Document 2.4.126, Revision 4
October 2017
23
Three-Phase Padmounted Distribution Transformers
terminal compartment at the bottom of the front
panel. Refer to “Sampling and Testing the Fluid”
and “Draining and Filling the Tank” for the fluid
sampling and draining procedures.
Automatic Pressure Relief Device
The automatic pressure relief device (PRD) is
designed to relieve excessive tank pressure that
might occur during operation of the transformer.
The device consists of a self-resealing, spring-
loaded diaphragm. Some PRD types also include a
re-settable visual flag to indicate that the valve has
operated. PRD’s are usually mounted on the front
panel inside the terminal compartment; however
some high flow-rate PRD’s are designed to be
mounted externally on the tank cover and enclosed
in a tamper-resistant housing.
When gas pressure in the tank exceeds the PRD’s
specified limit, the gas pressure will cause the valve
to open, venting the excess pressure. After the
internal pressure decreases below the PRD reseal
rating, the valve will automatically close and reseal
the transformer. For PRDs’ equipped with a visual
indicating flag, the flag must be manually reset.
Some PRD’s can be supplied with optional switch
contacts, which can be used to provide a remote
alarm of valve activation.
Internal Fault Detector
The Internal Fault Detector (IFD) is a mechanical
sensor that activates when sudden pressure
from an internal arcing fault occurs inside the
transformer. If an internal fault occurs, the IFD
releases a visible, non-resettable orange signal flag.
This signal flag alerts crews that the transformer is
faulted and should not be re-energized.
The IFD also includes a standard pressure relief
device that is integrated into the sensor to relieve
excessive tank pressures that might occur during
normal operation of the transformer.
The IFD incorporates a removable shipping lock
for transportation and storage. The shipping lock
must be removed after transformer installation.
Always transport IFD-equipped transformers with
the shipping lock installed to prevent accidental
operation.
High-Voltage Bushings
Three-phase padmounted transformers with live-
front construction are normally supplied with
externally-clamped porcelain high-voltage bushings
for connection to the high-voltage source. Bushings
are usually supplied with tin-plated eye-bolt
terminals that are suitable for connection to both
aluminum and copper conductors.
Three-phase padmounted transformers with dead-
front construction are supplied with universal
bushing wells, one-piece (integral) bushings or
universal bushing wells with factory-installed
inserts. One-piece bushings and bushing inserts
are designed to interface with insulated elbow
connectors, and can be supplied as either load-
break or non load-break devices.
WARNING
The cause of PRD activation should always
be investigated, since pressure venting
may indicate a potential problem inside
the transformer. FAILURE TO FOLLOW THIS
INSTRUCTION MAY RESULT IN DEATH,
SEVERE PERSONAL INJURY AND/OR
DAMAGE TO THE EQUIPMENT.
WARNING
FAILURE TO FOLLOW THE INSTRUCTIONS
BELOW MAY RESULT IN DEATH OR SERIOUS
PERSONAL INJURY, AND/OR DAMAGE TO
THE EQUIPMENT.
• Do not re-energize a transformer if the
IFD has operated.
• Always assume that a transformer
might be faulted, even if the IFD has not
operated.
• Never rely solely on the IFD as an
indicator of transformer condition.
• Transformers should always be re-
energized according to the user’s
standard safety procedures.
