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Selected definitions are compliments of the Vibration Institute, Willowbrook, IL.
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Glossary of Technical Terms Related to Sensors, Controls, and Protectors

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A/C: Air conditioning.

AC: Air conditioning.

Accelerometer: A transducer whose output is n electrical/mechanical directly proportional to acceleration forces. The output is usually produced by force applied to a piezoelectric crystal which generates a current proportional to the applied force. This current is then amplified and displayed as a time waveform or processed by a Fourier transform to produce a frequency display. Single integration of the acceleration signal will produce a velocity display and double integration of the acceleration signal will produce a displacement display.

Acoustic Emission: The detected energy that is generated when materials are deformed or break. For rolling element bearing analysis, it is the periodic energy generated by the over rolling of particles or flaws and detected by the display of the bearing flaw frequencies.

Actuation Force: The force which must be applied to the button to cause the moveable contact to snap from the normally closed contact position to the normally open contact position.

Actuation Point: The position of the button at which the moveable contact snaps from the normally closed contact position to the normally open contact position.

Actuation Pressure: The pressure at which the electrical element is operated on increasing pressure.

AFCB: Arc–fault circuit breaker.

AFCI: Arc–fault circuit interrupter.

Algorithm: A specific procedure for solving mathematical problems. An FFT is an algorithm.

Alignment: A condition whereby the axes of machine components are either coincident, parallel or perpendicular, according to design requirements, during operation.

Ambient Compensation: A feature of some thermal circuit breakers that limits or eliminates thermal derating due to ambient temperature.

Amplitude: The measurement of energy or movement in a vibrating object. Amplitude is measured and expressed in three ways: Displacement (commonly in mils Pk–Pk); Velocity (commonly in In/Sec Pk); and Acceleration (commonly in gs RMS). Amplitude is also the y–axis of the vibration time waveform and spectrum, it helps define the severity of the vibration.

ASIC: Application–specific integrated circuit.

Automatic Reset: A type of thermostat that will reset itself at a specific temperature (setpoint minus differential = reset temperature).

Auxiliary Circuit: An integrally housed, electrically independent switch linked mechanically to operate with the main circuit breaker contacts.

Bearing: Primarily two types, rolling element and sleeve or plain bearing. Rolling element bearings consist of four parts: an inner race, an outer race, balls or rollers, and a cage to maintain the proper separation of the rolling elements. A sleeve bearing is a cylinder of alloy metal surrounding the rotating shaft. Contact between the shaft and sleeve is prevented by a lubrication film.

Bearing Frequencies: Faults in any of the four bearing components will generate specific frequencies dependent upon the bearing geometry and rotating speed:
  • BPFO: Ball Pass Frequency, Outer Race
  • BPFI: Ball Pass Frequency, Inner Race
  • BSF: Ball Spin Frequency
  • FTF: Fundamental Train Frequency
Bearing Misalignment: A misalignment that results when the bearings supporting a shaft are not aligned with each other. The bearings may not be mounted in parallel planes, cocked relative to the shaft, or distorted due to foundation settling or thermal growth.

Bearing Nomenclature: Each bearing manufacturer has specific codes applied as prefixes and suffixes to their bearings. These codes inform the user of the construction, materials, clearances, and other factors used in the construction of the bearing. Consult the individual manufacturer’s handbook for specific code meaning.

Bimetal Disc: The mechanism that opens or closes the electrical contacts. It consists of two dissimilar metals bonded together then formed with a specific curvature. When heated, the internal stresses of the bimetal cause the disc to reverse its curvature with a snap action at a fixed, preset temperature and operate the electrical contacts. The decrease in ambient temperature below the preset temperature of the disc relieves the internal stresses in the disc. The disc returns to its normal curvature and the contacts assume their normal operating position.

Burst Pressure: The maximum pressure to which the switch can be subjected without causing leakage. Permanent damage may occur to the unit if burst pressure is applied.

