The Base of Low Voltage Fuse

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.).

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 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.