Clause 3 – How to Understand the Definition of “Intentionally Weak Part”

intentionally weak part: A part intended to rupture under abnormal operating conditions to prevent the occurrence of a condition that could impair compliance with this standard.
NOTE: Such a part may be a replaceable component, such as a resistor or capacitor, or a part of a component to be replaced, such as an inaccessible thermal link incorporated into a motor.

Clause 24.1.8 of the standard states: The relevant standard for thermal links is IEC 60691. Thermal links that do not comply with IEC 60691 are considered intentionally weak parts for the purposes of Clause 19.

Combining the definitions in Clause 3 and Clause 24.1.8, it’s easy to conclude that if a thermal link complies with IEC 60691, it’s not an intentionally weak part. Conversely, if a thermal link does not comply with IEC 60691, it can be an intentionally weak part. However, Clause 19.1 states the following: If a heating element complies with IEC 60691, it is considered an intentionally weak part. If an element or an intentionally weak part becomes permanently open-circuited, the relevant test is repeated on a second sample. This second test shall be terminated in the same mode unless the test is otherwise satisfactorily completed.

Thus, we can understand this: for some protective devices, such as current fuses and thermal links, if these components do not have approval certificates according to their corresponding standards and therefore do not comply with the standards, but they provide protection during the abnormal tests in Clause 19, they can be considered intentionally weak parts. However, sometimes the protective function is not provided by protective devices such as current fuses and thermal links, but rather by resistors or capacitors. If these components exhibit an open circuit during testing, breaking the circuit and triggering protection, typically during the tests in Clause 19.11.2, then resistors or capacitors can be considered intentionally weak parts. With technological advancements and the reduction in the cost of protective devices, most manufacturers rarely use resistors or capacitors as intentionally weak parts. However, some IPM modules that power high-current inverter motors or inverter motor-compressors may become intentionally weak parts because the IPM will burn out during the lock-rotor test of such motors.