Contact assembly for bimetal thermal protectors: the silver contact pair and its carrier, rated from 10,000 cycles at 2.5 A down to 300 cycles at 7.5 A
The silver contact pair, carrier and snap element inside a bimetal thermal protector · whether motor current runs through the bimetal, how contact life falls as switched current rises, and contact resistance and bounce
Every bimetal thermal protector ends in a pair of contacts. The bimetal element senses temperature and snaps; the contact assembly is what actually carries and breaks the motor or coil current, and its design decides how much current the protector can switch, for how many cycles, and whether the current itself heats the bimetal. Two layouts appear in the sources: the movable contact fixed to the bimetal so that load current flows through it, and a separate spring disc that carries the current and the contact while the bimetal only pushes. The picture shows contact assemblies of the strip type held in fixtures on a rotary assembly table: copper-coloured contact arms with round silver contact buttons and a blue-tinted bimetal blade, with a gripper and a camera above one station.
Contact assemblies of bimetal thermal protectors and thermal cut-outs for motors, transformers and coils: stationary and movable silver contacts, contact carriers and spring discs, the joint to the bimetal disc or blade, and their switching ratings, cycle life, contact resistance and bounce. Bimetallic thermal overload relays for motor starters are covered on the Overload relay page; complete snap-disc protectors for hermetic refrigeration compressors, with their calibration and matching to the compressor, on the Refrigerator compressor thermal overload protector page. Contacts for contactors and relays, and contact rivet raw material, are outside this page.
CNFX editorial research · Sources checked October 13, 2026 · 3 sources
Through the bimetal or round it: two ways of carrying load current to the movable contact
Whether the load current flows through the bimetal decides whether the contact assembly makes the protector current-sensitive as well as temperature-sensitive.
In Texas Instruments' thermostatic motor protector patent (filed 1963) the stationary contact, of fine silver or fine silver over nickel, is welded or riveted to a terminal, and a fine silver movable contact is riveted or welded to one end of the snap-acting bimetal disc; the disc itself is connected to a tab through a welding slug, so the circuit runs through the disc. [3] The multicomp pro T1 switch works the same way: its two-contact system passes the current directly through the bimetallic snap disc, which gives combined temperature and current sensing and switches off quickly when a motor is blocked and draws a much higher current. [2]
Thermik's series 01 takes the opposite approach. A spring snap-in disc forms the current-transfer element: it carries the movable contact and keeps current and self-heating away from the bimetallic disc by exerting a steady contact pressure, while the bimetal disc is held on the movable contact without being welded or fixed and reacts only to the ambient temperature. The stationary contact is a silver contact integrated in an insulated steel cap that closes the conductive housing like a button cell. [1] When the switching temperature is reached the bimetal snaps over, pushes the spring disc down and the contact opens abruptly. [1]
Say whether the protector must respond to current as well as temperature; that decides between a contact on the bimetal and a contact on a separate current-carrying spring.
02
Switched current against cycle life: 10,000 cycles at rated current, 3,000 to 300 at the maximum
The contact pair's rating is a list of current, power factor and cycle count pairs. The higher the switched current, the fewer cycles the contacts are rated for.
Editorial calculation: raising the switched current on the Thermik S01 from 2.5 A to 6.3 A (cos φ 1.0) cuts the rated cycles from 10,000 to 3,000, and to 7.5 A cuts them to 300, a factor of about 33 for three times the current. On the T1, going from 6.3 A to 8 A cuts cycles tenfold. [1][2]
Editorial calculation: at the 50 mΩ contact-resistance limit, 6.3 A would dissipate about 2 W in the contact path (6.3² × 0.05); at 2.5 A about 0.31 W. In the T1, where current also flows through the bimetal, that heat adds to the self-heating that makes it current-sensitive; in the S01 the spring disc is there to keep it away from the bimetal. [2][1]
Rating
Thermik S01 (spring disc carries current)
multicomp pro T1 (current through bimetal)
Rated voltage
250 V AC (VDE), 277 V AC (UL); operating up to 500 V AC / 14 V DC [1]
70–180 °C in 5 °C steps, ±5 K; hysteresis 30 K ± 15 K [2]
Give the motor's running and locked-rotor current with power factor, the number of trips expected over the product's life, and whether the circuit is AC or DC.
03
Contacts, carrier and housing: what the assembly is made of and how it is held
The sources name the materials and joints that make up the switching assembly.
Both datasheets give an impregnation rating, because the protector is wound into motor or transformer coils before varnishing: Thermik rates the S01 suitable for impregnation, and multicomp pro rates the T1 tight against ordinary resins and lacquers. [1][2]
Part
What the sources state
Stationary contact
Fine silver or fine silver over nickel, welded or riveted to the terminal [3]; silver contact integrated in an insulated steel cap [1]
Movable contact
Fine silver, riveted or welded to the snap disc [3]; carried by a spring snap-in disc, with the bimetal disc held on it unwelded [1]
Disc connection
Bimetal disc joined to the terminal tab by a welding slug [3]
Housing and insulation
Conductive housing, Ø9.5 mm, installation height from 4.3 mm, 450 N pressure resistance, Mylar/Nomex insulation [1]; shrink cap or PPS housing, potted [2]
Leads
0.25 mm² / AWG22 [1]; AWG20 at 150 °C or AWG24 at 200 °C [2]
State the varnish or potting process the protector will see, the space available in the winding, and the lead length and insulation class.
04
Data for a thermal protector contact assembly enquiry
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Protected device: [motor; transformer; coil]
Switching: [NC/NO; auto-reset or latching]
Current path: [through bimetal; spring disc]
Rated current and power factor: [A; cos φ]
Maximum switching current and cycles: [A; cycles]
Voltage: [V AC/DC]
Switching temperature and tolerance: [°C; K]
Contact resistance limit: [mΩ]
Installation: [size; insulation; impregnation]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
Does load current flow through the bimetal disc?
It depends on the design. In the multicomp pro T1 and in Texas Instruments' patented protector it does; in Thermik's series 01 a separate spring disc carries the current and the movable contact. [2][3][1]
How many cycles are the contacts rated for?
Thermik rates the S01 for 10,000 cycles at 2.5 A (cos φ 1.0) but only 300 cycles at 7.5 A; the T1 is rated 10,000 cycles at 6.3 A and 1,000 at 8 A. [1][2]
What contact resistance is specified?
Thermik gives ≤ 50 mΩ for the S01; the T1 datasheet gives below 50 mΩ including 100 mm of wire. [1][2]
What are the contacts made of?
Texas Instruments' patent names fine silver for the movable contact and fine silver or fine silver over nickel for the stationary one; Thermik integrates a silver stationary contact in the cap. [3][1]
Sources and what still needs a direct check
Sources checked 2026-10-13. Three documents from three publishers: Thermik's specification page for the S01 thermal protector, the multicomp pro T1 thermostat switch datasheet published by Farnell, and Texas Instruments' patent US3280285A on a thermostatic motor protector.
