Interpretation ID: aiam3641
CPSM
Product Safety Engineer
Grumman Flxible Corporation
970 Pittsburgh Drive
Delaware
OH 43015;
Dear Mr. Sidner: This responds to your March 31, 1983, letter asking about the lo-ai pressure warning device that is required by Standard No. 121, *Air Brake Systems*. In particular you ask whether the requirement must be met with two switches, one on the primary and one on the secondary air tank, or whether one switch on the supply tank would be sufficient.; S5.1.5 of the standard states that a warning signal shall be provide to give a continuous warning of a loss of pressure in the service reservoir system below 60 p.s.i. The intent of the section clearly is to provide a warning to a driver of any instance that results in a loss of pressure in either the primary or secondary service reservoir system.; Depending upon the design of any individual brake system, it might b necessary for more than one switch to ensure that the driver will be alerted to such an air loss. For example, a single switch on the supply tank might be inadequate to signal a loss of pressure in the primary or secondary service reservoir system if the action of the compressor masks such loss of pressure. It is up to the manufacturer to determine whether its system is designed in a way that requires more than one switch.; Sincerely, Frank Berndt, Chief Counsel