Calculated peak: Term used to describe the spectral overall RMS level multiplied by sqrt (2). Sometimes referred to as “derived peak” or “pseudo peak.”

Circuit Breaker: A device Designed to carry a specific value of current and automatically open a circuit upon overloads or short circuits.

Close on Rise: When the operating temperature is reached, electrical contacts close to complete the circuit. It is also referred to as a Fan Type Device.

Common Source Impedance Feedback: Occurs when an arc–fault in one circuit affects the source voltage enough to impose an arc signal onto any circuits that run in parallel to the arcing circuit. Feedback is likely to occur on lines with lower power rated supplies, and if not considered can also cause an unnecessary power interruption on the non arcing circuit.

Contacts: For low level applications (as defined in MIL-S-8805), we recommend the use of gold plated contacts. These can be substituted in all of our configurations.

Contact Erosion: A general loss of material from one or both surfaces of a pair of mating contacts. This is a result of switching electrical loads.

Contact Materials: The size, shape, and material determine the performance ability of a contact in a switch. AT style switches utilize silver contact material due to its excellent conductivity, low electrical resistance and superior wear characteristics. Gold plating is also used on all of our 10AT and some of our AT switches.

Contact Resistance: The resistance offers to a flow of current during its passage between electrical contacts. For practical reasons, lead and terminal resistance may be included in the actual measurement, as well as the contact resistance proper. Also: The electrical resistance of a pair of closed contacts, measured between switch terminals.

CSA: Canadian Standards Association.

DFPS: Dual–function pressure switch.

Deactuation Pressure: The pressure at which the electrical element is operated on decreasing pressure.

Deadband: The difference in pressure between the actuation and deactuation setpoints. it is also known as the differential.

Dielectric Withstanding Voltage: The maximum voltage that a switch can withstand between specified points without leaking current exceeding a specified value.

Dielectric Strength: A device’s ability to withstand a deliberate application of pre–determined over–voltage for a specified time.

Differential: The delta between operating temperature and reset temperature or between open temperature and close temperature.

Discrete: With reference to a spectrum, discrete means consisting of separate distinct points, rather than continuous. An example of a discrete spectrum is a harmonic series. An FFT spectrum, which consists of information only at specific frequencies (the FFT lines), is actually discrete regardless of the input signal. For instance, the true spectrum of a transient is continuous, and the FFT of a transient appears continuous on the screen, but still only contains information at the frequencies of the FFT lines. The input signal to an FFT analyzer is continuous, but the sampling process necessary to implement the FFT algorithm converts it into a discrete form, with information only at the specific sampled times.

Dynamic Range: The ratio in dB between the highest signal level that can be tolerated without distortion and the broadband noise level measured in the absence of the signal.

EMI: Electromagnetic interference.

Electrical Life: The number of cycles allowable for a switch under a specified combination of electrical load, actuation and environmental criteria. Usually specified for the maximum current and temperature rating of the switch.

Essential Equipment: Plant equipment whose failure, while not shutting down an entire operation/process/production line, will cause a major disruption to operations.


Fast Fourier Transform (FFT): The FFT is an algorithm, or digital calculation routine, that efficiently calculates the discrete Fourier transform from the sampled time waveform. In other words it converts, or “transforms” a signal from the time domain into the frequency domain.

FFT: Fast Fourier Transform.

Free Position: The position of the button when there is no external force other than gravity applied to it.

Ground Arc–Fault: An arc–fault between circuit and ground reference.

HVAC: Heating, ventilation, and air conditioning.

HVAC/R: Heating, ventilation, air conditioning, and refrigeration.

Hermetic Seal: A gas tight seal from the environment. Sealing is done via a gas-to-metal seal.

I/O: Input / output.

IC: Integrated circuit.

Indicating, Non–Indicating: Whether or not breaker gives prominent visual indication of opening, such as exposing a colored band around button, or moving toggle to off position.

