arpadveress 
Equipment assemblers and manufacturers have made requests to certify equipment with a rated ambient temperature range beyond that of some incorporated internal devices. An example of this is a control panel containing a power supply, an intrinsic safety barrier, and other switchgear. The power supply and IS barrier are rated (-20C to +40C), but the panel manufacturer wants the complete control panel to be rated for (-40C to +50C).
To mitigate this issue when the equipment is energized, manufacturers have proposed to install heaters or refrigerators with interlocked sensors, or other methods of ensuring that the internal devices (for example, power supply or IS barrier) cannot be energized unless the internal surrounding air is within the rated ambient range for those internal devices within the assembly.
However, this ambient temperature concern can also be an issue when the equipment is not energized, including when switched off. Damage to internal devices may occur when subjected to these temperatures, which may influence the type of protection for the equipment. Examples of possible damage include: components shearing from printed wiring boards due to contraction of protective encapsulation, cracking and loss of adhesion of encapsulation for “ Ex m” devices, damage to elastomers in “Ex t” enclosures, or cracking of a “Ex d” cemented joint.
Questions:
Consider a scenario in which an overall piece of equipment is intended for use in an ambient temperature range beyond that of some internal devices, and, if these internal devices were exposed to temperatures beyond their rated ambient temperatures, the Type of Protection for the equipment could be impaired.
Q1: Regarding when this overall piece of equipment is energized during normal operation, is it acceptable to incorporate an internal temperature monitoring and temperature control system, to ensure the internal devices remain within their rated ambient temperatures?
Q2: Regarding when this overall piece of equipment (including the internal temperature monitor and temperature control system) is not energized (for example when switched off), is it necessary to ensure all internal devices remain within their rated ambient temperatures specified by the internal device manufacturer(s)?
Answers:
A1: Yes, it is acceptable to incorporate a temperature monitoring and temperature control system to ensure internal devices remain within their rated ambient temperature range. The assessment would include assuring that the extra devices used for temperature monitoring and temperature control are rated for the application and that the devices are not operated outside of their ambient temperature specifications.
A2: Yes, it is necessary. In order to allow installation of devices in a temperature-controlled enclosure that will be installed in an area with ambient temperatures beyond the internal device ratings, the unpowered ambient and powered ambient must be within the internal device manufacturer’s specifications. The certifier’s assessment of the assembly includes consideration of the evidence of compliance with the internal device’s unpowered and powered ambient specifications as well as the effect of the internal device(s) on the enclosure internal temperature, should cooling or heating (as applicable) be lost.
Note: all safety devices shall be SIL rated if they are related to safe operation and act as a locking condition. All shall comply with EPL of the main installation.
Source: iecex extag
Keep up good work!
Arpad
