In a test plan for a 4 - 20 mA transmitter, forcing the transmitter to operate at 110% of scale is to detect:

• A. Power supply voltage droop
• B. Faults in intrinsic safety barriers
• C. Increased resistance in field wiring
• D. All of the above

Answer: (B)

When considering the scenario of a test plan for a 4 - 20 mA transmitter, forcing the transmitter to operate at 110% of scale is essential for detecting faults in intrinsic safety barriers. Intrinsic safety barriers are designed to limit the energy available in the event of a fault, ensuring that sparks or thermal effects that could ignite hazardous materials do not occur. By pushing the transmitter beyond its specified scale, one can assess whether these safety barriers function properly under abnormal conditions. If the barriers are functioning correctly, they will prevent excessive current from impacting safety.

This approach would not effectively address power supply voltage droop or increased resistance in field wiring. Voltage droop typically affects the operational range but does not specifically tie to the transmitter's percentage of scale in the same manner. Increased resistance in field wiring might cause a reduction in signal strength or affect accuracy but again does not isolate intrinsic safety barrier functionality. Forcing the transmitter to operate at 110% is a deliberate method to verify the integrity of the safety controls that are crucial in hazardous locations, making this the most relevant outcome of the testing method described.