Which gas is usually absent when certain faults occur in cellulosic insulation?

When certain faults occur in cellulosic insulation, acetylene is usually absent. This is primarily due to the type of decomposition that occurs in the insulation material when it is subjected to extreme conditions such as overheating or electrical arcing.

Cellulosic insulation is an organic material derived from plant fibers, and when it undergoes thermal decomposition, it generates various gases as byproducts. Acetylene is a hydrocarbon gas that typically forms during the incomplete combustion of organic materials at high temperatures. However, in the specific context of cellulosic insulation faults, the decomposition process may not favor the formation of acetylene, leading to its absence.

On the other hand, gases such as methane, carbon dioxide, and oxygen can be produced during the degradation of cellulosic materials. Methane can form during anaerobic processes, while carbon dioxide is a common byproduct of complete combustion. Oxygen, although consumed in reactions, might still be present in varying quantities pre- and post-fault conditions. Thus, the presence of acetylene is markedly low or non-existent when specific faults in cellulosic insulation occur, making it the correct answer in this context.