Pressure Relief Devices
These devices are the transformers’ last line of defense against excessive internal pressure. In the event of a fault or short circuit, the resultant arc instantly vaporizes surrounding oil, causing a rapid buildup of gaseous pressure. If the pressure relief device does not operate properly and pressure is not sufficiently relieved within a few milliseconds, a catastrophic tank rupture can result, spreading flaming oil over a wide area. Two types of pressure relief devices are discussed below. Consult your transformer’s instruction manual for specifics.
Never paint pressure relief devices because paint can cause the plunger or rotating shaft to stick. Then, the device might not relieve pressure, which could lead to catastrophic tank failure during a fault. Look at the top of the device; on newer units, a yellow or blue button should be visible. If these have been painted, the button will be the same color as the tank. On older units, a red flag should be visible; if it has been painted, it will be the same color as the tank.
If the pressure relief devices have been painted, they should be replaced. It is virtually impossible to remove all paint from the mechanism and be certain the device will work when needed.