FIELD OF INVENTION
The invention generally relates to electrical apparatus, such as motors and
generators operating on an electric supply system having operating voltage
between 6.6 KV to 11.0 KV. More particularly, the invention relates to an
improved flame proof terminal box for electric apparatus with a fault level
withstanding capacity of 500 MVA for 0.25 seconds.
BACKGROUND OF INVENTION
A manufacturing process of an electrical apparatus operating in hazardous area
generally includes the process of flame-proofing the electrical apparatus by
enclosing the same in such a way that the electrical apparatus is enabled to
withstand, without injury, any explosion of an inflammable gas that may occur
within the apparatus. The flame proofing prevents the transmission of flames
and thereby eliminates the possibility of igniting any inflammable gas which may
be present in the surrounding atmosphere. The flame-proofing in electrical
apparatus and in particular terminal boxes is realized by providing the joint gaps,
a length of path through and across any joint surface, or any opening in the
structure to a predetermined length.
A 3-phase to earth fault is the severest electrical fault that can occur in an
electrical apparatus. If an electric fault occurs within the terminal box or is
transmitted into the terminal box from the electrical apparatus, an enormous
amount of energy is dissipated instantaneously which needs to be contained. The

release of the energy in any form may endanger the working personnel in the
vicinity, or may lead to ignition of the surrounding gas. The problem has hitherto
been tackled from three particular angles; a) by increasing the strength of the
box; b) by reducing any likelihood of a fault occurring in the terminal box or
being transited into the terminal box; and 3) by providing a mechanical "fuse" to
release the energy in a selected direction.
With regards to the first approach, cast iron boxes which are susceptible to
shatter, have been replaced by fabricated steel boxes; but the flame originating
in the terminal box can still cut through the steel boxes allowing the emergence
of dangerous hot gas and flames.
In case of the second approach, earthed internal barrier shielding is provided to
prevent phase-phase faults so that the arching path and power of the arc is
reduced and insulating enclosure for each terminal is provided to reduce the
likelihood of a fault occurring between the phases and the earth. Cable size is
increased to minimize overheating and the terminals are mounted / held
between non-hygroscopic insulating material to minimize chances of occurrence
of fault. Even though inter-phase shields are employed, they are incapable to
fully prevent further flashover once an arc has been established between the
phase and earth.

In the third approach, a frangible panel formed of resinous insulating material is
positioned as a barrier such that the fragments due to any explosion would be
directed in a selected direction; the internal space in the terminal box below the
phase barrier is filled with a non-hardening plastic substance e.g. silicon
elastomer putty. In spite of such provisions, the resulting hot gas and flames can
still emerge from the terminal box.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved flameproof-
terminal box adaptable to electrical apparatus having a fault-withstand capacity
of 500 MVA for a duration of 0.25 seconds, which eliminates the disadvantages
of the prior art.
Another object of the invention is to propose an improved flameproof-terminal
box adaptable to electrical apparatus having a fault-withstand capacity of 500
MVA for a duration of 0.25 seconds, which is enabled to contain the hot air and
gas within the compartment of the terminal box without allowing dissipation of
energy in case of a 3-phase to earth fault.
Yet another object of the invention is to propose an improved flameproof-
terminal box adaptable to electrical apparatus having a fault-withstand capacity
of 500 MVA for a duration of 0.25 seconds, which is configured through selection

of the constituent components through the finite element analysis so as to
enable the terminal box to withstand the stress generated due to shock pulse
released in the event of a 3-phase to earth fault.
Yet another object of the invention is to propose an improved flameproof-
terminal box adaptable to electrical apparatus having a fault-withstand capacity
of 500 MVA for a duration of 0.25 seconds, which includes insulated terminal
bushings and insulated clamps to prevent catching of fire in case of a 3-phase to
earth fault.
A further object of the invention is to propose an improved flameproof-terminal
box adaptable to electrical apparatus having a fault-withstand capacity of 500
MVA for a duration of 0.25 seconds, which by its unique constructions
substantially eliminates the chances of occurrence phase to earth faults.
A still further object of the invention is to propose an improved flameproof-
terminal box adaptable to electrical apparatus having a fault-withstand capacity
of 500 MVA for a duration of 0.25 seconds, which is reliable, safe, and cost-
effective.
SUMMARY OF THE INVENTION
According to this invention, the terminal box is made up of a welded steel

construction which comprises a plurality of components made of steel, and
having optimum shape and thickness. The optimum shape and thickness of the
plurality of steel components are selected based on a finite element analysis and
evaluation of the releasable stress due to the energy shock pulse in case of a 3
phase to earth fault. The terminal bushings (separate patent filed) and insulated
cable clamps are formed of flameproof epoxy resins which are fire retardant and
prevent catching of fire. The hot air and gases are contained within the body of
the terminal box and prevented from emerging out, thereby eliminating the
possibility of any accident to the surrounding personnel or ignition of the
surrounding gas.
The improved terminal box according to this invention can withstand fault level
of 500 MVA for 0.25 secs. The size, location and shape of the constituting
components of the terminal box and the location of welding is so selected that
the terminal box is enabled to withstand shock pulse released at the instant of a
3-phase to earth fault. A first compartment (1) of the terminal box is designed to
withstand the resultant hoop stress. All holes configured for screws inside the
terminal box are plugged / sealed with copper screws. The hot air and gas are
contained within the body of the terminal box and they do not emerge out,
thereby eliminating the danger to the working personnel or ignition of the
surrounding gas.
In the improved terminal box, no component is used which is flammable or can
sustain a flame. The terminal bushings (separate patent filed) and the insulated

cable clamps are made of special flameproof epoxy resins which are fire
retardant and do not catch fire.
The terminals are air separated and the clearance between the terminals and
that between the terminal and earth is kept adequate to match the operating
voltage class. Thus chances of occurrence of phase to phase or phase to earth
fault is reduced.
The terminals are air separated and no earthed inter-phase barriers have been
provided which minimizes the chances of phase to earth faults.
Further, no insulating barriers/ enclosures have been provided around the
terminals to minimize phase to earth faults, which interlia eliminates use of
material which are flammable and which can sustain flame in the event of
occurrence of fault.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The embodiments of the invention are illustrated herein with the help of the
accompanying drawings in which:
Figure - 1 is a sectional side elevation view of a terminal box through a first
compartment according to the invention

