US3445679A - Intrinsically safe electrical control system - Google Patents

Description

May 20, 1969 c. F. MEYER ETAL I INTRINSICALLY SAFE ELECTRICAL CONTROL SYSTEM Filed Dec. 15, 1966 INVENTOR.

CHARLES F. MEYER ROBERT A.WIECZOREK E/M J United States Patent 3,445,679 INTRINSICALLY SAFE ELECTRICAL CONTROL SYSTEM Charles F. Meyer, Wauwatosa, and Robert A. Wieczorek, Milwaukee, Wis., assignors to Square D Company, Park Ridge, [1]., a corporation of Michigan Filed Dec. 15, 1966, Ser. No. 602,089 Int. Cl. H01h 9/30 US. Cl. 307136 9- Claims ABSTRACT OF THE DISCLOSURE The disclosure pertains to an intrinsically safe circuit wherein the transmission of electrical energy to a switching device located in a hazardous area is limited to a value insufficient to cause an explosion in the hazardous area upon any type of malfunction of the circuit. Malfunction of the circuit from any cause is prevented by spirally wound resistors, a grounded shield transformer, and a light coupling between the circuit in the hazardous area and the remainder of the circuit located in the nonhazardous area through a photocell and a lamp to prevent transmission of dangerous electrical energy to the switching device in the hazardous area from the switching circuits in the nonhazardous area.

The present invention relates to electrical control systems and is more particularly concerned with an intrinsically safe electrical switching system for use in connection with areas wherein explosive atmospheres are present.

The requirements of an intrinsically safe control system are generally expressed in the National Electric Code as follows:

Equipment and associated wiring approved as intrinsically safe may be installed in any hazardous location for which it is approved and the provisions pertaining to conventional explosion-proofing procedures need not apply to such installation. Intrinsically safe equipment and wiring is incapable of releasing suflicient electrical energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture. Abnormal conditions will include accidental damage to any part of the equipment or wiring, insulation or other failure of electrical components, application of over-voltage, adjustment and maintenance operations, and other similar conditions.

It is an object of the present invention to provide an improved acceptable intrinsically safe control system.

Another object of the present invention is to provide a control system wherein an intrinsically safe circuit in a hazardous area is electrically isolated from an output circuit in a nonhazardous area by a light emitting and a light sensitive element.

A further object is to provide an intrinsically safe switching system for controlling the operation of a switching device located in a nonhazardous area from a control station located in a hazardous area where explosive mixtures of gases or dust may be present to eliminate the requirement that electrical control equipment in the hazardous area be enclosed in an explosion-proof housing or be prevented from igniting the explosive mixtures, as by purging, sealing and the like, and to couple the circuit in the hazardous area with a supply circuit for a switching device in the nonhazardous area through a photo sensitive device to electrically isolate the circiut in the hazardous area from the supply circuit and to supply the circuit in the hazardous area with an intrinsically safe amount of energy through resistors of the spirally wound type from a secondary winding of a stepdown type transformer that has a grounded shield interposed between the primary and secondary windings to prevent current from a high energy supply to the primary winding of the transformer from flowing in the circuit in the hazardous area in event of a short between the primary and second ary windings of the transformer.

Further objects and features of the invention will be readily apparent to those skilled in the art from the specification and the appended drawing illustrating a preferred embodiment in which an intrinsically safe electrical control system incorporating the features of the present invention is diagrammatically shown.

In the drawing, a barrier wall 10 separates an area 12 wherein explosive gases or dust are present from an area 14, having a nonhazardous atmosphere. Included within the area 12 is a control station 16 and included within the area 14 is a control circuit indicated by a numeral 18. The control circuit 18 has a pair of input terminals 20 and 22 connected via a pair of leads 24 and 26 to a source 28 of alternating current located in the nonhazardous area 14. The control circuit 18 also has a pair of output terminals 30 and 32 arranged to supply an output signal through a lead 36- to an external load 38, not specifically shown. The control circuit 18 is enclosed within a housing 40 that has an internal barrier 42 physically dividing the housing 40 into two compartments indicated by the numerals 44 and 46. The terminals 20, 22, 30 and 32, and a G terminal, as well as a switching device shown as a relay 48, are positioned within the compartment 46. The relay 48 has a normally open contact 49 connected across the terminals 30 and 32 which close and complete a circuit to the load 38 upon energization of the relay 48. Three terminals 50, 51 and 52, located within the compartment 44, are connectible by leads 54, 55 and 56 to a switching circuit 60 within the control station. The leads 54, 55 and 56 each extend through a suitable atmospheric seal 58 in the barrier Wall 10 to the switching circuit 60 located within the control station 16. The atmospheric seal 58 prevents passage of the explosive atmosphere in the hazardous area 12 to the nonhazardous area 14. The switching circuit 60 is provided with a switch 62 having normally open contacts and a switch 63 having normally closed contacts. The switches 62 and 63 may be of the standard push button variety, the contacts of a conventional limit switch, or the contacts of a metering device or the like, and the control station 16 may or may not have an explosion proof enclosure, as desired. The leads 54, 55 and 56 are not required to have any type of explosion proof covering and may extend a considerable distance from the control station 16 to the control circuit 18. The barrier 42, the control station 16, as well as the enclosure 40, are all illustrated by broken lines. The barrier 42 is positioned within the enclosure 40 to have a top edge engageable with a bottom surface of a cover, not shown, for the enclosure to physically separate the compartments 44 and 46 and prevent the cover from being secured to the enclosure in event the leads 54, 55 and 56 are attached to any one of the terminals 20, 22, 30 or 32 and/or the leads 24, 26 and 36 are secured to the terminals 50*, 51 and 52. In the event that accidentally during installation any one of the leads are incorrectly thus positioned, the cover will be prevented from being secured to the enclosure by the leads which extend over the top edge of the barrier 42. Thus the control circuit 18 will be required to be properly connected before the circuit is placed in operation.

