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. It
should be noted that, aside from the incorrect operation just described, these pressure-type relays
can only detect faults inside the transformer tank. Faults on the bushings, and connecting leads, do
not create an arc in the insulating oil or gas, and must be protected by differential relays. In fact, a
combination of pressure relays and differential relays provides an excellent protection system for
a power transformer. In the presence of gas pressure relays, the differential relays can be made
less sensitive. Indeed, one may attempt reclosing on those faults which cause the operation of the
differential relay, but not of the pressure relay.