Induced Soft Foot: A type of soft foot that is caused by external forces (coupling, pipe strain, etc.) acting on a machine independent of the foot to baseplate connection.

Inductive: Inductive loads result in the tendency most contact erosion. This is due to the tendency of the current to continue travelling across the contact while opening, causing arcing.

Inductively Coupled Cross–Talk: Occurs when an arc–fault in one circuit causes an arc signal to be injected onto a closely coupled circuit. Cross talk is likely to occur on lines that are physically collocated (run in the same bundle for example), and if not considered can cause an unnecessary power interruption on the non–arcing circuit.

Insulation Resistance: The resistance offered by the insulating members of a component part to direct voltage tending to produce a leakage current through or on the surface of these members. Insulation resistance should not be considered the equivalence of dielectric withstanding voltage. Also: Measures the resistance of the insulating member of the device to a target voltage.



Lamp: In–rush current is the condition which results in the lamp load rating. the resistance rating of a lamp is normally specified when the lamp is hot. However, when the lamp is cold the resistance is low. This in–rush current can be 10 times the normal current, resulting in welding of contacts.

Limit Device: When operating temperature is reached, electrical contacts open to break the circuit. It is also referred to as Open On Rise.

Loads: Three types of loads associated with electrical switches are: resistive, inductive and lamp. Of concern when discussing loads are their effect on the switch contacts.

Low–Level Circuit Applications: AT series switches are ideal for use in low level circuit applications where maximum reliability is needed. Low level circuits are classified in MIL-8805 as those with resistive loads less than 10mA at 30mV. The use of gold plated contacts and the hermetic enclosure of the switch contacts afford an ideal environment for switching low level loads.

Manual / Automatic Reset: Method of accomplishing reclosure after circuit interruption has occurred.

Manual Reset: A type of thermostat that must be reset manually, such as by depressing a push button.

Manual Trip: Ability of breaker to be opened manually.

Maximum Overtravel Position: The position of the button beyond which further overtravel would damage switch.

Maximum Ultimate Trip: Current rating at which breaker must trip within a certain period (usually 1 hour) at a specified temperature.

Mechanical Life: The number of cycles allowable for a switch with no electrical load, and a specified combinator of actuation and environmental criteria.

Minimum Ultimate Trip: Current rating at which breaker will not trip within a certain period (usually 1 hour) at a specified temperature.

Movement Differential: The distance the button moves from the actuation point to the release point.

Normally Closed: (A thermostat switching action.) A limit type application; contacts open on temperature rise at a predetermined temperature to de–energize circuit. Contacts automatically re–close as the device cools to a pre–determined temperature.

Normally Open: (A thermostat switching action.) A fan type application; contacts close on temperature rise at a pre–determined temperature to energize the circuit. Contacts automatically re–open as the device cools to pre–determined temperature.

OEM: Original equipment manufacturer. A company that includes hardware components from other companies in its own product.

Open On Rise: When operating temperature is reached, electrical contacts open to break the circuit. It is also referred to as a Limit Device.

Operating Temperature: (Thermostats) Temperature at which normally closed contacts open or normally open contacts close.

Overtravel: The distance that the button can travel beyond the actuation point.

Parallel Arc–Fault: An arc–fault between two parallel portions of a circuit (including arcing to adjacent active lines), where the arcing current bypasses the circuit load (generally higher current).

Peak: The maximum positive or negative dynamic excursion from zero (for an AC coupled signal) or from the offset level (for a DC coupled) of any time waveform. Sometimes referred to as "true peak" or "waveform peak."

Peak-to-peak: The amplitude difference between the most positive and most negative value in the time waveform.

Peak Scaling, Peak-to-Peak Scaling, RMS Scaling: Method to display the amplitude axis of a spectrum. The methods are Peak Scaling, Peak-to-Peak Scaling, RMS Scaling.

Peak-to-Peak Scaling: Method to display the amplitude axis of a spectrum. The methods are Peak Scaling, Peak-to-Peak Scaling, RMS Scaling.

