Disclaimer: NFPA79 is a legal requirement only if (1) your area government has adopted it which is not commonly done or (2) your company has published that it shall comply which is fairly common. Publishing corporate compliance to NFPA79 permits OSHA to legally bind your company to the standard including both civil and criminal liability.
Short Answer: VIOLATION - But see disclaimer.
See NFPA79:2007 9.1 Control Circuits.
Especially NFPA79:2007 9.1.2 Control Circuit Voltages.
"... shall not exceed 120 volts, ac single phase." Exceptions are listed in the standard.
As per VinceS many in the industry are moving toward 24VDC for easier compliance with NFPA70E issues. Safety rated relays and safety rated contactors are used to break the higher voltages but the operator controls must comply with 9.1.2 at 120VAC 1ph or less.
petersonra: Since NFPA79 was noted in the OP I would tend to presume that it is a part of his company's policies and therefore legally binding on him. Section 18.104.22.168 and other sections go extensively into the proper wiring and operation of the Estop circuit. This is far from "is there to stop the machine." or "that is up to you."
A rose by any other name is tax deductible [1978 Wayne Wilcox]
People who read too many books get quirky. [2000 John Taylor Gatto]