FIELD OF THE INVENTION
The invention relates generally to manufacture of medium voltage electrical equipment and in
particular to epoxy seal off bushing for 33 KV air insulated switchgear. It pertains to the insulation
and high voltage connection support deployed in such electrical equipment.
BACKGROUND AND PRIOR ART
Air insulated switchgear comprises of switchgear panel which has three high voltage chambers
named as -Busbar Chamber, Cable termination chamber, and breaker chamber.
- Busbar chamber houses bus bars, droppers, and top fixed contacts .
- Cable termination chamber (CT Chamber) houses CTs, PTs, cables & bottom fixed contacts .
-Breaker chamber in houses vacuum circuit breaker trolley (VCB) having vacuum interrupter as
the current interrupting device & the operating mechanism for tripping & closing of vacuum
interrupter. VCB trolley has top and bottom contacts named as Moving contacts . This breaker
trolley is inserted in the breaker chamber of the switchgear panel and the 'Moving contacts '
engage with the respective 'Fixed contacts of the CT chamber and the Busbar chamber forming
the complete switchgear.
The position of the breaker trolley in the switchgear panel, when its contacts are engaged with
the fixed contacts of the panel is the 'Service position' of the breaker. Fixed contacts and the
overlapping zone of fixed & moving contacts in the switchgear panel form one of the most critical
zones from the high voltage point of view, in which phase to phase or phase to earth fault can
occur if the dielectric strength is not sufficient. Required dielectric strength can be achieved :
- Either by providing the sufficient air gap in-between the Fixed contacts of the three phases and
between the contacts and the earth,
- or by providing sufficient insulation in between them.
Increasing the air gap will result in the increased overall size of the switchgear. These critical
areas are required to be completely covered by an insulated component such that necessary to
phase & to earth creepage is achieved . If insulated partition sheets are provided for the purpose,
it will not completely cover the critical areas and chances of failure will be there.
To meet these requirements , an epoxy seal off bushing has been designed to provide adequate
insulating & mechanical strength to the switchgear.
Prior to the invention of the claimed epoxy seal off bushing , the six nos Fixed Contacts & the
overlapping zone of the fixed & moving contacts were covered by means of polycarbonate
shroud, separate shroud for each phase . The top and bottom fixed contacts were supported by
support insulators.
Above arrangements had following shortcomings:
a) Insulation in the high voltage critical areas was just sufficient to achieve the required
dielectric strength.
b) Sealing between various high voltage chambers could not be achieved.
The design of the polycarbonate shroud was such that the sealing between various high voltage
chambers could not be achieved .
c) Separate arrangement for safety shutters & its operating mechanism adds complicacy to the
design & increases the assembly time .
d) Fixed contacts do not have sufficient rigidity and fixed contacts and they tend to bend on
engagement of moving contacts at the time of insertion of breaker trolley .
e) Additional Setting fixture is required for the setting of Top and bottom contacts.
0 Two different components are provided for insulation and support of the contacts .
The present invention seeks to overcome above shortcomings of the prior art.
The epoxy seal off bushing according to the invention was designed with a technique which
provides insulation as well as support to the contacts and also safety shutters in the 33kV
switchgear.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS.
FIG-1 Shows complete assembly of switchgear showing various high voltage chambers , Fixed
contacts & vacuum circuit breaker trolley with epoxy seal off bushing according to the invention.
FIG-2 Shows conventional insulation and contact support method in medium voltage electrical
equipment as specified in the prior art.
FIG-3 Shows the view of epoxy seal off bushing along with the safety shutters according to the
invention
Fig-4 3D view of seal off bushing
The invention will now be described in an exemplary embodiment as depicted in the
accompanying drawings. There can be other embodiments of the same invention, allof which are
deemed covered by this description.
DETAILED DESCRIPTION OF THE INVENTION
This invention proposes an epoxy seal off bushing (12) for 33KV medium voltage switchgear.
Invention takes into consideration the dielectric strength required at the specified locations, prior
method of insulation at these locations alongwith creepage distance to earth, the material most
suited to the requirement, availability of space , mounting arrangement ,and requirement of
mechanical strength etc.
Epoxy has been used as this has high dielectric properties and has the capability to provide
rigidity also along with the insulation . The invention comprises of various parts as specified
below best seen in Fig.l and Fig.3
- Hollow bushing (14):
This provides the phase to phase & phase to earth insulation to fixed contacts and to the
overlapping zone of the fixed and the moving contacts , required creepage between contacts and
panel flanges used for mounting . It also provides insulation to the joint between contacts &
droppers coming from main busbar of switchgear.
-Safety shutter (13):
Safety shutters have been made the part of the seal off bushing & have been mounted on the
front face of the bushing . This provides simplicity to the design .
- Middle Flange (15):
This flange is used for the mounting of the brazed assembly of top & bottom fixed contacts .
