Mike:
No, they generally don't emit fumes. The ventilation requirements as called out in Art. 450 of the NEC deal primarily with heat dissipation from operation and ventilation of smoke and heat from a fire within the vault.
I do recall reading that all oil filled transformers, when there is internal arcing, will build up hydrogen and acetyline (sp?) gasses which could explode/ignite.
That same book also explained why oil/liquid filled transformers often fail after an extended outage. It seems that the heat and core/winding vibrations keep any contaminants (usually water and gasses) of the cooling/insulating fluid in suspension. After an extended cool-down, the contaminates and trapped gasses in the fluid form bubbles on the windings and internal conductors. This severely reduces the insulating values of the fluid, and when re-energized it results in arcing and failure. Many distribution and a few large substation transformers have failed, often violently, from this situation.
[This message has been edited by mxslick (edited 03-30-2006).]
