Sorry I guess I shouldn't have start switch. The start is a N.O. contact from a PLC so its maintained.
There are two concepts involved here, "2 wire control" and "3 wire control". The wire numbering system, the "1, 2, 3" is a small part of a standard configuration that has been around since before WWII, developed by what was called the "JIC", Joint Industries Council, which no longer exists. The purpose of the standard was so that electricians, wherever they worked, were going to be able to quickly identify and troubleshoot motor starter circuits in machine tools, regardless of the differences in component design or company standards. It came out of the need for (mainly) Automotive factories to have the ability to use (steal) electricians from each others facilities and feel confident in them not making mistakes. The JIC standards were supplanted by NFPA-79 a long time ago, but the NFPA-79 does not dictate a wire numbering scheme any longer. Still, many of the "old guard" NEMA manufacturers still use it.
In a standard JIC 3 wire control scheme, the start-stop function is done with momentary control devices, which makes it safer in the event of a power failure. 2 wire control schemes are rarely used for that reason, except when it was IMPORTANT for equipment to re-start automatically after a power failure.
The advent of PLCs left many people in the dust on that issue however. Traditionalists (like me) who cut our teeth on the old JIC wiring conventions, typically use a combination of a "Local-Off-Remote" selector switch and a momentary Start-Stop button that only functions in "Local". But when in Remote, then the PLC contact is a 2-Wire control scheme AS WIRED AT THE STARTER, but it is up to the
programmer to cover the re-start contingencies in the PLC program. Unfortunately that issue is often forgotten in the programming process.
When I do that, I add another NO aux. contact off of the starter that is feeding nothing but the pilot light and sourced from AHEAD of the rest of the circuit, i.e. terminal 1 in this case. That way if the light is on, it can ONLY be on if the contactor is pulled in.
Side note to modernize the concept: If there is a PLC involved and no Local mode required, I have taken to only using Solid State OL relays that are communicating to the PLC. The "seal-in" is now done NOT by looking at the status of the motor starter aux. contact, but rather at the FLOW OF CURRENT through the OL relay. That way you don't just know if the contactor is closed, you actually KNOW that the motor is running.