- Mounting flanges (16):
Two flanges are used for mounting of bushing in the switchgear panel . These two are kept in
different planes so as to provide adequate cantilever strength to the bushing & to the contacts
.The locations of the contacts gets fix with the mounting of the bushing and which in turn
eliminates the requirement of additional setting fixture for contacts .
Safety shutters is the mandatory requirement of the IEC, which should be provided in the
switchgear to protect the personnel from any flashover occurring between the live bus bars and
the breaker contacts, when the breaker trolley (2) is in the TEST or is in the 'Withdrawn'
position for maintenance purpose .
The safety shutters (13) have been mounted on the front face of the bushing and are made of
insulating material . These shutters lift upwards with the movement of the circuit breaker trolley
(2) inside the switchgear panel & move downwards when the trolley is taken out from the Service
position to the Test position . In the Test position and withdrawn position, shutters provide
insulation between the fixed contacts and the moving contacts thereby eliminating the chances of
any accident while the personnel is doing maintenance of the breaker.
Shutter has been made the part of the bushing hence it is very small in size , it eliminates the
requirement of additional shutter and its operating mechanism thereby reducing the space
required for the arrangement and also the assembly time & its complications.
In the epoxy seal off bushing according to invention, the mounting arrangement of bushing is
unique. It has been done in two different planes so as to enhance the cantilever strength of the
bushing & as a result provides required mechanical strength to the fixed contacts (6,7). ( Refer
Fig-1).
The epoxy seal off bushing (12) according to invention provides sealing between three high
voltage chambers namely Busbar chamber (3), Cable termination chamber (4) and the breaker
chamber (5). This sealing is required to prevent the travelling of arc from one chamber to the
other on occurrence of fault in any of these chambers, thereby protecting the other chambers
from arc.
The bushing has a middle flange (15) which acts as sealing between the high voltage chambers .
When the brazed fixed contacts (6,7) are fixed on the middle flange , the communication
between the chambers is totally eliminated (Refer Fig-1).
The epoxy seal off bushing itself acts as the support to the fixed contacts (6,7) and no separate
support insulators are required for this purpose ( Refer Fig-1).
Fixed contacts (6,7) of the switchgear should be supported rigidly so as to to avoid their bending
when the 'moving contacts' (8,9) of the circuit breaker trolley (2) engage with these fixed
contacts.
The epoxy seal off bushing also acts as the setting fixture for the incoming & outgoing fixed
contacts (6,7).
The position of the six nos fixed contacts (3 top & 3-bottom) inside the switchgear panel is
located automatically once the bushing is fixed in the panel. Hence no additional setting fixture is
required .
In absence of these bushings, after complete assembly a setting fixture for switchgear panel and
circuit breaker trolley was used in prior art to fix the location of the fixed & moving contacts for
smooth engagement of these contacts .
The seal off bushing according to the present invention does away with the setting fixture
required in the prior art.
We claim:
1. An epoxy seal off bushing (12) for 33KV air insulated switchgear comprising of
hollow bushing (14)
safety shutters (13)
middle flange (15) and
mounting flanges (16),
characterized in that the seal off bushing provides sealing between busbar chamber (3), cable
termination chamber (4) and breaker chamber (5) of the air insulated circuit breaker and prevents
traveling of arc from one chamber to the other on occurrence of fault in any of these chambers.
2. The epoxy seal off bushing as claimed in claim 1, wherein the bushing is provided with
unique mounting arrangement in two different planes to enhance cantilever strength of the
bushing which provides requisite mechanical strength to the fixed contacts (6,7) of the
switchgear.
3. The epoxy seal off bushing as claimed in claim 1, wherein the safety shutters (13) are
made of insulating material and are mounted on the front face of the seal off bushing (12).
4. The epoxy seal off bushing as claimed in claims 1 and 3, wherein the said shutters lift
upwards when the circuit breaker trolley (2) moves inside the switch gear panel and move
downwards when the trolley is taken out from the service position to the Test position so that the
shutters provide insulation between the fixed contacts and the moving contacts for protection of
maintenance personnel.
5. The epoxy seal off bushing as claimed in claim 1, wherein the middle flange (15) acts as
sealing between the high voltage chambers and when the brazed fixed contacts (6,7) are fixed on
the middle flange, the communication between the chambers is totally eliminated.
6. The epoxy seal off bushing as claimed in claim 1, which itself acts as the support to the
fixed contacts (6,7) and eliminates separate support insulators which are required in prior art.
7. The epoxy seal off bushing as claimed in claim 1, which itself acts as the setting fixture for
the incoming and outgoing fixed contacts (6,7) so that additional setting fixture is not required for
fixing the location of the fixed and moving contacts of the switchgear for smooth engagement.

An epoxy seal off bushing (12) for 33KV air insulated switchgear comprising of hollow bushing
(14) safety shutters (13) middle flange (15) and mounting flanges (16), characterized in that the
seal off bushing provides sealing between busbar chamber (3), cable termination chamber (4) and
breaker chamber (5) of the air insulated circuit breaker and prevents traveling of arc from one
chamber to the other on occurrence of fault in any of these chambers.