January/February 2013
Certification Review

ISA Certified Control Systems Technician (CCST) program

Certified Control System Technicians (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 main purpose of an optical isolation circuit on a digital input card in a PLC I/O system is:

A.     to provide a common reference point for DC signals.

B.     to isolate the low-voltage circuitry on the digital input card from the field device voltage.

C.     to block light from the surroundings in order to prevent the digital input card from overheating.

D.     to isolate the positive and negative terminals on the digital input card.

CCST answer

The correct answer is B, "to isolate the low-voltage circuitry on the digital input card from the field device voltage." An optical isolation circuit is used to provide a barrier between the field wiring and associated field wiring issues (shorts, ground loops, transients, etc.)  In most PLC systems, this is accomplished with a special high-precision LED as a light source and a phototransistor as a receptor. The dielectric barrier between the two provides physical separation of the LED circuit (connected to the field device in a digital input circuit) and the phototransistor (connected to the PLC card circuit.)

Answer A is not correct. This choice describes a feature of PLC cards, which do not have channel-to-channel isolation.

Answers C and D do not have any relation to the "optics" or "isolation" that is provided by an optical isolator. The "optics" in answer C refers to environmental light, which is not a factor in a PLC input circuit, and answer D refers to the normal isolation of positive and negative terminals in an electrical circuit.

Reference: Goettsche, L. D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.