US 3624449 A
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United States Patent Martin J. Morgan 72] inventor Rochester, NY. [211 App]. No. 66,372  Filed Aug. 24, 1970  Patented Nov. 30, 1971  Assignee Sybron Corporation Rochester, N.Y.
(54] INTRINSICALLY SAFE TRANSMITTER SYSTEM 10 Claims, 1 Drawing Fig.  US. Cl 317/16, 317/20, 317/31, 317/40A  Int. Cl 1102b 3/20  Field oi Search 3l7/l6,40 A, 31, 20, 18 C  References Cited UNITED STATES PATENTS 2,976,462 3/1961 Miller 317/18 C 3,339,114 8/1967 Kelleyetal. 317/31 X 3,440,492 4/1969 Carmody.... 317/31 X 3,496,416 2/1970 Agnew et a1. 318/18 C Primary Examiner-James D. Trammell Attorney-Theodore B. Roessel ABSTRACT: A DC regulator-type transmitter in a hazardous area is connected by a pair of electrical barriers to a DC source in a safe area to define a closed DC loop in which the current is controlled by the transmitter in proportion to a variable condition in the hazardous area. One barrier limits the voltage with respect to ground of metering resistance in the safe area of the loop connected to the source side grounded in the safe area. The other barrier limits the source voltage with respect to ground. Voltage drop across the metering resistance provides a measure of the variable condition.
PROCESS PAIENIEnmv so 1911 PROCESS ATTORNEY INTRINSICALLY SAFE TRANSMITTER SYSTEM CROSS REFERENCES TO RELATED APPLICATIONS B. L. Hallenbeck application for U.S. Letters Pat. Ser. No. 829,491, filed June 2,l969, assigned to the assignee of the present application, describes the barrier and intrinsic safety considerations referred to herein.
BACKGROUND OF THE INVENTION If, by reason of limiting the level at which electrical energy can be supplied thereto, electrical elements have parts which could produce hazardous effects, such as shocks, sparks and/or the like, unless said energy is so limited, apparatus having said elements is considered intrinsically safe when said energy is so limited. In practice, intrinsic safety is defined relative to specific situations, and reflects legislative and underwriters standards, and the like, as well as strictly technological considerations.
In any event, the aforesaid apparatus is normally part of an electrical system. For the purposes of the present invention, an intrinsically safe electrical system has a safe part, a barrier, and a hazardous part, each of these parts including electrical elements.
The safe part is deemed safe because damage caused by fault or defect therein usually will be confined to the safe part. In general, only the safe part can be, or provide, actually or potentially, a source of electrical energy for the safe part.
The hazardous part is deemed hazardous because fault may cause damage to its environment if, as a resultof the fault, electric energy in excess of a certain level obtains in the safe part. In general, the hazardous part must not be, or provide, actually or potentially, a source of excess electrical energy.
The barrier is considered a barrier because its function is to limit the level of electric energy transferable from the safe part to the hazardous part. In general, the barrier also interconnects safe part and hazardous part for transfer therebetween of electrical energy at safe levels.
Geographically, the hazardous part may be as far as miles from the barrier part. Functionally, however, there is a safe area ending at the barrier and a hazardous area beginning at the barrier. Characteristically, the safe area contains barriers and such energy sources such as mains AC, and the like. Characteristically, the hazardous area contains no barriers and no such sources without explosion-proofing means and, in general, no electrical energy is admitted thereto without explosion-proofing means except via a barrier. Electrical energy source here means any entity which is literally a source or can be made to act as such, either accidentally, as by short-circuiting or the like, or routinely, as by switching or the like.
The measure of intrinsic safety is the probability of a barrier failing to prevent energy levels in the hazardous area from exceeding safe values. More particularly, this measure is given by the probability that one or another, or several elements can become defective or fail in such manner as to leave the barrier unable to prevent safe values of energy levels from being exceeded. In practice, it is possible to construct a barrier such that the probability of this sort of defect, fault or failure, is less than It should be observed concerning this probability that it is not affected by the probability of such faults or defects as are fail-safe," i.e., do not prevent the barrier from performing its function. Indeed, the barrier may use an element chosen for this property, e.g., a fuse. In any event, the above-identified Hallenbeck discloses a barrier providing the above-specified measure of intrinsic safety.
