A partial discharge is an electrical discharge that only partially bridges the insulation between conductors. Such a discharge is generally considered to take place as a precursor to total insulation failure but may exist for a long period of time, possibly years, before total breakdown occurs. In some circumstances the existence of the discharge will modify the stress distribution so as to initially reduce the tendency to total breakdown. In time, however, total breakdown will always result, often because the discharge itself leads to chemical breakdown of the insulation which reduces its electrical strength. Clearly, in a healthy transformer under normal operating conditions the only acceptable level of partial discharge is nil.