NEC Commercial Electrical Practice Test

At what minimum voltage must secondary circuits of current and potential instrument transformers be grounded?

1. 100 volts

2. 200 volts

3. 300 volts

4. 400 volts

The correct answer is: 300 volts

The minimum voltage at which secondary circuits of current and potential instrument transformers must be grounded is specified as 300 volts. This requirement is in place to ensure safety and proper operation within electrical systems. Grounding transformer circuits at this voltage level helps prevent dangerous voltages from appearing in case of insulation failure or other electrical faults. By establishing a grounding point, it also facilitates the proper operation of protective devices and helps limit transient overvoltages that can occur in the system. Additionally, this grounding practice is aligned with the NEC (National Electrical Code) standards, which aim to provide a safe working environment and protect both equipment and personnel from electrical hazards. This is particularly important in commercial and industrial settings where large equipment and higher voltages are prevalent, increasing the risk of electrical shock or fire. The other voltage options do not meet the NEC standards for minimum grounding requirements, as they are either below the threshold needed to ensure safety or do not align with industry practices regarding voltage levels in transformer circuits.