PIND: Particle impact noise detection

Pole: The number of completely separate circuits contained in a switch. A single pole switch can control only one circuit at a time. A double pole can control two independent circuits at the same time.

Pretravel: The distance the button moves from the free position to the actuation point.

Proof Pressure: The maximum momentary pressure, including surges, that a switch can withstand without causing permanent degradation to the switch setpoints.


RAC: Room air conditioner.

RFI: Radio frequency interference.

REF: Refrigeration.

RMS Scaling: Method to display the amplitude axis of a spectrum. The methods are Peak Scaling, Peak-to-Peak Scaling, RMS Scaling.

RPC: Remote power controller.

Release Force: The level wo which the force on the button must be reduced to allow the contacts to snap from the normally open position to the normally closed contact position.

Release Position: The position of the button at which the moveable contact snaps from the normally open contact position to the normally closed contact position.

Reset Temperature: The temperature at which the contacts will return to the normal position. Subtract the differential from the operating temperature to determine the reset temperature.

Resistive: Purely resistive loads are the easiest to switch. Contact life is maximized when switching purely resistive loads because contact erosion is minimized.

Spectrum: The spectrum is the result of transforming a time domain signal to the frequency domain. It is the decomposition of a time signal into a collection of sine waves. The plural of spectrum is spectra. Spectrum analysis is the procedure of doing the transformation, and it is most commonly done with an FFT analyzer.

SPDT: Single Pole / Double Throw: an electrical switch capable of controlling two different circuits. Also: An electrical switch with one set of terminals that terminate ar actuate a single circuit.

SPST: Single Pole / Single Throw: an electrical switch with one set of terminals that terminate or actuate a single circuit.

SSPC: Solid–state power controller.

Series Arc–Fault: An arc–fault between two series portions of a circuit, where the arcing current is limited by the circuit load (generally low current).

Set Point: Operating temperature: temperature at which the disc changes its curvature and snaps to open or close electrical contacts.

Termination Leads or Connectors: Klixon® uses 20 gauge wire per MIL-W-16878. Alternative gauges, MIL specs and color coding schemes are available upon request. Sensata Technologies will also provide switches with connectors to meet customer application needs.

Thermal Circuit Breaker: A circuit breaker that senses a current overload based on thermal heating of the sensing element.

Thermal Derating: Tendency of a breaker to trip at lower current levels due to higher ambient temperatures, and to trip at higher levels due to lower ambient temperatures.

Throw: The number of circuits that each individual pole of a switch can control. The number of throws is completely independent of the number of poles.

Time–Current Curve: An approximate graph showing the minimum and maximum time a specific breaker will take to trip on various degrees of overload.

Tolerance: The maximum allowable deviation from the nominal setpoint under all specified environmental and operational conditions. It represents the total of setting deviations due to calibration, manufacturing, environmental and temperature changes, etc. Also: The allowable range above or below the set point or reset temperature.

Transducer: Any device that translates the magnitude of one quantity into another quantity. Four of the most common transducers used in measurements are pressure transducer, accelerometer, velocity transducer, and eddy current probe.

Trip–Free: Feature of certain breakers that makes it impossible to hold the breaker closed against an overload. Their contacts cannot be held in the closed position by the reset button if overload conditions persist.

UL: Underwriters Laboratory.

Velocity Transducer: An electrical/mechanical transducer whose output is directly proportional to the velocity of the measured unit. A velocity transducer consists of a magnet suspended on a coil, surrounded by a conductive coil. Movement of the transducer induces movement in the suspended magnet. This movement inside the conductive coil generates an electrical current proportional to the velocity of the movement. A time waveform or a Fourier transform of the current will result in a velocity measurement. The signal can also be integrated to produce a displacement measurement.

Voltage Drop: The voltage decrease across the breaker due to internal resistance of the device.

Wetted Material: That portion of the pressure switch which comes in contact with the fluid media.



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