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Item # SZ1, Type SZ1 Defined Current Sensitive Thermal Protector with Insulation Cap

Application: The current sensitive thermal protector type SZ1 with normally closed contacts is utilised for overheating protection of all electric equipment or devices, especially to safeguard transformers according to EN 60 742. The type SZ1 is sensitive for temperature as well as for currents. Therefore it is recommended in all cases of a dramatic rise of temperature, due to an overload current, e.g. in case of short circuit.

Design: The thermal protector SZ1 is based on the well proven mechanism of Thermik’s product series 01. A defined current-sensitivity is reached by an integrated series resistor RS.
Version :
  • SZ1 with insulation cap

    Operation: If, in the case of overheating or increase in current, the rated switching temperature of the bimetallic disc is reached, this suddenly snaps over and opens the contact. Automatic resetting after a significant temperature drop.

    Series Resistor RS: The series resistor RS has to be adapted to each application. To protect an electrical apparatus, the response time should be less than 20 seconds in case of a short circuit. In order to guarantee this ideal response time, a heating performance of RS between 5 W and maximal 10 W is recommended. The table on the next side will help to find an optimal resistor RS. Due to different possibilities of adapting the protector to the electrical device, experimental tests with different protectors are recommended.

  •  Specifications
    The data of this table refers to the standard version. For others - please inquire. 

    Diameter d

    9.4 mm

    Height h

    5.0 mm

    Lengths of Insulation Cap I

    19.0 mm


  • Quick response sensitivity - featured by the metal housing and small protector mass
  • Defined response time - < 20 s , due to series resistor RS
  • Excellent long term performance - due to fine-silver contacts. Reproducible switching temperature values due to tempered, electrically and mechanically unstressed bimetallic disc
  • Instantaneous switching - with constant contact pressure over the whole temperature range
  • Very short bounce times - < 1 ms
  • Temperature resistance - by use of high temperature resistant materials and components
  • Contact Type

    Normally Closed (NC)

    Nominal Switching Temperature1

    70 - 160 ºC

    Standard Tolerance

    ±5 K

    Resetting Temperature (RST)

    > 35 ºC

    Rated Voltage2

    250 V AC (VDE)

    Rated Current3

    2.0 A cos φ = 1.0
    1.6 A cos φ = 0.6

    Max. Switching Current at 250V AC4

    4.0 A cos φ = 1.0 over 3,000 switching cycles

    Current Sensitivity at INom


    Contact Bounce Time

    < 1 ms

    Impregnation Resistance5


    Contact Resistance6

    < 50 mΩ (without series resistor RS)

    Vibration Proof (at 10 to 60 Hz)

    100 m/s²

    Series Resistor RS

    Adapted to each application.
    Recommended heating performance in case of short circuit: 5 W - 10 W

    Switch Insulation7

    insulation cap Mylar - Nomex ®

    Dielectric Strength of the Basic Insulation Cap

    2 kVr.m.s

    Standard Connection Leads

    multi strand wire 0.25 mm² , length 100 mm, colour: white, constant operating temperature min. 180 ºC, insulation class H. Other connections upon request.

    Approvals Acc. to Design for

    VDE acc. to EN 60730-1; EN 60730-2-9 (VDE 0631)
    CB **) acc. to IEC 60730-1; IEC 60730-2-9

    **) The "European Accreditation CB Scheme" Certificate, named CB- Certificate, covers virtually the most national approbations.

    Marking Example

    1 in steps of 5K
    2 other values upon request
    3 depending on series resistor RS
    4 depending on series resistor RS
    5 according to Thermik test
    6 with reference to MIL-STD. R 5757
    7 ® Trade-mark Du Pont


    ·  TP of Type SZ1


    Defined Current Sensitive Thermal protector of Type SZ1 Technical Data Sheet
    (PDF, 320KB)