Protection
The sudden pressure relay, device 63, trips a transformer when internal pressure rises abruptly, as it does in an arcing fault. It is a mechanical device, so testing uses a pressure source instead of electrical injection, then proves the contacts and the trip circuit.
Device 63 in transformer protection is the sudden pressure relay, also called a rapid pressure rise relay. It is mounted on the tank of an oil-filled power transformer, either in the gas space above the oil or below the oil level on the side of the tank, and it responds to how fast the internal pressure rises, not to how high it is. It is a mechanical device with a sensing bellows or diaphragm, a small equalizing vent and a switch contact. Unlike an electrical relay, it takes no current or voltage from the system, so it is not tested by secondary injection. The field test checks that the unit responds to a rapid pressure change, that it ignores a slow one, that its contacts and wiring are sound, that the housing is sealed against moisture and that the contact drives the trip or alarm circuit as designed.
An internal arcing fault in a transformer vaporizes oil in a fraction of a cycle and creates a pressure wave that can rupture the tank, ignite the oil and spread the damage to neighboring equipment. Electrical protection such as differential relays can see the fault, but the sudden pressure relay adds an independent, fast sensing method that responds to the physical effect of the fault, and it can catch faults that produce little change in terminal currents, such as a local arc inside a winding. A unit that is clogged, seized, corroded or wired to a dead circuit will not respond when needed, and moisture in the housing can cause false operation or failed contacts. Because the device rarely operates, only periodic testing shows whether it is still alive.
The relay depends on a difference in speed. A fault produces a rapid rise in pressure, and the bellows or diaphragm inside the relay is pushed with a force that its small equalizing port cannot relieve in time, so the sensing element moves and actuates the contact. Normal changes in pressure, from load cycling, ambient temperature and thermal expansion of the oil, occur slowly, and the vent bleeds them off so the pressures on both sides of the element stay equal and the contact does not move. The device is therefore a rate-of-rise detector: it is calibrated against a pressure ramp and not against a pressure level. Gas-space and oil-space versions use the same idea, with differences in mounting and in how the internal volume is arranged, and the manufacturer's data give the response characteristic for each model.
The test is performed with the transformer de-energized and isolated, and the trip circuit to the breaker or lockout relay is blocked or the links opened so that the test cannot trip live equipment. The relay is inspected for oil leaks, a broken sight glass, a damaged gasket and moisture in the terminal enclosure, and its wiring is checked for tight, clean terminations and sound conduit seals. Many relays have a test fitting or a test valve, and a small hand pump or regulated pressure source, together with a gauge, is connected to apply a rapid pressure change. The pressure is applied quickly enough to represent a fault, and the contact is watched with a continuity tester or an indicator to confirm that it operates, and that it reopens or can be reset afterward. A slow pressure application confirms that the equalizing vent keeps the contact at rest. The contact insulation and the circuit it drives are checked, and the trip or alarm indication is confirmed. The blocked links are restored and the restoration is recorded.
The record states the relay type and location, its condition on inspection, the way pressure was applied and the instrument used, whether the contact operated on a fast change and stayed at rest on a slow one, and the response of the alarm or trip circuit. It also notes the insulation or continuity checks on the wiring, the state of seals and gaskets, any repair or replacement of parts, the links blocked and restored, and the calibration status of the gauge or tester.
The unit passes when it operates reliably on the rapid pressure change the manufacturer specifies, remains at rest on a slow change, resets as intended and drives its circuit correctly. Results are compared with the manufacturer's response data for the model, with earlier tests of the same unit and with identical units on sister transformers. A contact that sticks, a bellows that leaks or a housing that admits moisture is a reason to repair or replace the device. ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 cover the sudden pressure relay with the protective relay tests. For the values and tolerances, refer to your own licensed copy of those standards and to the relay instruction book.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It is a mechanical device that senses how fast tank pressure rises, and it takes no current or voltage from the system. The test therefore uses a controlled pressure source and checks the contact and wiring.
Both are mechanical transformer protections, but a Buchholz relay sits in the pipe to the conservator and reacts to gas and oil flow, while the sudden pressure relay reacts to a fast rise of pressure in the tank.
The vent lets slow pressure changes from temperature and load equalize on both sides of the sensing element. Only a fast rise, as in a fault, overcomes it and moves the contact.
Common problems are moisture in the terminal housing, corroded or loose terminals, a clogged vent, a sticky bellows and trip wiring that was disconnected after earlier work. Inspection plus a pressure test finds most of them.
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