What is NOT a method of testing protective relays as per NEC guidelines?

The correct answer is based on the practices outlined in the NEC guidelines regarding the testing of protective relays. Protective relays serve crucial roles in the protection of electrical systems, and testing them is typically aimed at verifying their operational reliability and accuracy.

Visual inspection for correct polarity, which is identified as the method that is NOT recognized as an appropriate testing method, pertains more to ensuring that the installation adheres to electrical codes and standards rather than a means of functional testing. While checking for correct polarity is important for ensuring safety and proper operation, it does not assess the operational effectiveness or responsiveness of the protective relay itself.

In contrast, the other methods listed are actively utilized to assess the performance of protective relays. Injecting different voltages at the outputs allows for observing how the relay responds under various conditions. Using current injection is a technique that tests the relay's ability to react to overcurrent situations. Observing timing and sequencing during operation ensures that the relay responds within the required time frames and sequences as specified in its design. These methods are integral to demonstrating that the relays will function correctly in real-world scenarios, hence why they are accepted practices for testing protective relays as per NEC guidelines.