Field of Invention
The present invention relates to a high current proof terminal bushing used in low and medium voltage electrical appliances like motors, generators etc. The invention particularly relates to flame proof, explosion proof terminal bushings capable to withstand high fault currents and generally used in high inflammable environments viz., petrochemical plants, oil refineries, gas pipelines, etc.
BACKGROUND OF THE INVENTION
Conventional terminal bushings [01] use a central high tension (HT) conductor [02] integrated with a radial built up insulation of epoxy [03]. The length of the bushing on either side of the system commensurates with the medium (air or oil). The bushing assembly is housed in a terminal box with suitable fixing arrangement. The reliability of the above system is governed by following parameters:
- Quality of the metal conductor and connectors during service.
- Void free preparation of the epoxy insulation.
- Heat conduction and thermal expansion based incompatibilities.
- Life of epoxy insulation.
Since conventional terminal bushings are moulded as a single piece i.e. integrated in nature, it is difficult to replace or to maintain if there is a minor damage to the conductor or to the insulator. The conventional bushing cannot withstand for higher fault currents as the HT conductor is integrated with epoxy
insulation. This is mainly due to difference in thermal expansion of epoxy and copper. Alternatively, isolating HT conductor and epoxy insulation can improve performance of the terminal bushing for high fault currents. However such an arrangement may not be able to withstand for medium voltage systems due to partial discharges in service.
It is therefore the object of the present invention to propose a high-fault current proof, flame proof, explosion proof terminal bushing assembly which does not have the disadvantages of the prior art.
A further object of the present invention is to propose a high-fault current proof, flame proof, explosion proof terminal bushing assembly which can withstand higher fault currents in the range of 50KA and above.
Still another object of the present invention is to propose a high-fault current proof, flame proof, explosion proof terminal bushing assembly which is simple in design and takes less time for manufacture and assembly;
Yet another object of the present invention is to propose a high-fault current proof, flame proof, explosion proof terminal bushing assembly which replaces the conventional integrated bushing assembly by an inexpensive terminal bushing assembly in which the HT-conductor is removable/replaceable.
Accordingly, there is provided a high current proof terminal bushing assembly for low and medium voltage electrical appliances comprising an epoxy body and a high tension (HT) conductor the epoxy body accommodes a high tension (HT) sleeve and a low-tension (LT) insert the LT insert being configured on an outer surface of the epoxy body, the HT-conductor being inserted through the epoxy body, to serve as an electrical connection between an electric appliance and a power supply source, characterized in that the HT-conductor is configured as a separate replaceable component being assembled with the HT-sleeve by means of locking means.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS.
Fig. 1: Assembly of the conventional terminal bushing.
Fig. 2: HT sleeve of the high current proof terminal bushing.
Fig. 3: HT conductor of the high current proof terminal bushing.
Fig. 4: LT insert of the high current proof terminal bushing.
Fig. 5: Epoxy body of the high current proof terminal bushing.
Fig. 6: Assembly of the proposed high current proof terminal bushing.
Detailed description of the preferred embodiment.
The high tension (HT) sleeve [04] made of brass or stainless steel is arranged at the center of the mould. The periphery of the sleeve is provided with number of step cut surfaces [05] to enhance the mechanical strength of the bushing at the interface of epoxy and sleeve. The HT sleeve is provided with a hexagonal locking means [06,07] on either side matching with a locking means [09] made of copper provided to the HT conductor [08].and with a lock nut [10] respectively. The HT conductor is designed in such away that one of its ends enters inside the terminal bushing. A hexagonal locking means configured at the frontal portion of the HT conductor [08] is so designed that it shall match with the hexagonal locking means [06, 07] provided at either end of the HT sleeve [04].
The LT insert [11] is proposed on the outer surface of the epoxy body. The inner surface of the LT insert is provided with number of step cut surfaces [12] to enhance the mechanical strength of the bushing at the interface of epoxy and LT insert. The outer surface of the LT insert is provided with a locking means [13] to ensure fastening to connecting modules like terminal box or mounting flanges.
The HT conductor, which is not an integral part of the epoxy body [14] of the proposed terminal bushing [15] is inserted through the epoxy body to serve as an electrical connection between the electrical equipment and the incoming/outgoing of the power supply. After inserting the HT conductor
through the bushing, the locknut [10] is placed and securely tightened with a brass nut [16]. The unique arrangement arrests any movement of the copper conductor during service. Since the HT conductor is not an integral part of the bushing, this arrangement facilitates easy assembly, maintenance and replacement.
The moulding of the terminal bushing is carried out through a process as described in the Patent Application No. 1218/ DEL/2002 dt. 09.12.2002. Brominated epoxy resin is thoroughly mixed with fillers, like Aluminium trihydrate and silica powder for about 8 hours in a chamber which is maintained under vacuum of 5 torr and at a temperature of about 65°C. The hardener used in the present invention is carboxylic acid anhydride. Hardener is also kept in another chamber maintained under the same conditions. This operation of maintaining the vacuum and temperature in both the chambers for about 8 hours is necessary to remove any dissolved gases present in the materials. After this operation, the filler mixed resin and the hardener are blended to get a homogenous material. For 100 parts by weight of brominated epoxy resin, 135 parts each of fillers aluminium trihydrate and silica powder are added and 65 parts by weight of carboxylic acid anhydride is used. For moulding of this material into a bushing of the present invention, a mould is fabricated to the required dimensions of the high current proof bushing [15]. The LT insert [11] and HT sleeve [04] are placed nside the mould. The homogenous resin mix, which will be in semi-solid
state, is pumped into the mould under pressure of 2 to 3 atmospheres. The temperature of the mould is constantly maintained at about 130°C to 150°C and pressure of 2 to 3 atmospheres. Under these conditions of pressure and temperature, the resin mix is kept in the mould for 3 to 5 hours for curing. The cured epoxy body [14] is then removed from the mould and kept in an air-circulating oven at a temperature of 130°C to 150°C for 07 to 09 hours.

We Claim:
1. A high current proof terminal bushing assembly for low and medium voltage electrical appliances comprising an epoxy body (14) and a high tension (HT) conductor [08] the epoxy body [14] accommodes a high tension (HT) sleeve [04] and a low-tension (LT) insert [11], the LT insert [11] being configured on an outer surface of the epoxy body [ 14], the HT-conductor [08] being inserted through the epoxy body [14], to serve as an electrical connection between an electric appliance and a power supply source, characterized in that the HT-conductor [8] is configured as a separate replaceable component being assembled with the HT-sleeve [04] by means of locking means [06, 07;09} 10].
2. The high current proof terminal bushing assembly as claimed in claim 1, wherein the said HT sleeve [04] is made of brass or stainless steel, and wherein a plurality of cut surfaces [05] is configured on the periphery of HT-sleeve [04].
3. The high current proof terminal bushing assembfy as claimed in claim 1, wherein the said LT insert [11] is provided with a grove [12] on the inner surface, and wherein a locking means [13] is provided on the outer surface of the LT-insert [11].
4. The high current proof terminal bushing assembly as claimed in claim 1, wherein said HT-sleeve [04] is provided with locking means [06, 07] on each side matching with correspondingly matching locking means [09,10] provided at a frontal portion of the HT-conductor [08].
5. The terminal bushing assembly as claimed in claim 1, wherein the HT-conductor [08] is provided with a brass nut f 16j being detachabiy attachable with said locking means [10].