Included within the compartment 44 is a step-down type transformer 61 having a primary winding 64, a secondary winding 66, and an iron core 68. The primary winding 64 and the secondary winding 66 are each wound upon the core 68 and are electrically separated by a metallic shield 70 having a connection with a ground G. The primary winding 64 is connected to the terminals and 22 to be supplied with alternating current from the source 28.

Also included within the compartment 44 is a full wave bridge rectifier 72, a capacitor 74, a resistor 76, a Zener diode 78, a light emitting element shown as a lamp 82, and a light sensitive element 84, such as a nonpolarized semiconductor whose resistance decreases when subjected to light, a resistor 88, an electronic switch such as a silicon controlled rectifier 90 having an anode electrode 91, a cathode electrode 92 and a gate electrode 93, and a firing and filter circuit for the rectifier gate circuit including the resistor 95 and a capacitor 94.

The rectifier 72 has a pair of input terminals 96 and 97 connected to receive the output of the secondary winding 66 and diodes polarized to provide a pair of output terminals 98 and 99 with full wave rectified current making the terminal 98 positive in polarity relative to the terminal 99. The capacitor 74 has one side connected to the terminal 98 and another side connected through a junction 100 to the terminal '99. The Zener diode 78 has an anode connected to the junction 100 and a cathode connected through a junction 102 to the terminal 50. The resistor 76 is connected between the terminal 98- and the junction 102 in parallel with the capacitor 74 and the Zener diode 78.

The controlled rectified 90 has its anode 91 connected to the terminal 52, its cathode 92 connected through the lamp 82 to the terminal 99 and its gate electrode 93 connected through the resistor 88 to the terminal 51. Connected in parallel between the cathode 92 and the gate electrode 93 are the resistor 95 and the capacitor 94. The light sensitive element 84, which is physically located to receive light from the lamp 82, is connected in series circuit with the operating coil of the relay 48 across the alternating current supply 28. Included in the switching circuit 60 is a junction 103 which is directly connected by the lead 56 to the terminal 52. Additionally, the junction 103 is connected through the closed contacts of the switch 63 and the lead 54 to the terminal 50 and is connectible through the contacts of the switch 62, when the switch 62 is operated, and the lead 55 to the terminal 51.

The operation of the circuit shown in the drawing is as follows. During standby conditions the switch 62 is in a circuit opening condition and the switch 63 is in a circuit closing condition. The voltage of the source 28, as reduced by the primary 64 and the secondary 66 windings of the transformer 61, rectified by the rectifier 72, and filtered by the capacitor 74, appears as a full wave rectified filtered voltage between the terminals 98 and 99. The voltage between the terminals 102 and 99 is limited by the Zener diode 78. In the event the voltage between the terminals 102 and 99 exceeds the break-down characteristics of the diode 78, the diode 78 will conduct, in the reverse direction, causing current flow from the terminal 98, through a circuit which includes the resistor 76, the junction 102, the diode 78 and the junction 100 to the terminal 99. Also during standby conditions, the controlled rectifier 90 is nonconducting and the lamp 232 extinguished. Thus the light sensitive element 84 will be de-energized and have a high resistance causing the relay 48 to be de-energized so its contact 49 interrupts the circuit to the load 38.

