Tech Tip No 1:- Temperature Rise of LEDs in “Ex” Equipment

LED’s are increasingly being used in hazardous area lighting. The main types of protection that are being targeted for certification are “Ex e”, “Ex i” and “Ex m”. However when the next edition of the “Ex e” standard (IEC 60079-7) is published it is very likely to be no longer available as an option.

This will cause a problem for designers trying to comply with the standards, IEC 60079-11 (Ex i) and IEC60079-18 (Ex m). At least one fault for “Ex ib” and “Exmb” has to be applied and still achieve a suitable maximum surface temperature. For testing purposes the fault is mimicked by applying a worst case matched power to the LED to simulate failing to a theoretical worst case condition.

Clearly this will cause an abnormally high temperature rise of the LED and most likely exceed any desirable temperature class.

Clause 7.6 d) of IEC 60079-11:2011 states that diodes including LED’s “operated within the requirements of 7.1 shall only be considered for the power they shall dissipate in the forward conducting mode, or Zener mode, if applicable.”

This means if the LED has a safety factor of at least 1.5 on its rated current, the maximum power dissipation is calculated from the product of the LED forward volt drop and its forward current. You could then describe it as an infallible LED. This will clearly give a much better result.

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