Temperature Assessment for “m” Encapsulation Protection Equipment
The temperature assessment for “m” encapsulation protection equipment consists of two parts: first, measuring the maximum compound temperature and the maximum surface temperature of the equipment under “normal operating” conditions; second, measuring the maximum surface temperature of the equipment under “fault conditions.”
I. Measurement Under “Normal Operating” Conditions – Maximum Compound Temperature and Maximum Equipment Surface Temperature
According to the definition in GB/T 3836.1, equipment operating within the specified supply voltage variation range and any other operating tolerances is considered to be under “normal operating” conditions.
The reference working temperature for the thermal stability test of the compound (Clause 8.2.3 of GB/T 3836.9) is determined under “normal operating” conditions.
When determining the working temperature of the compound (Clause 6.1 of GB/T 3836.9), the maximum compound temperature under “normal operating” conditions may be used as a substitute to assess the Continuous Operating Temperature (COT) of the compound.
As defined in GB/T 3836.1, variations in supply voltage within the specified range and any other operating tolerances are all considered part of “normal operating” conditions.
The standard introduces the concept of compound thermal conductivity, expressed in W/(m·K), which represents the amount of heat directly conducted per unit time through a unit area and unit length of material under a unit temperature difference. The higher the thermal conductivity, the better the material’s heat dissipation capability. When measuring the maximum compound temperature, the standard requires measurement of the compound temperature nearest to the hottest component. Testing typically involves embedding thermocouples before encapsulation. However, if the manufacturer can provide documentation confirming that the thermal conductivity of the compound exceeds that of air (0.25 W/(m·K)), the maximum compound temperature may be determined by measuring directly on the hottest component before encapsulation.
II. Temperature Determination Under Fault Conditions
For equipment with “ma” and “mb” protection levels, the maximum surface temperature must also be measured under fault conditions. The “ma” protection level considers two countable faults, the “mb” protection level considers one countable fault, and the “mc” protection level considers no faults.
Understanding Countable Faults
Countable faults apply only to clearances/creepage distances; there is no concept of a “countable fault” for components.
For components, they are classified only as either reliable components or non-countable faults. Faults include any component short circuit, any component failure, or printed circuit board faults, but do not include open circuits in printed wiring. A “countable fault” only occurs when there is insufficient clearance/creepage distance AND a component fails simultaneously.
Understanding the Most Unfavorable Load
For “m” type equipment without external loads, relevant tests are conducted directly in accordance with GB/T 3836.1. For “m” type equipment with external loads, such as control modules, equipment with “ma” and “mb” protection levels shall have the current adjusted to the highest value that does not cause the protective device to operate, or the manufacturer shall specify a load range in the documentation and test according to the load limits. For the “mc” protection level, testing shall be conducted under the specified load parameters.
With over a decade of experience, he is a seasoned Explosion-Proof Electrical Engineer specializing in the design and manufacture of safety and explosion-proof products. He possesses in-depth expertise across key areas including explosion-proof systems, nuclear power lighting, marine safety, fire protection, and intelligent control systems. At Warom Technology Incorporated Company, he holds dual leadership roles as Deputy Chief Engineer for International Business and Head of the International R&D Department, where he oversees R&D initiatives and ensures the precise delivery of design documentation for international projects. Committed to advancing global industrial safety, he focuses on translating complex technologies into practical solutions, helping clients implement safer, smarter, and more reliable control systems worldwide.
Qi Lingyi