Bending Lead Wires – Cautions

Improper bending of the lead wires on a thermal cutoff could result in improper functioning of the TCO. Both Uchihashi and NEC/SCHOTT, the manufacturers of the Elcut and SUFFUSE brand of thermal cutoffs provide detailed cautions on lead wire bending and should be strictly followed.

Since the SEFUSE SF thermal fuses operate differently than the Elcut brand, the lead wires bending cautions are slightly different.

Improper lead bending/handling include

  1. Cracked or broken seals on the lead wire/body junction
  2. Broken lead wire-to-thermal element welds
  3. Damaged fuse bodies

NEC/SCHOTT SEFUSE brand (Cautions from NEC/SCHOTT):
When bending the lead wire, it is important not to apply excessive pressure to the root of the lead wire. The lead wire should be secured close to the case and bent (not twisted) at a distance 3 mm or more from the body of the fuse. It is important to secure the lead during bending so as to prevent excessive forces on the TCO body and lead-wire to body connection. The fuse body should never be exposed to crushing forces,

thermal fuses

The tensile strength applied to the lead wire should be 49N or less for SF types.

The force applied to the body of the thermal cutoff should be 98N or less for SF-type. With regards to the SF-type, deformation of the case may change the location of the sliding contact during operation and could lead to the thermal cutoff operating only at temperatures lower than the normal operating temperature range. The thermal cutoff may also not operate even if the thermal cutoff’s operating temperature is exceeded.

If using SF-type thermal fuses, the lead on the resin-sealed side must not be allowed to touch the case since the case is electrically conductive. This would cause the current to flow from the lead on the resin-sealed side to the opposite lead resulting in a non-functioning thermal cutoff.

thermal fuses

Note that the body of the SF-type is the same in potential as the circuit. Therefore, it must be electrically isolated from other metallic parts.

Uchihashi Elcut brand Thermal Cutoff Installation
Caution to be observed when installing thermal cutoffs

  • Do not twist the body or lead wires (i.e. do not turn or rotate lead wires respect to the body). This could result in breaking the lead wire-to-thermal element weld or may compromise the epoxy seal.
  • Do not push lead wired toward, or pull them away from the thermal cutoff body at axial stresses exceeding those shown in Table 1 (room-temperature reference values). Values in Table 1 were calculated from lead wire diameter according to IEC60691.
  • Tangential forces on the leads must be avoided (i.e. pushing or pulling on the leads at an angle to the thermal cutoff body) as such forces may damage the thermal cutoff’s seals.
  • When bending a lead wire for installation, fix the part of the lead between the body and the lead section to be bent using a tool, and gently bend the lead section that is at least 3 mm from the body. Never hold the body with a tool.
  • Do not damage lead wires by holding with a sharp instrument, and do not bend at a sharp angle.
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    thermal fuses

  • After a thermal cutoff has been connected, do not apply excessive force that will crush the thermal cutoff body, sealant, or lead wires, and ensure that leads are not subjected to tension, pressing, or twisting forces with respect to the thermal cutoff body
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