IEC 60079 28 Edition 2 0 2015 05 INTERNATIONAL STANDARD NORME INTERNATIONALE Explosive atmospheres – Part 28 Protection of equipment and transmission systems using optical radiation Atmosphères explos[.]
General
Three types of protection can be applied to prevent ignitions by optical radiation in explosive atmospheres These types of protection encompass the entire optical system
The types of protection for optical radiation include: a) inherently safe optical radiation, classified as type of protection "op is"; b) protected optical radiation, known as type of protection "op pr"; and c) optical systems with interlocks, referred to as type of protection "op sh".
In cases where the ignition hazard assessment in Annex C indicates a potential risk of ignition from optical radiation, the protective measures outlined in Table 1 must be implemented.
Table 1 – EPLs achieved by application of types of protection for optical systems
Ga, Da, Ma Gb, Db, Mb Gc, Dc
Inherently safe optical radiation “op is” (see 5.2)
The system is deemed safe with two faults when utilizing an optical source based on the thermal failure characteristics outlined in sections 5.2.2.2 item 3) or 5.2.2.3 item 3) Conversely, it is considered safe with one fault under the same thermal failure characteristics, although it does not meet the safety criteria for two faults.
– safe in normal operation No No Yes
Ga, Da, Ma Gb, Db, Mb Gc, Dc
Protected fibre optic media with ignition capable beam “op pr”
– with additional mechanical protection No Yes Yes
Fibre optic media designed for standard industrial applications must adhere to manufacturer specifications, but they do not require additional mechanical protection In the event of fibre breakage, an ignition-capable beam is interlocked, as outlined in section 5.4.
– Protected fibre optic cable “op pr” for Gb/Db/Mb + shutdown functional safety system based on ignition delay time of the explosive gas atmosphere
– Protected fibre optic cable “op pr” for Gc/Dc + shutdown functional safety system based on eye protection delay times
– Unprotected fibre optic cable (not “op pr”) + shutdown functional safety system based on eye protection delay times (IEC 60825-
None (unconfined, ignition capable beam) No No No
1) Shutdown system safe with one fault
Requirements for inherently safe optical radiation “op is”
General
Inherently safe optical radiation refers to visible or infrared radiation that cannot provide enough energy to ignite an explosive atmosphere under normal or fault conditions This safety concept focuses on limiting beam strength, as ignition of a target absorber by optical radiation necessitates minimal energy, power, or irradiance from known ignition mechanisms within the visible and infrared spectrum Importantly, this concept applies to unconfined radiation and does not require an absorber-free environment.
Continuous wave radiation
Either optical power or optical irradiance shall not exceed the values listed in Table 2, Table 3 and Table 4, categorized by equipment group and temperature class
As an alternative to compliance with Table 2 the following options are available:
For irradiated surface areas exceeding 400 mm², the highest temperature recorded on the irradiated surface will determine the temperature class, regardless of the irradiance level Temperature measurements must account for potential variations in beam strength.
For irradiated areas not exceeding 130 mm², the maximum radiated power values, which differ from those specified in Table 2 for temperature classes T1, T2, T3, and T4, as well as Groups IIA, IIB, or IIC, are outlined in Table 4.
– Passing the ignition tests in accordance to with 5.2.4
Table 2 – Safe optical power and irradiance for Group I and II equipment, categorized by Equipment Group and temperature class
Optical radiation sources with Can be used for the following atmospheres (temperature classes in combination with equipment groups)
(no irradiance limit applies) (no radiated power limit applies) mW mW/mm 2
≤ 150 IIA with T1, T2 or T3, and I No limit to the involved irradiated area
Class, IIC with T1, T2, T3 or T4, and I
No limit to the involved irradiated area
≤ 15 All atmospheres No limit to the involved irradiated area
≤ 20 IIA with T1, T2 or T3, and I Irradiated areas limited to ≤ 30 mm 2
≤ 5 All atmospheres No limit to the involved irradiated area
The optical power and irradiance values in this table are determined by the equipment's gas group subdivision and temperature class, as the ignition process from small hot particles relies on these factors This assessment is separate from the electrical equipment group and its temperature class It is crucial to understand that the term 'temperature class' has a different meaning in the context of optical radiation protection techniques, referred to as "op," compared to other electrical equipment protection methods, such as flameproof enclosures ("d") or intrinsically safe apparatus ("i").
