What is the NEMA or industry standard symbol used for an NCTO set of timing relay contacts?

The NEMA (National Electrical Manufacturers Association) standard symbols for relay contacts are integral for understanding how these components are represented in electrical schematics. In the case of an NCTO (Normally Closed Timing Open) set of timing relay contacts, the designation is specifically represented by the letter 'C'.

This symbol 'C' indicates that the relay is designed to open its contacts after a predetermined delay, reflecting the timed functionality that is central to its design. In practice, this means that when the relay is energized, the contacts remain closed for a specific time before they open. This function is critical for timing applications in various electrical systems, where ensuring a delay before circuit interruption is necessary.

Understanding this standardization helps technicians and electricians easily identify relay functions on schematics and ensures consistency in electrical design and troubleshooting across various applications. The other letters correspond to different types or configurations of relay contacts that do not match the functionality of an NCTO setup, hence making 'C' the correct and relevant choice for this question.