INTRINSICALLY SAFE CIRCUIT

• US 3,611,053 A
• Filed: 10/10/1969
• Issued: 10/05/1971
• Est. Priority Date: 10/10/1969
• Status: Expired due to Term

• Current Assignee: William G. Rowell
• Original Assignee: William G. Rowell
• Inventors: Rowell, William G.
• Primary Examiner(s): Caldwell, John W.
• Assistant Examiner(s): Myer, Daniel
• Application Number:
  US04/865,457
• Time in Patent Office: 725 Days
• Field of Search: 340/255,213,248 174/52.6 336/96 323/60,61 317/9AC,11,186,DIG.9,123 307/92-94,136
• US Class Current: 361/160
• CPC Class Codes: G08B 17/04 Hydraulic or pneumatic actuation of the alarm, e.g. by change of fluid pressure

Abstract

A circuit for use with a sensor located in a hazardous area such as in an atmosphere of explosive gases, organized to prevent electrical power at the sensor from exceeding prescribed levels even upon reasonably foreseeable failures of parts of the circuit. The circuit, which is located in a nonhazardous area, includes an indicator, such as a relay-controlled alarm, to respond to the sensor, and terminals for connection to a source of line voltage. Two voltage stepdown transformers have their secondaries connected in series with one another and with the sensor. The primary of one transformer is connected to the line voltage source; and the primary of the other transformer is connected to the indicator. Each transformer secondary lies intermediate a pair of current-limiting resistors, and the primary and secondary windings of each transformer are isolated by means of a grounded conductive sheet. Each transformer, along with its associated current-limiting secondary resistors, is embedded in potting material to form a self-contained isolating circuit module. The circuit is intended to be connected to a remote ground terminal as well as the ground terminal of the line voltage source. A ground integrity detector circuit interrelates these three terminals by means of resistances and a neon glow tube is provide a signal whenever one of the ground circuits is lost, as well as a signal upon failure of one of the resistances forming the ground integrity detection circuit. The remote ground connection has a path extending through the circuit chassis to insure that its ground is maintained.

Claims

1. A circuit for monitoring a sensor located in a hazardous region comprising:

conductors extending from the sensor to a nonhazardous region;

a source of line voltage in the nonhazardous region;

a first voltage stepdown transformer having its primary connected to the voltage source;

indicating means responsive to the Sensor and located in the nonhazardous region, a second voltage stepdown transformer having its primary connected to said indicating means;

the secondaries of said first and second transformers being connected in series with each other and in series with the sensor through said conductors;

whereby faults arising in either the indicating means, the transformer primaries, or in the sensor or its conductors, present a substantially reduced energy discharge in the hazardous region.

2. A circuit according to claim 1 wherein in each stepdown transformer the primary winding is separated from the secondary winding by a sheet of material acting as a barrier to prevent the primary voltage from appearing across the secondary winding.

3. A circuit according to claim 2 wherein said barrier sheet is conductive and is connected to ground.

4. A circuit according to claim 1 wherein each of said transformers comprises a conductive core connected to ground.

5. A circuit according to claim 1 wherein said indicating means comprises a relay having its winding connected in series with said second transformer primary.

6. A circuit according to claim 1 wherein said sensor comprises a switch adapted to be opened and closed in said hazardous region.

7. A circuit according to claim 1 further comprising current-limiting resistances located in said nonhazardous area and connected in series with each of said transformer secondaries.

8. A circuit for monitoring the condition of a sensor located in a hazardous region by means of an indicator responsive to the sensor, comprising:

a first isolation module having input terminals for connection to a source of voltage, and output terminals;

a second isolation module having input terminals for connection to said indicator, and output terminals;

said first and second isolation modules being located in a nonhazardous region; and

conductors extending from said sensor to said nonhazardous region and being connected to said isolation module output terminals;

each of said isolation modules comprising a voltage stepdown transformer having its primary across the module input terminal;

the secondary of said transformer being connected in series between current-limiting resistances to the module output terminals;

said transformer and said current-limiting resistances being embedded in potting material.

9. A circuit according to claim 8 wherein each of said isolation modules comprises a conductive sheet separating its secondary and primary windings, said sheet being grounded, and wherein each of said transformers comprises a grounded conductive core.