The term 'temperature class' in the context of equipment does not refer to the maximum temperature of the equipment itself, but rather to the ignition properties of the gases associated with different equipment groups For IIA and IIB equipment, temperature classes T5 and T6 are not relevant, as there are no gases in these categories with corresponding auto-ignition temperatures Additionally, for IIC equipment, T5 is not applicable since there are no IIC gases with that auto-ignition temperature, while carbon disulfide is the sole IIC gas that has a T6 auto-ignition temperature.
When using the table for IIB equipment, there is a single option for optical power or optical irradiance values, specifically T1 to T4 In contrast, for IIA equipment, the manufacturer specifies an “op is” temperature class corresponding to the equipment group gases for the intended installation, which can be T1 to T3 or T4 For IIC equipment, the manufacturer may indicate temperature classes from T1 to T4, or T6 if carbon disulfide is part of the intended installation application.
Table 3 – Safe optical power and irradiance for Group III equipment
Equipment Group IIIA, IIIB and IIIC
Radiated power (no irradiance limit applies) mW ≤ 35 ≤ 35 ≤ 35
Irradiance (no radiated power limit applies) mW/mm 2 ≤ 5 ≤ 5 ≤ 10
Table 4 outlines the safe limit values for the intermediate area, specifically for Group I or II equipment operating under constant power in T1 – T4 atmospheres This data, derived from Figure B.1 and incorporating a safety factor, specifies the maximum radiated power values in milliwatts (mW) for equipment groups IIA, IIB, or IIC, along with the corresponding irradiated area in square millimeters (mm²).
For irradiated areas equal to or above 130 mm 2 the irradiance limit of 5 mW/mm 2 applies
To ensure compliance with Table 2, Table 3, or Table 4 based on maximum optical power values, it is essential to measure maximum optical power using one of the specified test methods, while maintaining the same or equivalent thermal dissipation conditions as those in the intended application.
1) The actual driver circuitry is used to power the optical device, with maximum optical power measured under fault conditions in accordance with the over-power / energy fault protection criteria according to 5.2.5 and the respective EPL at ambient temperature between 21 °C and 25 °C If the optical power is higher at the foreseen ambient temperature range of the equipment, the measured value at room temperature shall be adjusted according to the temperature coefficient taken from the data sheet If no information is given in the data sheet then the measurement shall be done additionally in the lowest and highest values of the temperature range specified for the equipment Separate samples shall be taken for each of the 3 tests if the optical device is subjected to input parameters which are higher than its maximum rating The number of test samples depends upon the number of fault conditions to be applied
2) The maximum input parameters to the optical device from the actual driver circuitry are calculated based on analysis of the driver circuitry schematic This analysis shall include consideration of fault conditions in accordance with the over-power / energy fault protection criteria according to 5.2.5 and the respective EPL One test sample of the optical device without the driver circuitry is then connected to a separate variable source of supply and subjected to input parameters equal to the maximum calculated input parameter values Maximum optical power is measured with the optical device at ambient temperature between 21 °C and 25 °C If the optical power is higher at the foreseen ambient temperature range of the equipment, the measured value at room temperature shall be adjusted due to the temperature coefficient taken from the data sheet If no information is given in the data sheet then the measurement shall be done additionally in the lowest and highest values of the temperature range specified for the equipment Separate samples shall be taken for each of the 3 tests if the optical device is subjected to input parameters which are higher than its maximum rating
3) The actual driver circuitry is replaced with a separate variable source of supply This source of supply is then used to provide variable inputs to the optical device, with maximum optical power measured No faults are considered Ten samples of the optical device are to be tested at ambient temperature between 21 °C and 25 °C The maximum optical power is then taken from the highest power that can be measured at the ten samples before the optical device shuts down or folds back
When replacing the actual driver circuitry with a separate variable power supply, the maximum optical power is defined as the power measurable before the optical device enters shutdown or fold-back mode During these conditions, significant variations can occur between different samples of the same optical device To mitigate this issue, testing is conducted on 10 samples to determine the maximum optical power However, such variance is not a concern when the optical device is evaluated with its original driver circuitry.
4) Calculation of maximum optical power based on the electrical power supplied to the optical device as described in 2) For the optical output values the data sheet specifications shall be taken into account, together with the calculated power supplied, and if applicable distances provided by construction from the radiating surface
The following is applicable to whichever of the above test conditions is selected:
An optical detector, such as a semiconductor sensor for nearly monochromatic radiation, an optical power meter, or a thermopile sensor for non-monochromatic or spectrally variable optical sources, is utilized to measure optical power effectively.
