Air purged portable electric lamp

A portable electric lamp suitable for use in hazardous locations has the fixture purged with breathable air a predetermined time before power is coupled to energize the lamps. The fixture is continuously purged during operation. If the internal pressure falls below a predetermined minimum level, or exceeds a predetermined maximum during operation, power to the lamps is shut off, and the complete start cycle, including the time delay, must be undertaken before the lamps can be re-started. All leads and components are either potted or operated at an intrinsically safe power level.

FIELD OF THE INVENTION 
The present invention relates to portable electric lamps, and more 
particularly to a portable electric lamp suitable for use in hazardous 
locations. The term "hazardous location" is a term of art, and it is well 
known to those in the art. It includes operation in potentially volatile 
environments, such as in oil refineries, certain manufacturing locations 
which use solvents or other combustible materials, such as airplane 
manufacturing facilities, and chemical production facilities, among 
others. 
BACKGROUND OF THE INVENTION 
Portable lighting is often used in hazardous locations. In the past, 
incandescent lamps have been widely used in hazardous locations. However, 
since incandescent lamps may break during a fall, thereby exposing the 
heated filament and the electrical power lead, such lamps have been 
thought of as creating a potential for an explosion, depending upon the 
conditions in the environment in which they are used. Thus, attempts have 
been made to make incandescent lamps "explosion proof". This has required 
expensive and elaborate provisions for shielding, enclosing and 
reinforcing the enclosure for the lamps. For example, in one commercial 
incandescent lamp designed for use in hazardous locations, a very thick 
and strong globe of special explosion-proof glass surrounds the lamp, and 
a metal framework is placed around the globe for coupling to the base of 
the fixture. These units are expensive, and it is time-consuming to 
replace a burned-out lamp due to the construction of the unit. 
It is known that fluorescent lamps are more efficient in producing light 
than incandescent lamps, that they operate at a much lower temperature, 
and that they generally have a much longer useful life. However, to 
provide a fluorescent lamp with an explosion-proof transparent housing 
such as described above for incandescent lamps is deemed prohibitive, from 
a manufacturing as well as a cost standpoint. 
SUMMARY OF THE INVENTION 
The present invention provides a portable fluorescent electrical lamp 
fixture including a housing which surrounds and encloses the fluorescent 
lamps. Power is coupled to the interior of the housing by a sheathed cord 
which extends through a rubber end cap. A flexible tube from a source of 
pressurized breathable air is also fed into the housing through the rubber 
end cap and coupled to a pressure regulator. Air pressure within the 
housing is monitored by a low pressure switch and a high pressure switch. 
A control circuit includes a timer circuit which commences timing when 
power is applied to the fixture. The pressurized breathable air is applied 
to the fixture at the same time. The time duration of the timer is set as 
a function of the air volume within the fixture, and it is of sufficient 
duration that approximately four times the volume of the interior of the 
fixture will be purged. The control circuit energizes a yellow indicator 
to indicate that the pressure inside the fixture has reached the 
predetermined minimum design level for purging. In a preferred embodiment, 
the control and timing circuit is provided in duplicate for redundancy to 
increase reliability. When the timer times out, a green indicator is 
energized, and power is then coupled to the lamp ballasts for energizing 
the lamps as line voltage. 
The fixture is vented at a location remote from the inlet for the 
pressurized air so that purging is continuous. Air pressure is maintained 
within the fixture during the entire operation of the lamps. If at any 
time the air pressure falls below a predetermined lower level, or exceeds 
a higher predetermined level, power to the lamps is shut off. When power 
is shut off, the lamp must go through a complete start-up cycle, including 
the time delay, before the lamps can be re-started. Moreover, the lead-in 
power cables are coupled directly to the control circuit and the juncture 
between the lead-in cables and the control circuit, as well as the control 
circuit, are completely potted so that they are not exposed to even the 
environment within the fixture. The pressure switches and their associated 
leads are operated at an intrinsically safe power level, thus 
substantially increasing the safety of operation. The term "intrinsically 
safe" is also a term of art known to those skilled in this art, and as 
used herein, it means that a mechanical switch is operated at a power 
level below 0.9 milliwatts. Operation at or below this power level will 
prevent the occurrence of sparks. 
Other features and advantages of the present invention will be apparent to 
persons skilled in the art from the following detailed description of a 
preferred embodiment accompanied by the attached drawing wherein identical 
reference numerals will refer to like parts in the various views.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring first to FIG. 1, reference numeral 10 generally designates an 
air-purged portable electric fluorescent lamp fixture constructed 
according to the present invention. The fixture 10 includes a transparent 
housing 11 with its ends received in left and right end caps 12, 13. The 
fixture also includes a frame 14 on which first and second ballasts 15, 16 
are mounted. The ballast 15 is used to energize a first pair of 
fluorescent lamps 17, 18; and the ballast 16 energizes a second pair of 
fluorescent lamps 19, 20. Further details of the mechanical mounting of 
the ballasts and lamps, as well as the structure of the frame 14 and its 
mounting within the end caps 12, 13, can be found in my U.S. Pat. No. 
