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The on-load tap changer (OLTC) is a crucial and expensive component of every power transformer. Its primary function is the dynamic adjustment of load voltage to specified levels without interrupting load current. Apart from thermal limitations at each tap position, the main concern in correctly rating an OLTC is the basic impulse insulation level (BIL). The reason for this is that during the standard BIL test on a power transformer, damaging overvoltage may develop on the regulating winding, which is transferred to the OLTC, which may damage the OLTC, and as a result, fail the BIL test on the power transformer. In regard to the lightning impulse voltage test, since the overvoltage that develops is a consequence of the distributed capacitance and inductance of the regulating winding, and as there are several types of regulating windings, each with different values of distributed capacitance and inductance, the correct rating of the OLTC will depend on the type of regulating winding used. Therefore, the transformer designer must be able to estimate the transferred impulse overvoltage, for the purpose of not only choosing the right type of regulating winding, but also to limit the overvoltage through passive or active means, for example incorporating electrostatic shielding in the regulating winding or installing a MOV surge arrestor. With this aim, computational tools are a powerful aid for assisting the transformer designer, since they allow the computation of distributed capacitance and inductance of the regulating winding, as well as estimating the distributed impulsive voltage. On the other hand, in regard to voltage withstand at power frequency, the induced AC voltage test is also important for determining the BIL of an OLTC. This article presents a practical case as to the steps that must be taken by a transformer designer in rating a power transformer OLTC.