In most power transformers, the low voltage winding is placed closest to the core, and the high voltage winding is wound outside of it. This arrangement improves insulation efficiency, since lower voltage windings have less insulation requirement to the grounded core, and high voltage windings can be insulated more easily from the lower voltage layer and tank. It also helps with mechanical stability against short circuit forces. In some special designs the order may differ, but low voltage near the core is the standard configuration.