SUMMARY OF THE INVENTION One of the consequences of using a barrier is that it introduces impedance in the interconnections between safe part and hazardous part. In the present invention, such interconnection is essentially a lengthy two-conductor transmission line coupling a two terminal DC regulator in the hazardous area, with a two-terminal DC source in the safe area. In the system thus defined, the regulator terminals float off-ground on transmission line impedance. One of the transmission line conductors includes so-called metering resistance, which is one or more stable, precision resistors, the voltages across which are accurate measures of the current in the closed circuit defined by source, regulator and transmission line. The regulator is constructed to act like a variable resistance terminating the transmission line, and to regulate the DC independently of source variations, and the like, to a certain-extent. Its effective resistance is modulated by means responsive to a variable condition such as flow, liquid level, or the like obtaining in the hazardous area, whereby the transmission line current becomes a measure of the condition, which measure can be evaluated or otherwise utilized by sensing voltages across metering resistance in the transmission line.
For intrinsic safety in the hazardous area, the present invention uses two barriers, one for each of the transmission line's conductors, because the regulator's terminals are off-ground. The reason they are off-ground is that the one end of the metering resistance and one terminal of the DC source are grounded in a safe area. The other end of the metering resistance is connected to the safe area terminal of one of the barriers, and this particular barrier is designed to limit the drop across the metering resistance to a safe value, much smaller than the value of the voltage normally across the source terminals.
The current passing through the metering resistance is less then that passing through the regulator, due to losses through the stray coupling of the corresponding side of the transmission line to ground. However, the major part of the DC source voltage is on the other side of the transmission line, and isolated from the metering resistance side thereof by the relatively large effective resistance of the regulator. Said effective resistance is much larger than the metering resistance and very much larger than the resistance added by the barrier to the transmission line on the metering resistance side. Hence, only a minor part of the DC source voltage is on the metering resistance side of the transmission line and relatively little of the metering resistance current is bypassed via stray coupling to ground.
On the other hand, on the said other side of the transmission line substantial current due to stray couplings can be tolerated as the path of last said current lies neither through the metering resistance nor through the regulator, but constitutes merely additional DC source loading, and the regulator tends to maintain its efiective resistance at a value independent of all but the variable conditions's magnitude. The second barrier, i.e., one on said other side of the transmission line, may be allowed to add substantial amounts of resistance to this side of the line, and to limit the voltage at its hazardous end to about the value of the normal source voltage.
BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE of drawing is a schematic diagram of an intrinsically safe system in accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT In the FIGURE, reference numerals I0 and 20 denote respective barriers. Barrier 10 is identical in reference characters, construction and parts values to that shown in the drawings of the above-identified Hallenbeck application, with the exception of present numeral 10 and the subscript of the reference character F.
Barrier 20 is identical in construction to barrier 10, but its parts differ in reference character and value. Reference characters in barrier 20 are those of barrier 10, but numerically augmented by 10. As for parts values, in the present invention, the parts of barrier 20 generally run about an order of magnitude different from the corresponding parts of barriers 20. Thus, wire-wound, :5 percent resistor R, of barrier 10 corresponds exactly to resistor R of barrier 10, except that the latter is 1 ohm, not 10 ohms. Again, fuse F, is a 1.6 ohm, one-eighth ampere, :20 percent fast-blow fuse. Fuse F is the same, except that it is 0.5 ohm and rated at one-half ampere. The zeners of barrier 20 are the same as those of barrier 10, except that the former have zener voltages of 5 volts. R and R are 3 and ohms, respectively, so the total series resistance of barrier is 14.5 ohms as opposed to 151.0 ohms total series resistance of barrier 10.
Like barrier 10 encapsulated in a casing 5, barrier 20 is encapsulated in its own casing l5, both casings being indicated as simple dashed-line boxes. The two barriers could share a common encapsulation and easing, but preferably do not.
The operation of barrier 20 is exactly as described in the above-identified Hallenbeck application, except that voltage levels involved difi'er, as is obvious from the parts value, and barrier 20 is treated in every way like barrier 10. Each barrier obviously operates exactly as in the Hallenbeck application.
However, it is convenient to describe briefly operation and use of the barriers as such. The function of the barriers is to prevent electrical energy from being available above a certain level at terminals 3 and 13. The zener diodes do this temporarily and. practically instantaneously, by firing and diverting excess current to ground whereas the fuses will do the same, after some delay, by blowing. In short, the diodes, when firing, divert current from whatever is connected to terminals 3 and 13. When a diode does this for a while, one of the fuses blows, and permanently disconnects one of terminals 3 and 13 from the source of current. Each barrier uses two zeners as a precaution against one zener failing by opening up. However, more likely zener failure is by short-circuiting, which is failsafe, since except for ruining the diode, it has otherwise substantially the same effect as firing the zener.
