The recommendations presented in this guide primarily apply to new large oil-immersed power transformers with high-voltage windings rated 69-287 kV inclusive. However, appropriate sections of the guide may be applied to lower voltage transformers when similar conditions and similar transformer construction exist, and may also be used in servicing older transformers which have been opened for maintenance or repair work. Increasing application of larger capacity high-voltage transformers with reduced insulation has brought forth the parallel need for increasing care during installation and service. To maintain dielectric strength of the insulation it is essential to avoid moisture and entrapped air or gas bubbles. Since oil is an essential portion of the insulation structure, oil quality and care in handling are extremely import ant .