The optical detector must be positioned at an appropriate distance from the optical device's output to capture the entire beam diameter, adhering to the optical detector's guidelines If the optical device is recessed within an enclosure that does not emit optical radiation, the detector can be placed at the specified distance, provided the enclosure meets recognized protection standards for electrical equipment designed to contain internal ignition, such as flameproof "d" enclosures per IEC 60079-1, or if there are no absorbing targets inside, as determined by the ignition hazard assessment Additionally, the maximum measured optical power must not exceed the applicable limits outlined in Tables 2, 3, or 4.
To assess compliance with the 'Optical irradiance' requirements outlined in section 5.2.2.3, the maximum measured optical power must be equal to or less than the applicable maximum optical power values specified in Table 2, Table 3, or Table 4.
Pulsed radiation
The optical pulse duration for Gc or Dc equipment can be calculated using the modulation frequency and duty cycle ratings provided by the manufacturer Specifically, the pulse duration, or 'on-time,' is determined by multiplying the period, which is the inverse of the frequency, by the duty cycle.
The optical pulse duration for Ga, Gb, Da, Db, Ma, or Mb equipment must be assessed under fault conditions, adhering to the over-power/energy fault protection criteria for optical devices designed with inherent safety features To measure the pulse duration of the voltage at the input of the optical device during each fault condition, an electrical oscilloscope can be utilized.
The flow diagram in Annex E shows the assessment procedure for Group II
5.2.3.2 Optical pulse duration of less than or equal to 1 s for Group II
For optical pulse durations under 1 ms, the energy of the optical pulse must remain below the minimum ignition energy (MIE) for the specific explosive gas environment, as dictated by the relevant equipment protection standards.
For optical pulse durations ranging from 1 ms to 1 s, the energy of the optical pulse must not exceed 10 times the Minimum Ignition Energy (MIE) of the explosive gas atmosphere, in accordance with the applicable equipment protection level.
For a single pulse, optical pulse energy is equal to the product of the average power and the optical pulse duration of that single pulse
NOTE In accordance with the ‘Comparison of measured minimum igniting optical pulse energy (Qe,pi,min) at
The minimum ignition energy (MIE) and auto ignition temperatures (AIT) for a 90 àm beam diameter are derived from literature, as shown in Table B.2 The relevant MIE values are determined according to the equipment group subdivision.
The MIE values for the application of this standard are:
5.2.3.3 Optical pulse duration greater than 1 s for Group II
For optical pulse durations exceeding 1 second, peak power must be measured according to the 'Continuous Wave Radiation' standards and must not surpass the safety limits established for continuous wave radiation (refer to sections 5.2.2, Table 2, or Table 4) These pulses, irrespective of the involved EPL, are classified as continuous wave radiation.
5.2.3.4 Additional requirements for optical pulse trains for Group II equipment
For optical pulse trains involving pulse duration less than or equal to 1 s, the following applies:
1) For all repetition rates, compliance with the single pulse criterion applies for each pulse
2) For repetition rates above 100 Hz, the average power shall not exceed the safety levels for continuous wave radiation in Table 2 or Table 4
3) For repetition rates at or below 100 Hz, the average power shall not exceed the safety levels for continuous wave radiation in Table 2 or Table 4 unless demonstrated to not cause ignition by tests according to Clause 6
5.2.3.5 Additional requirements for optical pulses for Group I and Group III equipment
The output parameters for optical sources in EPL Ma or Mb and Da or Db equipment must not exceed 0.1 mJ/mm² for pulse lasers or pulsed light sources, with pulse intervals of at least a specified duration.
The output parameters of optical sources of equipment of EPL Dc shall not exceed 0,5 mJ/mm 2 for pulse lasers or pulse light sources
Radiation sources with pulse intervals of less than 5 s are regarded as continuous wave sources.
Ignition tests
Ignition tests to demonstrate inherent safety may be performed for Group II in special cases such as:
• beams of intermediate dimensions or pulse duration that may exceed the minimum optical ignition criteria but are still incapable of causing ignition;
• beams with complex time waveforms such that pulse energies and/or average power are not easily resolved;
• specific atmospheres, targets, or other specific applications that are demonstrably less severe than test conditions studied to date
NOTE 1 These tests will be used only in very rare cases since they are quite expensive and require special test equipment Not all testing stations working with this standard will have the necessary test equipment for ignition tests
The test will be conducted according to Clause 6, utilizing 10 samples of the optical radiation source under the most challenging ambient conditions The test is deemed successful if no ignition occurs during the evaluation.
