Interpretation ID: nht94-4.19

TYPE: Interpretation-NHTSA

DATE: September 2, 1994

FROM: John Womack -- Acting Chief Counsel, NHTSA

TO: John Collins -- Senior Vice President, Government Affairs, American Trucking Associations (Alexandria, VA)

TITLE: None

ATTACHMT: Attached to letter dated 4/18/94 from John Collins to John G. Womack (OCC 9876)

TEXT:

This responds to your letter about a recent amendment to the supply line retention requirements in Standard No. 121, Air Brake Systems. I apologize for the delay in our response. You requested an interpretation of the test procedure set forth in S5.8.2 . That provision states, in relevant part that,

A trailer shall meet the above supply line retention requirement with its brake system connected to the trailer test rig shown in Figure 1, with the reservoirs of the trailer and test rig initially pressurized to 100 psi, and the regulator of the test rig set at 100 psi.

Specifically, you believe that this provision means that (1) the test rig remains connected to the shop air, as shown in Figure 1, for the duration of the test, (2) the shut-off valve of the test rig remains open for the duration of the test, and (3) the pressure in the test rig's 1000 cubic inch reservoir is maintained at 100 psi for the duration of the test. I will address each of these suggested interpretations below.

The basic issue raised by your questions is whether the supply line retention test is conducted with air flowing from the test rig (simulating the flow of air from a tractor), or with the supply of air cut off. It is our opinion that this test is conduc ted with the air flowing from the test rig.

As you suggested in your letter, this result is implied by the language of S5.8.2. That section states that a trailer must meet the supply line retention requirement with its brake system connected to the trailer test rig, and with the regulator of the test rig set at 100 psi. There would be no reason to set the regulator at 100 psi if air was not flowing from the test rig. We therefore agree with the first of your three suggested interpretations, that the test rig remains connected to the shop air f or the duration of the test.

We similarly agree with the second of your suggested interpretations, that the shut-off valve of the test rig remains open for the duration of the test. S5.8.2 does not specify that this valve be shut, and such shutting would be inconsistent with conduct ing the test with air flowing from the test rig.

Your third suggested interpretation is that the pressure in the test rig's 1000 cubic inch reservoir is maintained at 100 psi for the duration of the test. We note that no special efforts are made to maintain this pressure at 100 psi. As indicated above, S5.8.2 specifies that the reservoir of the test rig is INITIALLY set at 100 psi; the pressure could vary during the test. However, as indicated above, 100 psi air pressure would continue to flow through the regulator during the test in the direction of the reservoir.

I hope this information is helpful. If you have any questions about NHTSA's safety standards, please feel free to contact Marvin Shaw at this address or by telephone at (202) 366-2992.