Thermal fuse
Encyclopedia
A thermal cutoff is an electrical
safety
device that interrupts electric current
when heat
ed to a specific temperature
.
. Unlike the thermal switch which automatically reset
s itself when the temperature drops, the thermal fuse is more like an electrical fuse
: a single-use device that cannot be reset and must be replaced when it fails or is triggered. A thermal fuse is most useful when the overheating is a result of a rare occurrence, such as failure requiring repair (which would also replace the fuse) or replacement at the end of service life
.
One mechanism is a small meltable pellet that holds down a spring. When the pellet melts, the spring is released, separating the contacts and breaking the circuit. The NEC Sefuse SF/E series, Microtemp G4A series and Elmwood D series, for example, use pellets that contain Copper
, Beryllium
, and Silver
.
Thermal fuses are usually found in heat-producing electrical appliance
s such as coffeemaker
s and hair dryers. They function as safety devices to disconnect the current to the heating element
in case of a malfunction
(such as a defective thermostat) that would otherwise allow the temperature to rise to dangerous levels, possibly starting a fire
.
Unlike electrical fuses or circuit breaker
s, thermal fuses only react to excessive temperature, not excessive current, unless the excessive current is sufficient to cause the thermal fuse itself to heat up to the trigger
temperature. Such an arrangement may be found in a surge protector. The thermal fuses are wired in series with the varistors; when the varistors conduct, the fuse heats up and fails, which eliminates the risk of fire which can occur when the varistors are overloaded.
or short circuit
. Unlike the thermal fuse, it is reusable, and is therefore suited to protecting against temporary situations which are common and user-correctable. Thermal switches are used in power supplies in case of overload, and also thermostats in heating and cooling systems.
Thermal switches are included in some light fixture
s, particularly with recessed light
s, where excessive heat is most likely to occur. This may lead to "cycling", where a light turns off and back on every few minutes. Christmas lights take advantage of this effect. Flasher bulbs interrupt power when heated. Twinkle/sparkle mini-bulbs momentarily shunt
current around the filament.
Thermal switches are part of the normal operation of older fluorescent light fixtures, where they are the major part of the starter
.
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
safety
Safety
Safety is the state of being "safe" , the condition of being protected against physical, social, spiritual, financial, political, emotional, occupational, psychological, educational or other types or consequences of failure, damage, error, accidents, harm or any other event which could be...
device that interrupts electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
when heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
ed to a specific temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
.
Thermal fuse
A thermal fuse is a cutoff which uses a one-time fusible linkFusible link
Fusible links include mechanical and electrical devices.A mechanical fusible link is a device consisting of two strips of metal soldered together with a fusible alloy that is designed to melt at a specific temperature, thus allowing the two pieces to separate...
. Unlike the thermal switch which automatically reset
Reset
Reset may refer to:* Ramsey RESET test , a general specification test for the linear regression model* Reset , to clear any pending errors or events and bring a system to normal condition or initial state...
s itself when the temperature drops, the thermal fuse is more like an electrical fuse
Fuse (electrical)
In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...
: a single-use device that cannot be reset and must be replaced when it fails or is triggered. A thermal fuse is most useful when the overheating is a result of a rare occurrence, such as failure requiring repair (which would also replace the fuse) or replacement at the end of service life
Service life
A product's service life is its expected lifetime, or the acceptable period of use in service. It is the time that any manufactured item can be expected to be 'serviceable' or supported by its manufacturer....
.
One mechanism is a small meltable pellet that holds down a spring. When the pellet melts, the spring is released, separating the contacts and breaking the circuit. The NEC Sefuse SF/E series, Microtemp G4A series and Elmwood D series, for example, use pellets that contain Copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, Beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
, and Silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
.
Thermal fuses are usually found in heat-producing electrical appliance
Small appliance
Small appliance refers to a class of home appliances that are portable or semi-portable or which are used on tabletops, countertops, or other platforms in the United States of America...
s such as coffeemaker
Coffeemaker
Coffeemakers or coffee machines are cooking appliances used to brew coffee without having to boil water in a separate container. While there are many different types of coffeemakers using a number of different brewing principles, in the most common devices, coffee grounds are placed in a paper or...
s and hair dryers. They function as safety devices to disconnect the current to the heating element
Heating element
A heating element converts electricity into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element....
in case of a malfunction
Malfunction
A malfunction is when something functions wrongly or does not function at all.Some types of malfunctions are:*Malfunction , malfunction of a parachute*Sexual malfunction, also called "sexual dysfunction"**See also dyspareunia...
(such as a defective thermostat) that would otherwise allow the temperature to rise to dangerous levels, possibly starting a fire
Fire
Fire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....
.
Unlike electrical fuses or circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
s, thermal fuses only react to excessive temperature, not excessive current, unless the excessive current is sufficient to cause the thermal fuse itself to heat up to the trigger
Trigger
-Technology:* Trigger , a mechanism that actuates the firing of firearms* Image trigger, a device used in highspeed cameras* Schmitt trigger, an electronic circuit* Trigger circuit, IBM's name for a flip-flop...
temperature. Such an arrangement may be found in a surge protector. The thermal fuses are wired in series with the varistors; when the varistors conduct, the fuse heats up and fails, which eliminates the risk of fire which can occur when the varistors are overloaded.
Thermal switch
A thermal switch (sometimes thermal reset) is a device which normally opens at a high temperature (often with a faint "plink" sound) and re-closes when the temperature drops. The thermal switch is a bimetallic strip, often encased in a tubular glass bulb to protect it from dustDust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...
or short circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
. Unlike the thermal fuse, it is reusable, and is therefore suited to protecting against temporary situations which are common and user-correctable. Thermal switches are used in power supplies in case of overload, and also thermostats in heating and cooling systems.
Thermal switches are included in some light fixture
Light fixture
A light fixture, light fitting, or luminaire is an electrical device used to create artificial light and/or illumination, by use of an electric lamp...
s, particularly with recessed light
Recessed light
A recessed light or downlight is a light fixture that is installed into a hollow opening in a ceiling...
s, where excessive heat is most likely to occur. This may lead to "cycling", where a light turns off and back on every few minutes. Christmas lights take advantage of this effect. Flasher bulbs interrupt power when heated. Twinkle/sparkle mini-bulbs momentarily shunt
Shunt (electrical)
In electronics, a shunt is a device which allows electric current to pass around another point in the circuit. The term is also widely used in photovoltaics to describe an unwanted short circuit between the front and back surface contacts of a solar cell, usually caused by wafer damage.-Defective...
current around the filament.
Thermal switches are part of the normal operation of older fluorescent light fixtures, where they are the major part of the starter
Electrical ballast
An electrical ballast is a device intended to limit the amount of current in an electric circuit. A familiar and widely used example is the inductive ballast used in fluorescent lamps, to limit the current through the tube, which would otherwise rise to destructive levels due to the tube's...
.