5,088,015, "PORTABLE FLUORESCENT LAMP FIXTURE", the disclosure of which is 
incorporated herein by reference. 
Still referring to FIG. 1, a metal housing 22 is mounted to the frame 14 
for enclosing the control circuitry which, as mentioned, is potted in 
conventional epoxy pottery compound for hermetically sealing the circuit 
elements. An electrical power cord 24 extends from the line or other 
source of electricity, through the end cap 12 as will be described in 
connection with FIGS. 2 and 3, and fed into the fixture and to the housing 
22. 
A tube 25 serves as a conduit for pressurized breathable air from a source 
into the interior of the fixture 10, as also better seen and described in 
connection with FIG. 3. 
A mounting bracket 26 is mounted to the frame 14; and a high pressure 
switch 27 and a low pressure switch 28 are mounted to the bracket 26. The 
pressure switches 27, 28 are referenced against atmospheric pressure by 
means of a pair of tubes coupled to a T-fitting 30, the third port of 
which is in communication with the atmosphere by means of a tube 32 which 
passes through end cap 13 and is mounted in such as way as to be sealed to 
that end cap. The interior of the fixture is also vented to the atmosphere 
through end cap 13. 
Referring now to FIG. 2, a tubular housing 34 is provided with a top cap 35 
and a bottom cap 36 to form an enclosure for a conventional pressure 
regulator 37. Pressurized air from a source (not shown) supplied by the 
user is fed to the input of the regulator 37 by means of an 
adapter/connector 38. The output of pressure regulator 48 is coupled to 
the air conduit 25 by means of a coupling 39. 
Turning now to FIG. 3, the air inlet tube 25 may be routed with the 
electrical cord 24, protected by a spiral plastic wrapping 29. Adjacent 
the end cap 12, the tube 25 is separated from the electrical cord 24 and 
fed separately through the end cap 12. The electrical cord 24 is secured 
with an air-tight coupling to the end cap 12 by means of a threaded nipple 
molded in end cap 12. A threaded lock nut 40 secures a tapered grommet 41 
and a nylon washer 42 to the threaded nipple to secure the cord 24. The 
air tube 25 is also connected to the end cap 12 by means of a 
double-threaded nipple 43 which is secured and sealed by means of a lock 
nut 44, washer 45 and grommet 46. A lock nut 47 then secures the right 
side of the threaded nipple 43 to the end cap 12. 
Similarly, the tubing or conduit 25 passes through the end cap 12 and is 
mounted on a double-threaded nipple 40, bushing 41, washer 42 and lock nut 
43. 
Turning now to FIG. 4, the ballasts 15, 16 and the fluorescent lamps 17-20 
are shown connected in a conventional electrical circuit fed by input 
leads 51, 52 from the control circuit which is shown in FIG. 4 as included 
within the dashed block 55. The dashed block 55 also shows those circuit 
elements and components which are embedded in a potting compound for 
safety reasons and housed in housing 22. Included within the potted 
material are first and second timer circuits generally designated 57 and 
58 respectively. The timer circuits 57, 58, which will be further 
discussed below, may be identical. They are connected in redundant 
circuit, however, to provide for greater reliability. Further, a 
conventional surge suppression circuit may be included to reduce 
transients in the input power leads. 
Each timer circuit 57, 58 has seven terminals designated respectively X1 
through X7. The negative lead from the input power cord 25 is connected to 
the X1 terminal of the timer circuit boards 57, 58; and the positive power 
lead from the input power cord 25 is connected to the X2 terminal of the 
timer circuits 57, 58. The ground lead from the input power cord is 
connected to a ground terminal 59 which is connected by means of a screw 
to the frame 14. FIG. 4 also illustrates the electrical connections 
between the terminals X6, X7 of the timer circuits 57, 58 and the high 
pressure switch 27 and low pressure switch 28, previously described. Each 
of the pressure switches 27, 28 has a common terminal, a normally closed 
terminal and a normally open terminal. 
A more detailed circuit schematic of the control timer circuits 57, 58 is 
shown in FIG. 5. 
The input power is fed to a transformer 60 and thence to a diode bridge 61, 
the output of which feeds a conventional, commercially-available voltage 
regulator circuit 63. The output of the voltage regulator circuit 63 is a 
regulated DC voltage which supplies the B.sup.+ voltage for the remainder 
of the logic circuitry to be described. 
Turning now to the lower left-hand portion of the schematic diagram of FIG. 
5, the normally open contacts of the low pressure switch 28 are connected 
in series with the normally closed contacts of the high pressure switch 27 
when only a single control circuit is used. For redundant control 
circuits, the connections are described below. In FIG. 5, then, the two 
switches are connected in series between terminals X6 and X7 of the 
control circuit. Output terminal X6 is connected through a voltage divider 
network to the negative input of a comparator circuit 65. The output of 
the comparator 65 is coupled through a diode 66 to the junction between a 
capacitor 67 and a resistor 68. 
