Process control system for hazardous areas

Claim 2

An improved process control system of the type normally having control, record and measurement instruments together with a power source in a control room communicating with instruments in a process area, wherein the improvement for adapting the system for use in hazardous areas comprises: an interface for limiting the current in all signal paths leading to and from the process area, and a voltage limiter between the power source and the interface for limiting the voltage supplied to the interface means.

Claim 3

The improved process control system as claimed in claim 1 wherein the interface includes: amplifiers for processing signals being sent to and from the process area, and resistors for current limiting associated with the input side of each amplifier.

Claim 4

The improved process control system as claimed in claim 1 wherein the interface comprises: a control room portion, a process area portion, input resistors for signals originating in the process area in said process area portion, amplifiers for signals originating in the process area in said control room portion, input resistors for signals originating in the control room in said control room portion, and amplifiers for signals originating in the control room in said process area portion.

Claim 5

The improved process control system as claimed in claim 3 wherein the voltage limiter includes; a first cable for receiving power from the power source, a second cable for supplying power to the control room portion of the interface means, and a third cable for supplying power to the process area portion of the interface means.

Claim 6

The improved process control system as claimed in claim 3 wherein the voltage limiter further includes; series fuses of different values in said first and second cables.

Claim 7

An intrinsically safe interface assembly for use between a control area containing instrumentation and a process area containing field devices in a hazardous atmosphere, comprising: a control room side connected to the control area; a field side for providing signal and power connections to the process area; impedance devices connecting the control room side and the field side for limiting the current transferred therebetween; and a bi-directional voltage barrier for limiting the voltage applied to the interface having an input to which a source of power may be applied, a first output connected to the control room side and a second output connected to the field side, the voltage limit of the barrier being determined by the current limit of the impedance devices so that the energy transmitted to the process area is constrained below the level required for ignition of the hazardous atmosphere even during conditions of failure of any device in the power source, the control area, and the control room side and field side of the interface.

Claim 8

An intrinsically safe interface assembly for use between a control area containing instrumentation and a process area containing field devices in a hazardous atmosphere, comprising: a control room side connected to the control area; a field side for providing signal and power connections to the process area; impedance devices connecting the control room side and the field side for limiting the current transferred therebetween; and a bi-directional voltage barrier for limiting the voltage applied to the interface having an input to which a source of power may be applied, a first output connected to the control room side and a second output connected to the field side, the voltage limit of the barrier being determined by the current limit of the impedance devices so that the energy transmitted to the process area is constrained below the level required for ignition of the hazardous atmosphere even during conditions of failure of any device in the power source, the control area, and the control room side and field side of the interface.

Claim 9

The interface assembly of claim 6 wherein the barrier comprises: an SCR connected across the input of the barrier, and a triggering circuit connected thereto for sensing the voltage applied to the input and causing the SCR to conduct when the voltage limit is exceeded.

Claim 10

The interface assembly of claim 7 wherein the barrier further comprises: a first set of fuses in each input selected to burn out when the SCR begins to conduct.

Claim 11

The interface assembly of claim 8 wherein the barrier further includes a second set of fuses in series with the first set selected to burn out when the SCR conducts for a predetermined time.

Claim 12

The interface assembly of claim 9 wherein the SCR, triggering a circuit and second set of fuses are potted for environmental protection, whereby the barrier may be reused after the SCR was caused to conduct by replacement of the first set of fuses.