Zeners Z and Z, are nominally identical in firing voltage, but in practice are selected such that Z fires at a slightly lower anode voltage than 2,; likewise, as to Z and 2, The series resistance in each barrier cooperates with the zeners to provide current-limiting, and also for barrier testing. Thus, the terminals 1, 2, 3, 4, 11, l2, l3 and 14, which represent terminal hardware, are the only electrical parts of the barriers externally accessible without breaking up the casings and encapsulating material, so by suitable measurements of impedance at these terminals (as more fully set forth in the aboveidentified Hallenbeck application), internal faults in the bar riers can be detected nondestructively. In particular, the resistances R and R are only necessary for testing purposes, and could be eliminated. However, like many other specific features of the barriers, such as housing, terminal arrangement, and so on, all of which, from the bare function point of view, might appear to be candidates for elimination or modification, R and R are elements which contribute to meeting intrinsic safety demands, as viewed at this writing.
In the present case, the remainder of the system must accommodate itself to the electrical properties of the barriers as shown, in order to conform to underwriters requirements. However, this particular system has the property of including, as its hazardous part, apparatus that has to float above ground potential, whereas the safe part of the system is grounded.
The safe part of the system includes a battery 21 having its anode connected to a ground bus 22, grounded in the safe area. The cathode of battery 21 is connected to a power bus 22 to which in turn the cathodes of isolating diodes 23, 24, 25 and 26 are connected. Diodes 24, 25 and 26 connect by the anodes to apparatus B, C, D, this apparatus being any suitable entities which may utilize, as is commonly the case, the battery 21 as a source of electrical energy. 1
The anode of diode 23 is connected via resistor 27 to terminal l of barrier 10. Resistors 28 and 29 in series connect terminal 11 of barrier 20 to ground bus 22. The illustrated barriers are designed for a 28-volt battery, with resistors 28 and 29 being 62.5 ohms apiece, and resistor 27 being 50 ohms. Diode 23, for the polarities illustrated, has an effective resistance such that its voltage drop is approximately 0.7 volt.
The resistors 28 and 29 are metering resistors, so should have accurately known, stable resistance values, so that the drops across the resistors are accurate measurements of the current through the resistors. As illustrated, the drop across resistor 29 provides voltage to an amplifier A. Amplifier A represents part of a process control system, as indicated by its illustrated connection to a box marked process," and representing apparatus responsive to amplifier output to modify the value of a variable condition in such process. As shown, amplifier A has one input connected to the grounded end of resistor 29 via a pair of otentiometers 30 and 31. This connection is for the purpose of comparing the voltage across resistor 29 to a fraction of the voltage of a battery 32 common to the two potentiometers, so that the net voltage across the amplifier input reflects the diflerence between said fraction and the drop across resistor 29. Normally, there will be a local negative feedback loop 33 around the amplifier, constituted to define its gain and other characteristics, and operating to oppose net amplifier input from deviating from zero volts. The amplifier A preferably may be powered from and 32, and usually would be, just as devices B, C and D are.
Across the metering resistor 28 is connected a voltage measuring device, illustrated as a recorder drawing on a chart the time variation of the drop across resistor 8.
From the foregoing, it will be evident that the system being described is a full-fledged process control system wherein a variable condition, such as flow, liquid level, or the like, in a process, is being recorded and controlled as a function of the current through resistors 28 and 29. This current has battery 21 as its source but its magnitude is controlled by a transmitter T.
As shown, transmitter T is connected to the process, signifying that the operation of transmitter T is controlled by the variable condition under control in the process. The basic function of transmitter T is to.regulate the current in resistor 29 in accordance with the magnitude of the variable condition.
The transmitter T is shown merely as a variable resistor 35, signifying that if one looks into it via its connections to terminals 3 and 13, one sees what appears to be a DC resistance 35 which is a function of the value of the variable condition. As to overall operation of the system, change in the amplifier output changes the variable condition in the process in a sense such that resistor 35 varies, in consequence, in a sense such as to make the amplifier's output change again, but opposite in sense to its original change. Consequently, current through resistor 29 tends to maintain a predetermined value such as to keep amplifier net input substantially zero volts. This predetermined value depends on the settings of potentiometers 30 and 31.
The overall measuring and control system, as above set forth, is well known in the art, hence, no further explanation or description of system and system operation is necessary here.
