In the USA, the more common transformer construction is of the tank type with a gas cushion
at the top of the tank.5 In such a transformer, the pressure wave is detected by a SPR mounted on
the side of the transformer. The mechanical sensor consists of a bellows and a pressure equalizer,
which together is insensitive to slow changes of pressure, for example those caused by thermal or loading changes in the transformer. However, a pressure wave created by a fault is detected by the
relay, and can be used to trip or alarm. In fact, often these relays are found to be too sensitive, and
may respond to the movement of transformer windings caused by severe external fault currents