Bucholz Relay

A PRV (Pressure Relief Valve) opens when the internal pressure of the tank is greater than its setpoint; sometimes there is also a contact closure to indicate that it has operated.

A Bucholz relay's contact(s) close when the amount of gas, or rate of internal gas production exceeds its setpoint; sometimes it operates due to a rapid rise in tank pressure.

The third type of transformer pressure related relay is the SPR (Sudden Pressure Relay) that is supposed to operate only when the internal pressure's rate of rise exceeds its setpoint.

The words "sometimes" and "supposed to" are used because all of these relays are prone to misoperation and false trips due to externally generated events such as equipment vibration, very high magnitude short circuit current pass-through, or anything that mechanically makes the oil compress or slosh around unexpectedly.

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