NOTE 2 Ignition tests for Group I and III are currently not specified.
Over-power/energy fault protection
Optical devices designed with inherent safety features must include over-power and energy fault protection to avoid excessive beam strengths in explosive environments A thorough risk and hazard analysis will assess whether further limitations are necessary It is essential to evaluate the failure modes of the optical source, driver circuitry, and the intended Equipment Protection Level (EPL) during both normal operations and fault conditions to identify any additional limitations needed.
Optical sources like laser diodes, LEDs, and lamps are prone to failure when subjected to overheating due to over-power conditions Testing 10 samples can demonstrate that a defined fail-safe shutdown or foldback mechanism is in place, ensuring necessary over-power fault protection The maximum optical output power from these samples is considered the peak power or irradiance value The thermal failure characteristics of low-power optical sources are deemed sufficient to provide adequate over-power protection for any EPL.
5.2.5.3 Optical sources requiring power limiting circuitry
In cases where the optical device's beam strength is constrained by the driver circuitry, any faults should be attributed to the circuitry rather than the optical device itself.
An LED's current, regulated by the driver circuitry to stay within the specifications outlined in the data sheet, is not deemed to surpass the maximum forward voltage specified for that current.
When assessing faults in optical devices, it is crucial to consider the potential opening or shorting of components that may affect beam strength However, printed wiring board traces are exempt from shorting concerns, as they adhere to the creepage distance, clearance, and solid insulation standards outlined in relevant industrial regulations.
Electrical circuits can incorporate current and voltage limiters between the optical source and the electrical power source to ensure over-power fault protection This protection must align with the necessary degree for the intended Equipment Protection Level (EPL), as outlined in IEC 60079-11, although alternative methodologies may also be utilized For Ga, Da, or Ma equipment, these limiters must safeguard against over-power faults during normal operation and after one or two faults occur In contrast, Gb, Db, or Mb equipment requires over-power fault protection during normal operation and after one fault For Gc or Dc equipment, the rated electrical values should be considered without accounting for any faults.
Requirements for protected optical radiation “op pr”
General
This concept requires radiation to be confined inside optical fibre or other transmission medium based on the assumption that there is no escape of radiation from the confinement
The performance of the confinement is crucial in determining the system's safety level, specifically “op pr.” Applicable safety levels include EPL Gb, Gc, Db, Dc, and Mb (refer to Table 1) Two options are available for implementation: 5.3.2 or 5.3.3.
All optical components shall be suitable for the ratings and temperature range for which they are used
NOTE It is not a requirement of this standard that conformity to the specification of the components be verified.
Radiation inside optical fibre or cable
Optical fibre cables are designed to prevent the release of optical radiation into the atmosphere during normal operation For EPL Gb, Db, or Mb, these cables must be equipped with additional armouring, conduit, cable trays, or raceways Additionally, a pull test must be conducted for optical fibres or cables that exit the end-equipment enclosure, in accordance with IEC 60079-11.
Cables, whether internal or external, can be spliced or terminated from one fiber to another using specialized couplers or joining kits, ensuring a secure connection For external terminations, it is essential that the cable connection maintains mechanical strength comparable to that of the original cable Detailed instructions for performing field connections must be provided.
NOTE 1 This can be achieved by using mechanical clamping or snap connection
EPL Gc or Dc optical fibers and cables, along with internal pluggable factory connections, must adhere to relevant industrial standards Additionally, external optical fiber or cable field connections should meet the external plug and socket outlet requirements specified in IEC 60079-0, ensuring compatibility with the designated EPL.
For EPL Gb, Db, or Mb, optical fibers or cables connected through internal pluggable factory connections must adhere to the pluggable connection standards outlined in IEC 60079-15 Additionally, external optical fiber or cable field connections are required to meet the external plug and socket outlet specifications from IEC 60079-0 corresponding to the necessary EPL.
NOTE 2 Typical examples are connections in split-boxes
NOTE 3 Optical fibre or cable alone is not Ex equipment.