When the switch 62 is operated and its contacts are moved to a circuit closing position, a firing circuit for the controlled rectifier 90 is completed from the terminal 98 through the resistor 76, the junction 102, the terminal 50, the lead 54, the closed contacts of the switch 63, the closed contacts of the switch 62, the lead 55, the terminal 51, the resistor 88, the gate to cathode electrodes 93 and 92 of the controlled rectifier 90 and the lamp 82 to the terminal 99. The gate to cathode current through the .4 rectifier switches the rectifier 90 to a conductive state that is maintained as direct current flows from the terminal 98 through the resistor 76, the junction 102, the terminal 50, the lead 54, the closed contacts of switch 63, the lead 56, the terminal 52 and the anode 91 to cathode 92 of the rectifier 90 and the lamp 82 to the terminal 99. The anode to cathode current flow through the rectifier 90 causes: the lamp 82 to be illuminated, the resistance of the light sensitive element 84 to markedly decrease and the relay 48 to be energized so that the contact 49 closes and completes a circuit through the load 38. The conductive state of the rectifier 90 continues as long as the switch 63 is closed, permitting the switch 62 to be opened, after its initial operation, without de-energizing the circuit which includes the lamp 82. The de-energization of the load 38 is achieved when the controlled rectifier is switched to a nonconductive state by opening the switch contacts 63 to interrupt the anode circuit of the controlled rectifier 90. When the lamp 82 is de-energized as the rectifier 90 becomes nonconductive, the resistance of the light sensitive element 84 again increases to a value which causes the relay 48 to be deenergized and the contacts 49 to interrupt the circuit to the load 38.

The intrinsically safe features of the circuit are basically achieved by the combination of the spirally wound resistors, the grounded shield transformer and the isolation provided by the lamp-photocell in the circuit. The resistors 76, 88 and 95 each are spirally wound with a wire having a positive temperature coefficient without crossovers and sealed in a crack-free, fireproof ceramic case. Thus upon failure, the resistors will fail to an open circuit state. The transformer 62, by having a grounded shield 70 between the primary winding 64 and the secondary winding 66, is protected against short circuits between the primary and secondary winding and prevents possible high energy level currents from the supply 28 from flowing in the switching circuit 60. The isolation provided by the lamp 82 and the photocell 84 positively prevents transmission of dangerous currents from the relay 48 circuit to the switching circuit 60. Finally in the preferred embodiment, the components in the compartment 44 are protected from physical damage by encapsulating the same in a suitable epoxy resin, not shown.

What is claimed is:

1. An intrinsically safe switching system for controlling the operation of a switching device located in a nonhazardous area from a control station in a hazardous area where an explosive atmospheric mixture may be present, comprising:

(a) an actuable switching circuit in the hazardous area, said switching circuit having at least a portion that is not sealed or provided with explosion proof enclosing means or the like,

(b) a source of alternating current in the nonhazardous area,

(c) a control circuit in the nonhazardous area, said control circuit having a connection through a first circuit with the switching circuit and a connection through a second circuit With the switching device and isolated from the switching circuit in the hazardous area by a hermetic seal which prevents passage of the explosive mixture between the hazardous and the nonhazardous areas.

(d) said control circuit including: means in the first circuit for supplying a small and intrinsically safe amount of electrical energy through the switching circuit insufiicient to ignite the explosive mixture in the hazardous area when the switching circuit is actuated, and means including a light emitting element in the first circuit providing an output signal in response to the flow of electrical energy in the switching circuit and a light sensitive element in the second circuit for receiving the output signal from the light emitting element and causing the switching device to be energized from the source in response to the output signal and electrically isolating the second circuit from the first circuit.

2. The combination as recited in claim 1 including a transformer having a primary winding connected across the source, a secondary winding connected to supply the first circuit and the switching circuit with the intrinsically safe energy and the second circuit with energizing current and a ground shield interposed between the primary and secondary windings for preventing unsafe electrical energy from flowing in the switching circuit in event of a short circuit between the primary and the secondary windings.

3. The combination as recited in claim 1 including at least one spirally wound resistor connected in a series circuit between the source and the switching circuit for limiting current flow in the switching circuit.

4. The combination as recited in claim 2 including at least one spirally wound resistor connected in a series circuit between the source and the switching circuit for limiting current flow in the switching circuit.

5. The combination as recited in claim 2 including a full wave rectifier having a pair of input terminals connected across the secondary winding and a pair of output terminals connected to supply the switching circuit and the first circuit with the electrical energy.

6. The combination as recited in claim 5 including a spirally wound resistor connected in series circuit between the switching circuit and the output terminals of the rectifier.

7. The combination as recited in claim 5 including a silicon controlled rectifier having an anode and a cathode connected in a series circuit with the light emitting device and a gate and the cathode connected in a series circuit with the switching circuit for energizing the light emitting device with direct current when the switching means is actuated.

8. The combination as recited in claim 7 including a switch having normally open contacts connected in the switching circuit in series with the gate of the controlled rectifier.

9. The combination as recited in claim 7 including a switch having normally closed contacts connected in the switching circuit in series with the anode of the controlled rectifier.

References Cited UNITED STATES PATENTS 7/1965 Elliot 307136 3/1967 Elliot 307l36 US. Cl. X.R. 317123

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