Figure - 2 is a front elevation view of the terminal box to show the interior of a
second compartment of Figure - 1.
A terminal box (T) is made up of welded steel construction which comprises a
first compartment (1) holds a plurality of terminal bushings (2) and having a
cylindrical shape with a cut at bottom for entry of supply cables into the box. The
internal cables from the machines are soldered to the rear end of a conducting
rod (9) passing through the bushings (2) which are screwed to a rear wall (3)
adapted to be secured to a machine terminal pad (11). A front wall (4) having an
opening with a removable cover plate (5) is secured by several cap screws to the
front wall (4). The trifurcated supply cables are held in position by a plurality of
clamps (10) (made of special flameproof epoxy resins), and bolted to the rear
wall of a second compartment (6). The second compartment (6) is shaped like a
truncated pyramid with its base welded to an outer wall of the first compartment
(1) at the bottom, a truncated top with flange of the second compartment (6) is
covered by a gland plate (7) secured by means of several cap screws. The walls
of the second compartment (6) are made up of four plates welded together on
the front and rear wall sides with the plates of the side walls bent in V shape. A
plurality of stiffeners (8) welded one each on the outer walls and inside the
second compartment (6), the size and location of the stiffeners being designed
to withstand the stress generated due to a shock pulse in the event of a 3 phase
to earth fault. The supply cables enter the second compartment (6) through
several flameproof glands (not shown) screwed to the gland plate (7). All holes
for the screws inside the terminal box (T) are plugged / sealed with copper
screws (12).

WE CLAIM
1. An improved flameproof terminal box (T) adaptable to electrical
apparatus, having a fault-withstand capacity of 500 MVA for a duration of
0.25 seconds, comprising :
- a first compartment (1) substantially cylindrical in shape, accommodating
a plurality of terminal bushings (2), and having at least one opening at the
bottom to allow ingress of internal cables from the electric apparatus into
the compartment (1);
- a conducting rod (9) passing through the bushings (2), the internal cables
from the apparatus being soldered at a distant end of the conducting rod
(9) which being further screwably joined to a rear wall of the
compartment (1) which rear wall (3) is secured to a pad (11) of the
apparatus terminals;
- a second compartment (6) substantially shaped like a truncated pyramid,
having a rear wall and a front wall, the side walls of which being bent in
U-shape, the base of the second compartment (6) welded to an outer wall
of the first compartment (1) wherein the truncated shaped top portion of
the second compartment (1) having a flange is covered by a gland plate
(7), and wherein the supply cables are held in position by providing a
plurality of flameproof clamps (10), and bolted to the rear wall of the
second compartment (6); and
- a plurality of stiffeners (8) selected in respect of material composition,
size, and locations based on a finite element analysis being each rigidly
joined on the outer and inner walls of the second compartment (6)

which enable the terminal box (T) to withstand the stress when generated
due to a shock pulse in the event of a 3 phase to earth fault, the supply
cables ingressing the second compartment (6) via a plurality of flameproof
glands screwably jointed to the gland plate (7).
2. The terminal box as claimed in claim 1, wherein the first compartment (1)
comprising a front wall (4) with an opening which is closeable by a
removable cover plate (5) by using a plurality of cap screws.
3. The terminal box as claimed in claim 1, wherein the gland plate (12) is
secured to the truncated top with flange by means of a plurality of cap
screws.
4. The terminal box as claimed in any of the preceding claims, wherein
several holes are configured inside the terminal box (T), the holes being
plugged by means of a plurality of copper screws (12).
5. An improved flameproof terminal box (T) adaptable to electrical
apparatus, having a fault-withstand capacity of 500 MVA for a duration of
0.25 seconds, as substantially described and illustrated herein with
reference to the accompanying drawings.

The invention relates to an improved flameproof terminal box (T) adaptable to electrical apparatus, having a fault-withstand capacity of 500 MVA for a duration of 0.25 seconds, comprising a first compartment (1) substantially cylindrical in shape, accommodating a plurality of terminal bushings (2), and having at least
one opening at the bottom to allow ingress of internal cables from the electric apparatus into the compartment (1); a conducting rod (9) passing through the bushings (2), the internal cables from the apparatus being soldered at a distant end of the conducting rod (9) which being further screwably joined to a rear wall
of the compartment (1) which rear wall (3) is secured to a pad (11) of the apparatus terminals; a second compartment (6) substantially shaped like a truncated pyramid, having a rear wall and a front wall, the side walls of which being bent in U-shape, the base of the second compartment (6) welded to an
outer wall of the first compartment (1) wherein the truncated shaped top portion of the second compartment (1) having a flange is covered by a gland plate (7), and wherein the supply cables are held in position by providing a plurality of
flameproof clamps (10), and bolted to the rear wall of the second compartment (6); and a plurality of stiffeners (8) selected in respect of material composition, size, and locations based on a finite element analysis being each rigidly joined on the outer and inner walls of the second compartment (6), which enable the
terminal box (T) to withstand the stress when generated due to a shock pulse in the event of a 3 phase to earth fault, the supply cables ingressing the second compartment (6) via a plurality of flameproof glands screwably jointed to the gland plate (7).