Is the temperature class based on normal operation or also on faults?
Abstract
Both, depending on the protection level: Gc equipment is assessed in normal operation, Gb also in expected malfunctions, Ga also in rare malfunctions. The type of protection defines which faults count, such as two countable faults for Ex ia or a stalled Ex eb motor.
Both, and the equipment protection level (EPL) decides how far it goes. Equipment for EPL Gc must keep its temperature class in normal operation, Gb equipment also during expected malfunctions, and Ga equipment also during rare malfunctions. The type of protection standard then defines exactly which faults are applied, for example up to two countable faults for intrinsic safety "ia" or a stalled rotor for an increased safety "eb" motor.
Short answer
- The marked class is determined at the most onerous rated conditions: rated duty, the worst supply voltage within the tolerance, and the maximum ambient temperature.
- Gc: normal operation, plus some regular expected occurrences such as a lamp failure.
- Gb: normal operation and expected malfunctions.
- Ga: normal operation, expected malfunctions and rare malfunctions.
- The faults that count are defined per type of protection (Ex i, Ex e, Ex d, Ex h and so on), not left to judgement.
- Faults outside the equipment, such as a missing overload relay, a fire or an overheated process, are not covered by the marking; the installation and the plant's hazard assessment must handle them.
The principle: protection level sets the conditions
IEC 60079-0 defines the gas EPLs by the conditions under which the equipment must not become an ignition source. Ga equipment must not ignite in normal operation, expected malfunctions or rare malfunctions; Gb in normal operation or expected malfunctions; Gc in normal operation, with possible extra protection for regular expected occurrences. A hot surface is one such ignition source, so the surface temperature is assessed under the same conditions.
The general rule for electrical equipment is to determine the maximum surface temperature at rated duty, at the most unfavourable voltage (between 90 % and 110 % of rated in the public 2004 edition), at the maximum ambient temperature and with any rated external heating. Each type of protection standard adds its own overload or malfunction conditions.
What each type of protection includes
| Type of protection | Conditions included in the temperature assessment |
|---|---|
| Intrinsic safety ia (Ga) | Normal operation with none, one and two countable faults, each time plus the most onerous non-countable faults |
| Intrinsic safety ib (Gb) | Normal operation with none and one countable fault, each time plus the most onerous non-countable faults |
| Intrinsic safety ic (Gc) | Normal operation and the conditions the standard specifies; countable faults do not apply |
| Increased safety eb (Gb) | Normal operation and expected malfunctions; motors include the stalled rotor, luminaires include lamp ageing |
| Increased safety ec (Gc) | Normal operation and regular expected occurrences; no extra condition for motors |
| Flameproof d | External surface at rated conditions; for luminaires with magnetic ballasts, voltage 10 % above rated and a simulated lamp rectifier effect; no extra condition for motors |
| Type of protection n (Gc) | Normal operation and certain regular expected occurrences |
| Non-electrical Ex h | Gc normal operation; Gb plus expected malfunctions; Ga plus rare malfunctions, with a larger safety margin |
For intrinsic safety, IEC 60079-11 requires the temperatures of all surfaces the atmosphere can reach to be assessed after the faults of the level have been applied, with a safety factor of 1.0 on voltage and current for the temperature classification. For an "eb" motor, the stall is covered by time: the motor protection must disconnect a stalled motor before the time tE, the time the winding needs to heat from its rated service temperature to the limiting temperature. For mechanical Ga equipment, ISO 80079-36 limits the highest measured temperature to 80 % of the marked value, because rare mechanical malfunctions are hard to reproduce in a test.
Example: A flameproof motor Ex db IIB T4 Gb stalls. The flameproof standard adds no stall condition for motors, so the T4 rating relies on the installation: the motor protection required by the installation rules must disconnect it before its surface overheats. An Ex eb motor has the stall built into its rating, through tE, but only if the overcurrent relay is set to trip within tE.
What the marking does not cover
The equipment rating stops at the equipment boundary. It assumes the supply, protection and environment stated in the certificate and instructions. Not included are:
- a protective device that is missing, wrongly set or bypassed;
- heat from the process or external fire;
- operation outside the rated ambient range or with a different lamp, converter or load.
Plant-level rules address these. The German technical rule TRGS 723, for example, requires surface temperatures in Zone 0 to stay at or below 80 % of the ignition temperature even in rare malfunctions, and in Zone 2 not to exceed the ignition temperature in normal operation, which applies to process surfaces as well as equipment.
Common mistakes
- Assuming a T-class covers every possible fault. It covers the faults its EPL and type of protection define, and nothing else.
- Assuming a T-class covers only normal operation. For Ga and Gb equipment, the defined malfunctions are included.
- Setting motor protection without checking tE. For "eb" motors the trip time is part of the temperature class.
Related on excodex
- Temperature classes T1 to T6: matching surface temperature to autoignition temperature
- Equipment Protection Levels (EPL): Ga, Gb, Gc, Da, Db, Dc and how they match zones
- Do brakes, bearings and friction need to meet the temperature class?
- Does a variable speed drive change the temperature class of an Ex motor?
- Standards spaces: IEC 60079-0, IEC 60079-11, IEC 60079-7
Standards and further reading
- IEC 60079-0, equipment general requirements (EPL definitions, determination of maximum surface temperature). Public texts: IS/IEC 60079-0:2007 archive.org and IEC 60079-0:2004 law.resource.org
- IEC 60079-11:2011, intrinsic safety "i", levels ia, ib, ic and thermal ignition compliance: law.resource.org
- IEC 60079-7:2015, increased safety "e", conditions for the determination of maximum surface temperature and tE: law.resource.org
- IEC 60079-1:2014, flameproof enclosures "d", conditions for the determination of maximum surface temperature (replaced by IEC 60079-1:2026): law.resource.org
- IEC 60079-15:2017, type of protection "n": law.resource.org
- ISO 80079-36:2016, non-electrical equipment, EPL-dependent determination of surface temperature: law.resource.org
- TRGS 723 (Germany), avoidance of ignition sources, section 5 (hot surfaces): bgn-branchenwissen.de
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.