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December 06, 2019
Sensata Airpax hydraulic-magnetic circuit protectors. (Image courtesy of Sensata Technologies.)
(Sensata-Airpax hydraulic-magnetic circuit protectors. (Image courtesy of Sensata Technologies.)


Circuit protectors are components which protect electrical circuits from damage in case of faults in the system such as high temperature, high current or short circuits. In general, there are three types of protectors: fuses, thermal circuit breakers and magnetic circuit breakers. Fuses are an inexpensive solution for high current capacity, though they are imprecise, temperature-dependent and destroyed after protection. Thermal circuit breakers can be used as the main switch and be reset after tripping, but they are heat-sensing devices that are affected by changes in ambient temperature.


Hydraulic-magnetic circuit protectors have hydraulic-magnetic current sensing mechanisms which operate only to current changes in the protected circuit. These protectors are combined with a hydraulic delay to make them tolerant to inrush current. They combine power switching and accurate, reliable circuit protection with inverse time delays and trip-free features.


Due to their temperature stability, magnetic circuit protectors are unaffected by temperature changes. Unlike thermal circuit protectors, they do not change trip current over a wide temperature range. Because of this, the protectors can operate at any ambient temperature between -40° C and +100° C.


Another advantage of hydraulic-magnetic circuit protectors is their main safety feature: trip-free action. The trip-free delay mechanism senses the fault and blocks the circuit from closing in case of overload, even when the contacts are closed manually. This prevents the operator from damaging the protected circuit by holding the handle in the ON position.


Hydraulic-magnetic circuit protectors are a suitable solution for applications such as communication, navigation and aircraft instrumentation, radar and power supplies. These circuit protectors should be used whenever site conditions could influence the normal operation of circuit protectors, including low temperatures which could prevent thermal protection from tripping, high temperatures which could trip thermal protection without overload and moisture which can apsorb heat and affect the operation.




Hydraulic-magnetic circuit protectors can be designed with the following configurations: series trip, relay trip, remote indication and voltage trip. The schematic diagram and illustration of the different combinations can be found in the circuit protector’s data sheet.


Series trip is the most popular configuration for hydraulic-magnetic protectors. In this configuration, the sensing coil and contacts are in series with the protected load (Figure 1). The handle position conveniently indicates circuit status. In addition to providing conventional overcurrent protection, it’s simultaneously used as a power disconnect.

Figure 1. Series trip circuit protector configuration.
Figure 1. Series trip circuit protector configuration.


The shunt trip configuration is designed for controlling two separate loads with one assembly. This configuration provides overload protection for the critical load. When the load current becomes excessive and reaches the trip point, the protector will trip and remove power from both loads simultaneously. The total current rating of both loads must not exceed the maximum contact rating of the protector.

Figure 2. Shunt trip circuit protector configuration.
Figure 2. Shunt trip circuit protector configuration.


The relay trip configuration enables the overload sensing coil to be placed in a circuit which is electrically isolated from the trip contacts. The coil may be actuated by sensors monitoring pressure, flow, temperature, speed, etc. The trip is accomplished by voltage or current, which must be removed immediately upon tripping.

Figure 3. The relay trip circuit protector configuration.
Figure 3. The relay trip circuit protector configuration.


The remote indication includes electrically isolated contacts. These contacts provide SPDT switching action (Single Pole Double Throw—with one input terminal and two output terminals) which can be used to indicate “power-on” and “power-off” conditions or to actuate lights, alarms or timing devices.


The voltage trip (also known as the panic trip) configuration enables opening the main power contacts with lower power inputs from one or more sources. This configuration is becoming increasingly important for sensitive circuitry and denser packaging in automation systems. This solution is available in series, shunt or relay configurations.

Figure 4. Voltage trip circuit protector configuration.
Figure 4. Voltage trip circuit protector configuration.




The main characteristics of the hydraulic-magnetic circuit breaker are inrush pulse tolerance, time delay (depending on the current peak amplitude), nominal ratings, short circuit capacity and multi-pole combinations.


The pulse tolerance is defined as a single pulse of half sine wave peak current amplitude of 8ms duration that will not trip the circuit protector. The manufacturer, usually following the table, provides a comparison of inrush pulse tolerance with and without the inertial delay feature for each of the 50/60Hz delays. Also, the specifications provide the delay curves for standard instant, short and long delays.


The nominal ratings of the circuit protectors reperesent the defined values of the current, voltage and frequency in nominal protector operation. For example, Sensata AP protectors are available in current ratings from 50mA to 20A, 50Vdc or 240Vac maximum, and 60Hz or 400Hz.


