Products
-> Ancilliaries
-> Thermal Protection Devices
Thermal Cut-out
 These
are devices where the element responsible for operating
the device is composed of two different metals fused
together as the bimetal component. The metals have different
expansion properties that when heated cause the shape
of the bimetal component to deform. This physical deformation
is used to open or close an electrical contact.
Type 1 (Manual Reset)
These are trip devices that operate directly in the
supply circuit to the transformer. The unit is usually
mounted in the wall of the transformer enclosure so
the device can be manually reset after the fault or
overload condition has been removed and the unit cooled
down. They are supplied with a range of current ratings
up to about 50 Amps.
The device will trip at a specified temperature which
is reached as a combination of the ambient temperature
inside the enclosure from the heat generated by the
transformer and also the self heating from the current
that flows through it.
Type 2 (Automatic Reset)
These devices are constructed in numerous formats with
two options. “Normally Open” a device that
is open circuit until heated when it closes, and “Normally
Closed” which has the opposite characteristic.
The devices are manufactured to operate at a pre-determined
temperature and come in ranges of temperature options
typically between 60°C and 190°C. 
These devices can be either built into the body of
the transformer winding or they can be mounted on the
coil surface. Usually they will only handle small currents
up to about 10-15 Amps so they can be used direct in
the supply circuit of transformers rated up to 3KVA
or indirectly via some other tripping device e.g. an
MCCB or a contactor
When the unit cools to about 20°C below the tripping
temperature the device will automatically reset itself,
depending on the control circuitry this will permit
either automatic re-start of the supply or allow for
manual reset.
Thermal Fuse
 These
are “single shot” devices that again operate
directly in the supply to the transformer. They are
built into the windings of the transformer and become
open circuit when the temperature exceeds their thermal
rating.
The units are not repairable once the fuse has blown
PTCR
The Positive Temperature Coefficient Resistor (PTCR)
is a device manufactured such that at a predetermined
temperature the resistance of the component will change
from a low value (100’s to 5Kohm) to a high value
(10Kohm to 1Mohm). The resistance of the device is monitored
by a PTCR Relay that is used to interrupt the supply
to the transformer being protected. As the temperature
drops the PTCR resistance will switch back to a low
value. Depending on the control circuit this can be
used to automatically re-start the unit or leave it
in the off condition pending human intervention to re-start
Thermocouple Devices
 Thermocouples
are created by joining conducting wires of two differing
types. This joint will create a small voltage signal
the size of which varies as the temperature of the joint
changes. A suitably calibrated monitoring instrument
can measure this voltage and convert it to a temperature
reading which can be displayed. These monitoring units
can be programmed to operate switching and control gear
at pre-set temperatures.
In addition to providing safety protection by removing
the supply from a transformer that is overheating some
of these devices can also be used to operate cooling
fans on type ANAF transformers.
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