing current: this test shall be in accordance with BS 88-1.
The performance characteristics of the LV fuse cutouts themselves shall be verified by successful completion of the range of tests listed below. In order to complete the tests, all fittings and accessories comprising the fuse and load break system shall be required (that is: fuse transition pieces, solid links, etc.).
1.5.1 Temperature Rise Test
The unit shall be erected as in the service condition with normal 400A cartridge fuse fitted to the
400A unit and a 630A cartridge fuse to the 630A unit. Air movement of up to 0.6 metres per second shall be permitted in the region of the test unit. The test shall be carried out generally in accordance with the method contained in IEC 947-3 and the results shall meet the requirements of that standard.
1.5.2 Test of Dielectric Properties
This test shall be carried out on the unit which has just completed the temperature rise test after the temperature has returned to ambient (+ 5°C). The test method is as described in IEC 947-3. A
4kV, 50Hz voltage shall be maintained for one minute without flashover.
1.5.3 Wet Voltage Withstand Test
This test shall be carried out after the LV fuse cutout has satisfactorily completed the dielectric properties test. A 50Hz, 4kV, voltage shall be applied between phase and neutral poles. Artificial rain shall be directed onto the unit at 45° and falling at a rate of 3mm per minute (the specific resistance of water shall be 10,000 ohms/cub cm). No flashover shall occur within five minutes but a glow is acceptable.
1.5.4 Making and Breaking Capacities Test
The unit shall be erected as in the service condition with solid links fitted. The test method andrequirements shall be in accordance with utilisation category 21-B, as defined in table IIof IEC
947-3.
1.5.5 Short Time Current Withstand Test
This test is intended to prove the satisfactory operation of the short circuit device and fuse cutout contacts. The unit shall be mounted with the short circuit device in position. In accordance with IEC 947-3, five current pulses are applied. The value of these alternating current pulses shall be
20 times full load current for 1 second. The requirements shall be those of IEC 947-3.
1.5.6 Mechanical Endurance Test
The unit shall be mounted as it will be in service and shall be operated without presence of voltage or current. Three thousand operations shall be performed in the conditions specified in IEC 947-3 and the requirements shall be those stated in the same standard.
1.5.7 Electrical Endurance Test
The unit shall be mounted as it will be in service and shall be operated by hand. One hundred and fifty operations shall be performed on the energised units in the conditions specified in IEC
947-3 and the requirements shall be those stated in the same standard.
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