Do Machinery Rooms Require a Shunt Trip?
The term “shunt trip” is often used in discussions regarding an ammonia refrigeration machinery room e-stop. Technically speaking, a shunt trip is a device that is designed to trip a circuit breaker remotely. In the context of an ammonia refrigeration system, the shunt trip would be located outside the machinery room so that all power to the room could be killed during an emergency.
Is a shunt trip required?
From 1994-2009 the Uniform Mechanical Code (UMC) required that “A clearly identified switch of the break-glass type providing “off” -only control of electrically energized equipment and devices within the refrigeration machinery room shall be provided immediately adjacent to and outside of each refrigeration machinery room exit.” While the term “shunt trip” was not used in the language of the text, code officials often required a shunt trip to satisfy the UMC requirement.
In 2012 UMC, the requirement was changed to clarify that only the refrigeration-related equipment needed to be shut down, instead of all electrically energized equipment within the machinery room. 2012 UMC §1109.4 states that “A clearly identified switch of the break-glass type or with an approved tamper-resistant cover shall be provided immediately adjacent to and outside of the principal refrigeration machinery room exit. The switch shall provide off-only control of refrigerant compressors, refrigerant pumps, and normally closed, automatic refrigerant valves located in the machinery room.”
The less stringent requirement in the 2012 UMC was slightly modified in 2015 UMC (and moved to §1108.3), but included the same basic requirement. 2018 UMC §1102.2 punted completely regarding ammonia refrigeration and directed its readers to comply with IIAR Standards 2-5.
ANSI/IIAR 2-2014 has the same basic requirement as 2012 UMC. ANSI/IIAR 2-2014 §6.12.1 prescribes that “A clearly identified emergency shut-off switch with a tamper-resistant cover shall be located outside and adjacent to the designated principal machinery room door. The switch shall provide off-only control of refrigerant compressors, refrigerant pumps, and normally closed automatic refrigerant valves located in the machinery room. The function of the switch shall be clearly marked by signage near the controls.”
To satisfy the requirements of ANSI/IIAR 2-2014 §6.12.1 it is acceptable to de-energize the equipment by turning control power “off”. As such, installing a “shunt trip” is not required to comply with ANSI/IIAR 2-2014. However, in order to eliminate the potential of a spark from an electric motor, shunt tripping the entire machinery room (minus the emergency ventilation fan) is an effective solution and can be included as a consideration in system design.
 Requirement was in §1108.4 of 1994 and 1997 UMC. In 2000 UMC, the requirement was moved to §1109.4.
 2018 UMC §1102.2 Ammonia Refrigeration Systems. Refrigeration systems using ammonia as the refrigerant shall comply with IIAR 2, IIAR 3, IIAR 4, and IIAR 5 and shall not be required to comply with this chapter.