Fire in the test chamber of a rocket LH2 tank

During preparation of a spacecraft LH2 tank in an environmental test chamber, a liquid hydrogen fire occurred in the chamber. The tank was being "topped off" at the time. Damage was caused to the facility and test system.

[Ordin, NASA (1974)]

Air had entered the system through the vent line quick disconnects. Ignition occurred as a result of bare wires in the heater access hole. A protective insulation grommet had inadvertently been omitted. The discrepancy had not been identified by inspection. ROOT CAUSES are related to an error in the execution of one preparatory step and in failing to identify it during inspection. Moreover, vent are expected to be designed to avoid or limit as far as possible entrance of air, preventing formation of explosive gas mixture.

environnmental test chamber

ENVIROMENTAL TESTING CHAMBER FOR THE ARTEMIS
NASA used different environmental chambers for cryogenic and structural testing for the Saturn V liquid hydrogen stage, loacated atdifferent R&D centres . The source reporting this event does not clarify the type of tests performed. It cn be guessed that it was a cryogenic proof tests, Fior example, the 3rd March 1968, a Saturn V second stage (S-II-4) underwent its first cryogenic proof test at the Mississippi Test Facility, now NASA's Stennis Space Centre).

WHAT IS A GROMMET?
An insulation grommet is a small, ring-shaped component, usually made of rubber, that is inserted into a hole to protect wires, cables, or hoses passing through it from sharp edges and abrasion. They are also used for electrical insulation, shock absorption, and creating a seal against dust, moisture, and vibration.

test facility and test system damaged

On the recurrence of air into vent stacks during NASA programme, see for example Rhodes, R., Explosive lessons in hydrogen safety, Ask Magazine 2013 (https://appel.nasa.gov/wp-content/uploads/2013/04/513855main_ASK_41s_explosive.pdf).