Non-destructive pinhole testing of enamelled wire by a corona method

A `pinhole¿ in enamel on a wire is a spot of insufficient electric strength to pass certain tests. The B.S. method of counting pinholes requires the wire to be passed through a mercury bath. This is considered destructive because it contaminates the enamel surface. A non-destructive method is described in which the wire is run through an open tube held at a sufficiently high voltage for corona to occur on the passage of a pinhole. It may be applied on enamelling plant and winding machines. Corona onset conditions are discussed and a formula is derived for the minimum electric stress to be applied to the bare wire surface. Peek´s empirical law for the onset of visible corona between coaxial electrodes is recommended for calculating the maximum permissible surface stress. A suitable high-voltage supply unit and pulse-detecting and -counting circuits are suggested. Attention is drawn to the data-handling problem arising from the accumulation of a large amount of test information which must be reduced to an accept/reject decision. The solution adopted in a multi-channel instrument is based on the use of magnetic-core sealers and paper-tape distribution recorders.