Radiation inside enclosures
Ignition-capable radiation within enclosures is permissible if the enclosure meets recognized protection standards for electrical equipment designed to contain internal ignition, such as flameproof "d" enclosures per IEC 60079-1 Additionally, it is acceptable when the ignition hazard assessment indicates no absorbing targets are present inside the enclosure, including types like IP 6X, pressurized "p," restricted breathing "nR," and dust ignition protection by enclosure "t." However, any non-inherently safe radiation that may escape the enclosure must be adequately protected in accordance with these standards.
Optical system with interlock “op sh”
This protection is essential for radiation that is not inherently safe, as it ensures that radiation is contained within an optical fiber or similar transmission medium The principle relies on the assumption that, under normal operating conditions, there is no leakage of radiation from the confinement.
Depending on the EPL, “op sh” requires the application of “op pr” principles, along with an additional interlock cutoff, as follows (see also Table 1):
For applications involving Ga, Da, or Ma "op sh," it is essential to use protected fibre optic cable "op pr" for Gb/Db/Mb Additionally, a shutdown functional safety system that relies on the ignition delay time of the explosive gas atmosphere is necessary.
For Gb, Db, or Mb "op sh" applications, it is essential to use protected fibre optic cable "op pr" for Gc/Dc Additionally, implementing a shutdown functional safety system that adheres to eye protection delay times as specified in IEC 60825 is crucial for ensuring safety and compliance.
– For Gc or Dc “op sh” applications, unprotected fibre optic cable (not “op pr”), along with a shutdown functional safety system based on eye protection delay times (IEC 60825-2), is required
The interlock cut-off activates when confinement protection fails, allowing radiation to become unconfined within time frames shorter than the ignition delay or the eye protection delay.
The interlock cut-off delay time for equipment classified under Group I and Group IIA temperature classes T1 and T2 must be shorter than the boundary curve depicted in Figure 1, which is based on a curve fit to minimum ignition delays, incorporating a safety factor of 2.
Ignition delay times are specified only for Group I and Group IIA temperature classes T1 and T2, as shown in Figure 1 For other Group IIA applications, as well as for Group IIB and Group IIC applications, further testing and documentation are required to determine appropriate ignition delay times.
Figure 1 – Optical ignition delay times and safe boundary curve with safety factor of 2
The interlock cut-off must adhere to the specifications outlined in the risk analysis Equipment performance can be evaluated using methods from relevant standards such as IEC 61508 and IEC 61511 to ensure the necessary safety level As indicated in Table 1, the shutdown system is designed to function safely even in the presence of a single fault.
Test set-up for ignition tests
General
All gas-air-mixtures within the test vessel shall be maintained during the test at a temperature of 40 (±3) °C, or at the maximum temperature of the specific application
All gas-air-mixtures within the test vessel shall be maintained at an ambient pressure in accordance with IEC 60079-0.
Test vessel
A test vessel shall be used with a diameter greater than 150 mm, and a height above the absorber target (potential ignition source) greater than 200 mm
Criteria to determine ignition
Ignition is deemed to have taken place when a temperature increase of at least 100 K is detected by a 0.5 mm diameter thermocouple bead positioned 100 mm above the reference absorber, or when a flame is visually observed.
Verification of suitability of test set-up for type tests
Reference gas
To check whether the test set-up is suitable for type tests according to 6.3, ignition tests shall involve a propane-air-mixture in accordance with the following:
• For continuous wave radiation and for pulsed wave radiation above 1 s duration: propane- air-mixture of either 5 % or 4 % by volume, quiescent mixture
• For pulsed wave radiation equal to or less than 1 s and for all pulse trains: propane-air- mixture of 4 % by volume, quiescent mixture
See Table A.1 for additional background on the application of the propane-air-mixture
If the set-up is used only for either continuous wave or pulsed radiation, only the applicable of the two reference tests is necessary.
Reference absorber
Absorption at investigated wavelength above 80 %, to be applied on the transmission fibre tip (fibre optics), or compressed respectively applied to an inert substrate (free beam transmission)
NOTE Experiments show that for pulses in the micro and nanosecond range a carbon black absorber gives the lowest igniting pulse energies (absorption 99 %, combustible, high decomposition temperature) [1,4,61].
Reference test for continuous wave radiation and pulses above 1 s
The irradiated reference absorber must remain physically and chemically inert throughout the test duration, exhibiting high absorption characteristics to function effectively as a near-black body The testing setup will be conducted with the reference gas and absorber maintained at a temperature of 40 °C ± 5 K For fiber optics testing, the absorber should be applied in a very thin layer at the fiber tip.