Having thus described the invention, what is claimed as new 1. An intrinsically safe system having a DC regulator constructed and arranged for location in a hazardous area, said regulator having a pair of first terminals and being responsive to a variable condition for causing DC to flow from one said terminal to the other said terminal in proportion to the magnitude of said condition:
Said system also having a DC source constructed and arranged for location in a safe area, said source having a pair of second terminals, and
said system also having first DC conductive means interconnecting one said first terminal and one said second terminal, and second DC conductive means interconnecting the other said first terminal and the other said second terminal, for defining a closed circuit including said source and said regulator in series with each other, with DC flowing in said circuit via DC conductive means and in proportion to said magnitude;
said first DC conductive means being defined by metering resistance having one end directly connected to said one said second terminal and additional resistance having one end connected to the other end of said metering resistance, said additional resistance having its other end connected to said one said first terminal;
said second DC conductive means being defined by further resistance interconnecting said other first terminal and said other said second terminal;
saidone said second terminal being connected to ground,
and said first tenninals being normally isolated from said ground save through the resistances of said DC conductive means and through the resistance of said source;
and the improvement comprising first zener diode means interconnecting a point on said second conductive means and said ground, and having a zener voltage value corresponding to a voltage at said point that is substantially the largest voltage at that point as can be tolerated for intrinsically safe electrical conditions to obtain in said hazardous area with respect to said regulator; and second zener diode means interconnecting said other end of said metering resistance and said ground and having a zener voltage value that is substantially the value of the largest voltage at said other end of said metering resistance as can be tolerated for intrinsically safe electrical conditions to obtain in said hazardous area with respect to said regulator.
2. The system of claim 1, including additional zener diode means interconnecting a point on said additional resistance and said ground, said additional zener diode having a zener voltage corresponding to said second zener diode means said zener voltage, insofar as are concerned said intrinsically safe electrical conditions, but slightly lesser in value;
and also including further zener diode means interconnecting a point on said further resistance and said ground, said further zener diode means having a zener voltage corresponding to said first zener diode means said zener voltage, insofar as are concerned said intrinsically safe electrical conditions, but slightly lesser in value.
3. The system of claim 2 wherein the first said point on said additional resistance is separated by part of said additional resistance from the second said point on said further resistance;
and the first said point on said resistance also being separated by part of said further resistance from the second said point on said further resistance.
4. The system of claim 2, wherein said additional resistance includes a first fuse means and a further resistance includes a second fuse means; and
said first fuse means being located between the first said point on said additional resistance and the said other end of said metering resistance; said second fuse means being located between said other said first terminal and the first said point on said first resistance.
5. The system of claim 1 wherein said additional resistance includes first fuse means between said other end of said metering resistance and said point on said additional resistance; and
said further resistance including second fuse means between said other second terminal said point on said further resistance.
6. An intrinsically safe system having a DC regulator constructed and arranged for location in a hazardous area, said regulator having a pair of first terminals and being responsive to a variable condition for causing DC to fiow from one said terminal to the other said terminal in proportion to the magnitude of said condition;
said system also having a DC source constructed and arranged for location in a safe area, said source having a pair of second terminals, and
said system also having first DC conductive means interconnnecting one said first terminal and one said second terminal, and second DC conductive means interconnecting the other said first terminal and the other said second terminal, for defining a closed circuit including said source and said regulator in series with each other, with DC flowing in said circuit via said DC conductive means and in pro ortion to said magnitude; said first D6 conductive means being defined by metering resistance having one end directly connected to said one said second terminal and additional resistance having one end connected to the other end of said metering resistance, said additional resistance having its other end connected to said one said first terminal;
said second DC conductive means being defined by further resistance interconnecting said other said first terminal and said other said second terminal;
said one said second terminal being connected to ground,
and said first terminals being normally isolated from said ground save through the resistances of said DC conductive means;
and the improvement comprising a first barrier forming part of said additional resistance for preventing conduction of electrical energy, by said first conductive means, above a given level, and a second barrier forming part of said further resistance for preventing conduction of electrical energy, by said second conductive means, above said given level; and the resistance of said first barrier being large with respect to said second barrier.
7. The intrinsically safe system of claim 6, wherein at least one said barrier is of the type limiting current therethrough.
8. The intrinsically safe system of 6, wherein at least one said barrier is of the type limiting voltage due to current therethrough.
9. The intrinsically safe system of claim 6, wherein at least one said barrier is of the type limiting current therethrough and voltage due to said current.
10. The intrinsically safe system of claim 6, wherein at least one said barrier is of the type connected to ground for diverting current to ground if said level is exceeded.