ISA Certified Control Systems Technician (CCST) program

CCST

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

All electrical power and other paths must be eliminated when measuring _______ of an electrical component.

A. capacitance
B. resistance
C. voltage
D. current

CCST Answer

The correct answer is B: Resistance.

When measuring resistance, all voltage sources should be removed from the circuit whose resistance is to be measured. The multimeter will inject a known voltage into the circuit so that it can measure the resulting current. The multimeter then uses the voltage and current to calculate the resistance using Ohm’s Law. If there are any outside voltage sources remaining in the circuit, the voltage will not be constant, so the calculated resistance will be incorrect.

The resistance measurement displayed by a multimeter is the total resistance through all possible paths between the test lead probes. Measuring resistance across a component that is part of a circuit often requires removal of the component from the circuit.

The resistance of all components connected in parallel with a component being tested affects the resistance reading, usually lowering it. If the component is left in the circuit, the readings could be affected by other components in parallel with the component to be tested.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition (2005), ISA Press