The short circuit capacity interrupts the capacity of the circuit protector. This represents the maximum RMS current that would be present if all the conductors were to be connected directly to the fault location. For example, Sensata AP supplementary protectors have a short circuit capacity of 1000 amperes at 32Vdc, 120Vac, 240Vac, 60Hz or 400Hz. The MIL-PRF-39019 type is 500A, 50Vdc, 500A, 120Vac and 300A, 240Vac, 60Hz or 400Hz.


The protectors can be designed in single pole or multi-pole units. Some specific types are available in single pole units only, mostly because of the UL/CSA regulations which specify creepage and clearance distance. Other hydraulic magnetic circuit protector types are available in one, two and three pole series trip units, with or without auxiliary contacts. One appropriate representative of this protector type is the Sensata-Airpax AP-MIL.


A common trip linkage between all poles is an important safety feature for multi-pole circuit protectors, ensuring that an overload in one pole will trip all adjacent poles. Multi-pole circuit protectors offer different variations, from simple two pole units with one series breaker and a simple ON/OFF switch, to more complex three pole units with one series, one shunt and one relay configuration with auxiliary indicator circuit contacts.


Two pole protectors consist of two of the basic units combined in a sealed case to provide protection for dual circuit. A commonly used application of two pole protectors is to interrupt either or both sides of a power line.


Three pole protectors are used when three circuits need to be simultaneously protected. One actuating toggle handle closes all circuits and all poles trip in the presence of an overload on any circuit. Different variations are available. Some typical combinations could be three series poles; two series and one shunt; two series and one relay; or three series with a set of auxiliary contacts.




Circuit protectors must fully satisfy all requirements of corresponding applications. There are independent laboratories and regulations which certify that circuit protectors can safety operate in the given application and environmental conditions. For example, Sensata’s AP/UP hydraulic-magnetic circuit protector series is approved by two core certifications that allow it to operate in specialty applications: Qualified Products Listing (QPL) and FM approval (Factory Mutual Research). QPL is a military designation for military applications.


The Sensata-Airpax AP-MIL protectors are the only hydraulic magnetic circuit breaker on the market that is QPL designated. These protectors are produced in line with MIL-PRF-39019 and have a sealed case. Waterproof panel integrity is provided by an O-ring bushing seal and a silicon rubber gland within the bushing/handle assembly. Salt spray testing per MIL-STD-202, Method 101, Test Condition B, resulted in no evidence of corrosion, electrical or mechanical damage. It can operate in hazardous environments and is unaffected by dust and water. The circuit breaker is also unaffected by field conditions such as dust or rain, enabling it to stay combat-ready at all times.


FM Approvals/ATEX is a testing and certification service used to verify that products meet rigorous loss prevention standards of quality, technical integrity and performance for use in commercial and industrial facilities. This certification means that the circuit protectors can operate in Class 1/Div 2 zones, which include places where ignitable concentrations of flammable gases can be present under abnormal conditions.


The Sensata-Airpax UP circuit protector is a unique hydraulic magnetic circuit protector that can break both AC and DC currents with FM/ATEX approval. Without this approval, the alternative would be: A) creating an expensive sealed exposure which would add cost and complexity, or B) locating the breakers and associated electronics off-site. A typical application of this protector is on offshore oil rigs. In these applications, option B is not available. This certificate is very important for petrochemical applications.




Circuit protectors are fully adapted to their application and the specifics of the protected circuit. Customers need to know how to select the proper circuit protectors for their applications. Some manufacturers, such as Sensata, organize their products by different criteria to help customers select proper circuit protectors for their applications.


The following parameters need to be considered when selecting appropriate circuit protectors:

  1. Specifics of protected circuit (type of protector according the application)
  2. Number of poles
  3. Internal configuration
  4. Frequency (Hz) and delay
  5. Nominal amperage rating
  6. Approval certificates


The product name contains all necessary information about the circuit protector:

  1. AP - fully sealed magnetic circuit protector (e.g. used for military applications)
  2. 12 - Two pole unit
  3. 1 - Series internal configuration
  4. 51 - DC short time delay
  5. 203 – 20 Amperes
  6. FM – FM Approved


The figure below illustrates an example of one AP hydraulic magnetic protector: AP12 -1-51-203-FM.

Meaning of the product name. (Image courtesy of Sensata Technologies.)
Meaning of the product name. (Image courtesy of Sensata Technologies.)


For more information about hydraulic-magnetic circuit protectors, or Sensata’s AP/UP series, visit



Master Electronics has sponsored this post.  All opinions from Edis Osmanbasic (