September/October 2010

ISA Certified Control Systems Technician (CCST) program

CCSTs calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

The purpose of an intrinsic safety barrier is to:

A.        Provide a physical obstruction between the hazardous and non-hazardous areas.

B.         Establish a central grounding point in the hazardous area.

C.        Prevent excess voltage and current from reaching the hazardous area.

D.        Increase the resistance in the circuit to reduce the risk of explosion.

CCST answer

The correct answer is C, prevent excess voltage and current from reaching the hazardous area. In order for ignition to occur, three elements are required: Fuel (something to burn-gas, vapor, or powder), an oxidizer (air or oxygen), and an ignition source (electrical or thermal). An intrinsic safety barrier is designed to remove the last element-an ignition source-from the equation. Without an ignition source, there can be no ignition or combustion.

Answer A is incorrect because just providing a physical obstruction between the hazardous and non-hazardous area does not eliminate any of the three elements required for ignition.

Answer B is incorrect because a common ground does not guarantee that a voltage or current less than what is required for ignition will not be present.

Answer D is incorrect because resistance does not necessarily reduce the overall energy in the circuit.

Reference: ANSI/ISA-60079-0 (12.00.01)-2005 - Electrical Apparatus for Use in Class 1, Zones 0, 1 & 2 Hazardous (Classified) Locations.