When pressurized breathable air is first transmitted into the housing the 
fixture 10, the normally opened contacts of the low pressure switch 28 are 
open, as shown in FIG. 5. The output of the comparator circuit is a low 
voltage which clamps the positive terminal of capacitor 67 to a low 
voltage. When the pressure inside the housing reaches the first 
predetermined low pressure level (typically around 2.0 in. Hg.), the 
contacts 28 close and cause the output of comparator 65 to go positive. 
This permits the positive terminal of capacitor 67 to charge through 
resistor 68. The values of capacitor 67 and resistor 68 are selected to 
allow a charging time of approximately two minutes. 
The junction between the capacitor 67 and resistor 68 is connected to the 
one input of a second comparator circuit 70. When the charging voltage at 
the two minute interval reaches the design level, as determined by the 
resistive voltage divider coupled to the other input of the comparator 70, 
the output of the comparator 70 goes to a low voltage, thereby energizing 
an LED 72 of an optical coupler generally designated 73, as well as an 
indicator LED 74. The optical coupler is connected in the gate circuit of 
a switching Triac 76 (or other semi-conductor power switch) which then 
conducts, and establishes electrical continuity between the terminals X4 
and X5 of the control circuit. In the case where only a single circuit 
board is employed for the control circuit, a connection must be added as 
indicated by the dashed line 78, and the switching of the Triac 76 
thereupon causes power to be coupled from the leads X1, X2 to the leads X3 
and X4 respectively, thereby coupling power to the ballasts to excite the 
lamps. 
When the low voltage switch 28 closes, as the lower pressure threshold is 
reached, and the output of comparator 65 goes high, it also causes a 
transistor 80 to conduct, which in turn causes a yellow LED indicator 81 
to become illuminated, thereby signalling to a user that the interior of 
the housing is under pressure and that the pressure has exceeded the low 
pressure threshold level. 
The LED indicator 81 may be yellow so as to indicate, when it is 
illuminated, a "stand-by" condition; whereas the LED indicator 74 may be 
green to indicate, when it is illuminated, that power is applied to the 
lamp circuits. 
If the pressure at any time exceeds the high pressure level (5.0 in. Hg. in 
the illustrated embodiment), the normally closed contacts of the high 
pressure switch 27 open, reversing the state of comparator 65, and causing 
capacitor 67 to discharge immediately. This disables the optical coupler 
73 and causes the Triac 76 to become non-conducting, and thereby shuts off 
power to the ballasts immediately. Once the high pressure threshold has 
been exceeded, a full restarting cycle must be completed before power is 
again coupled to the lamps. This allows the user to check the vent which 
discharges to the ambient to be checked. 
All of the circuitry enclosed within the dashed line 55 of FIG. 5 may be 
mounted on a single circuit board and embedded in an epoxy resin, 
including the sheath of the power cord 25. The switches 27 and 28 are 
selected, and the voltage levels on the lines leading to the switches 27 
and 28 are designed such that the intrinsically-safe level of power 
defined above is not exceeded in the pressure switches 27, 28 and their 
associated leads. Thus, the circuit qualifies as an intrinsically safe 
circuit because the only power coupled to components not encased in epoxy 
is at an intrinsically safe level until the purging cycle is complete. 
Once the purging cycle is complete, there is no hazard, of course. 
Turning now to FIG. 6, there is shown a schematic diagram illustrating how 
two control circuit boards, 57, 58, each individual board including the 
circuitry shown in FIG. 5, may be wired together externally to provide a 
redundant control circuit. Again, a conventional surge suppression circuit 
may be employed. The input power leads are connected to both input 
terminals X1 and X2 respectively. The X3 and X4 inputs of the circuit 
board 58 are coupled to the lamp load for the circuit board 58, and the X3 
lead of circuit board 57 is coupled to terminal X5 of circuit board 57. 
The terminal X4 of circuit board 57 is connected to the terminal X5 of 
circuit board 58, and the terminals X6 and X7 are connected directly 
together, as illustrated in FIG. 6. Only one set of pressure switches is 
used and they are designated respectively 27 and 28 in accordance with the 
above disclosure. This arrangement also calls for the removal of a trace 
of copper to disconnect terminal X2 from the cathode of the Triac 76 
(represented by opening the lead 90 in FIG. 5), and the addition of jumper 
wire illustrated at 91 in FIG. 5 between the terminal X3 and the terminal 
X5 of control circuit 57. This arrangement thus places the Triacs in 
series circuit, and the timer circuits in parallel, while leaving the 
switch arrangement the same. In operation, the redundant control circuit 
arrangement is similar to that described above in connection with FIG. 5 
except that when the normally open switch 28 closes, both timer circuits 
time out simultaneously to the two minute interval and then switch their 
associated comparators 70, causing Triacs 76 to conduct. 
Having thus disclosed in detail a preferred embodiment of the invention, 
persons skilled in the art will be able to modify certain of the circuitry 
or structure which has been illustrated and to substitute equivalent 
elements for those which have been disclosed, while continuing to practice 
the principle of the invention; and it is, therefore, intended that all 
such modifications and substitutions be covered as they are embraced 
within the spirit and scope of the appended claims.