Straight from the USDOT FAA ATO Policy Document
Chapter 2 General Control
2-1-6. SAFETY ALERT
Issue a safety alert to an aircraft if you are aware the aircraft is in a position/altitude which, in your judgment, places it in unsafe proximity to terrain, obstructions, or other aircraft. Once the pilot informs you action is being taken to resolve the situation, you may discontinue the issuance of further alerts. Do not assume that because someone else has responsibility for the aircraft that the unsafe situation has been observed and the safety alert issued; inform the appropriate controller.
NOTE-
1. The issuance of a safety alert is a first priority (see para 2-1-2, Duty Priority) once the controller observes and recognizes a situation of unsafe aircraft proximity to terrain, obstacles, or other aircraft. Conditions, such as workload, traffic volume, the quality/limitations of the radar system, and the available lead time to react are factors in determining whether it is reasonable for the controller to observe and recognize such situations. While a controller cannot see immediately the development of every situation where a safety alert must be issued, the controller must remain vigilant for such situations and issue a safety alert when the situation is recognized.
2. Recognition of situations of unsafe proximity may result from MSAW/E-MSAW/LAAS, automatic altitude readouts, Conflict/Mode C Intruder Alert, observations on a PAR scope, or pilot reports.
3. Once the alert is issued, it is solely the pilot's prerogative to determine what course of action, if any, will be taken.
a. Terrain/Obstruction Alert. Immediately issue/initiate an alert to an aircraft if you are aware the aircraft is at an altitude which, in your judgment, places it in unsafe proximity to terrain/obstructions. Issue the alert as follows:
PHRASEOLOGY-
LOW ALTITUDE ALERT (call sign),
CHECK YOUR ALTITUDE IMMEDIATELY.
THE (as appropriate) MEA/MVA/MOCA/MIA IN YOUR AREA IS (altitude),
or if an aircraft is past the final approach fix (nonprecision approach),
or the outer marker,
or the fix used in lieu of the outer marker (precision approach),
and, if known, issue
THE (as appropriate) MDA/DH IS (altitude).
Seems pretty close to by the book