The following I cut and pasted from the 2002 NFPA 79 "Electrical Standard for Industrial Machinery". I'm not sure what other codes and standards exist out there on this subject. Interpretation of this code will probably also vary, depending on your local AHJ or OSHA Rep. Hope this helps;
9.2.2* Stop Functions. The three categories of stop functions
shall be as follows:
(1) Category 0 is an uncontrolled stop by immediately removing
power to the machine actuators.
(2) Category 1 is a controlled stop with power to the machine
actuators available to achieve the stop then remove power
when the stop is achieved.
(3) Category 2 is a controlled stop with power left available to
the machine actuators.
9.2.5 Operation.
9.2.5.1 General.
9.2.5.1.1 The necessary interlocks shall be provided for safe
operation.
9.2.5.1.2 Measures shall be taken to prevent movement of the
machine in an unintended manner after any stopping of the
machine (e.g., locked-off condition, power supply fault, battery
replacement, lost signal condition with cableless control).
9.2.5.2 Start.
9.2.5.2.1 The start of an operation shall be possible only
where all of the safeguards are in place and functional except
for conditions as described in 9.2.4.
9.2.5.2.2 On those machines where safeguards cannot be applied
for certain operations, manual control of such operations
shall be by hold-to-run controls together with enabling
devices.
9.2.5.2.3 Interlocks shall be provided to ensure correct sequential
starting.
9.2.5.2.4 On machines requiring the use of more than one control
station to initiate a start, the following criteria shall be met:
(1) Each control station shall have a separate manually actuated
start control device.
(2) All required conditions for machine operation shall be met.
(3) All start control devices shall be in the released (off) position
before a start operation is permitted.
(4) All start control devices shall be actuated concurrently.
9.2.5.3 Stop.
9.2.5.3.1 Each machine shall be equipped with a Category 0
stop.
9.2.5.3.2 Category 0, Category 1, and/or Category 2 stops
shall be provided where indicated by an analysis of the risk
assessment and the functional requirements of the machine.
Category 0 and Category 1 stops shall be operational regardless
of operating modes, and Category 0 shall take priority.
Stop function shall operate by de-energizing that relevant circuit
and shall override related start functions.
9.2.5.3.3 Where required, provisions to connect protective
devices and interlocks shall be provided. Where applicable,
the stop function shall signal the logic of the control system
that such a condition exists. The reset of the stop function
shall not initiate any hazardous conditions.
9.2.5.4* Emergency Operations (Emergency Stop, Emergency
Switching Off).
9.2.5.4.1 Emergency Stop. Emergency stop functions provided
in accordance with 9.2.5.3 shall be designed to be initiated
by a single human action.
9.2.5.4.1.1 In addition to the requirements for stop, the
emergency stop shall have the following requirements:
(1) It shall override all other functions and operations in all
modes.
(2) Power to the machine actuators, which causes a hazardous
condition(s), shall be removed as quickly as possible
without creating other hazards (e.g., by the provision of
mechanical means of stopping requiring no external
power, by reverse current braking for a Category 1 stop).
Reset of an emergency stop circuit shall not initiate a restart.
9.2.5.4.1.2 Where required, provisions to connect additional
emergency stop devices shall be provided in accordance with
Section 10.7.
9.2.5.4.1.3 The emergency stop shall function as either a
Category 0 or a Category 1 stop (see 9.2.2). The choice of
the category of the emergency stop shall be determined by
the risk assessment of the machine.
9.2.5.4.1.4 Where a Category 0 stop is used for the emergency
stop function, it shall have only hardwired electromechanical
components.
Exception: An electronic logic (hardware or software) system as well
as the communication network or link that complies with both 9.4.3
and 11.3.4 and is listed for Category 0 emergency stop function shall
be permitted. The final removal of power shall be accomplished by
means of electromechanical components.
9.2.5.4.1.5 Where a Category 0 or a Category 1 stop is used
for the emergency stop function, final removal of power to the
machine actuators shall be ensured and shall be by means of
electromechanical components. Where relays are used to accomplish
a Category 0 emergency stop function, they shall be
nonretentive relays.
9.2.5.4.2 Emergency Switching Off. Where the emergency
switching off function is used, it shall be initiated by a single
human action.