The test setup is deemed acceptable if the ignition values obtained do not exceed 20% above the reference values listed in Table A.1 Additionally, it is essential that the absorber remains undamaged following the test.
For testing free beam transmission, the beam's smallest diameter must strike a flat layer of the target material, either applied to a substrate or in pellet form Reference values for the respective beam diameter can be found in Table A.1 The test setup is deemed acceptable if the ignition values achieved do not exceed 20% above the data listed in Table A.1, and the absorber must remain undamaged after the test.
Reference test for pulsed radiation below 1 ms pulse duration
The irradiated reference absorber must undergo free beam irradiation from the front during all pulse tests For free beam transmission testing, the beam's smallest diameter should impact a flat layer of the target material, which can be applied to a substrate or in a compressed pellet form The reference values for a beam diameter of 90 µm are 499 µJ pulse energy for 90 ns pulses and 600 µJ for 30 ns pulses.
The test setup is deemed acceptable if the ignition values obtained do not exceed 20% above the reference data provided in Table B.1, with the reference gas and absorber maintained at a temperature of 40 °C ± 5 K.
NOTE Background information for the reference values are given in the bibliography [4].
Type tests
Ignition tests with continuous wave radiation and pulses above 1 s
The ignition tests for continuous wave radiation and for pulsed wave radiation above 1 s duration shall involve a gas-air-mixture in accordance with the following:
• For T6/IIC atmospheres: CS 2 in air, 1,5 % by volume, and Diethyl ether, 12 % by volume
If only diethyl ether is used, the minimum ignition powers or irradiances obtained shall be divided by a factor of 4 when applying the acceptance criteria
• For T4/IIA, T4/IIB and T4/IIC atmospheres: diethyl ether, 12 % by volume
• For T3/IIA and I atmospheres: propane in air, 5 % by volume
• For special applications: the atmosphere under consideration.
Ignition tests with single pulses less than 1 ms duration
The ignition tests for pulsed wave radiation less than 1 ms duration shall involve a gas-air- mixture in accordance with the following:
• For IIC atmospheres: H 2 in air, 12 % and 21 % by volume, or CS2 in air, 6,5 % by volume
• For IIB atmospheres: ethene in air, 5,5 % by volume
In I and IIA atmospheres, the acceptable concentration of diethyl ether is 3.4% by volume, while propane in air should be at 4% by volume When using propane in air, it is important to divide the minimum ignition energies obtained with propane by 1.2 to meet the acceptance criteria.
• For special applications: the atmosphere under consideration.
Tests for pulse trains and pulses from 1 ms to 1 s duration
The ignition tests for pulsed wave radiation from 1 ms to 1 s and for all pulse trains shall involve a gas-air-mixture in accordance with the following:
• ignition tests performed with gas-air-mixtures in accordance with the above “pulsed wave radiation above 1 s duration”, followed by
• ignition tests performed with gas-air-mixture in accordance with the above “pulsed wave radiation less than 1 ms duration”.
Absorber targets for type tests
The absorber target shall be maintained at the same temperature as the gas-air-mixture
The absorber target must remain physically and chemically inert during the irradiation test, ensuring it exhibits extremely high absorption properties to function effectively as a near-black body.
For effective optical transmission, the absorber target must exhibit an absorption property exceeding 80% at the relevant wavelength Further details regarding the criteria for selecting the reference absorber are provided below.
The absorber target must be placed as close as possible to the output of the optical source For optical fiber transmission sources, a thin layer of the reference absorber should be applied directly to the fiber tip In the case of free beam transmission sources, the reference absorber should either be applied in a thin layer to an inert substrate or compressed into a pellet, and positioned at the output of the optical source.
For optical sources positioned at a specific distance within an enclosure, the absorber target can be placed at the same distance from the optical source It is essential that the absorber is applied as a thin layer on an inert substrate or compressed into a pellet, maintaining this distance from the optical source's output This method is viable only if the enclosure meets recognized protection standards for electrical equipment designed to contain internal ignition, such as a flameproof "d" enclosure per IEC 60079-1, or if the ignition hazard assessment indicates that absorbing targets are not expected inside the enclosure, as seen in IP 6X, pressurized "p," or restricted breathing "nR" enclosures.
Application of this very thin layer shall be achieved by having the absorber begin as a powder in suspension, and then dried afterwards at a recommended thickness of approximately
NOTE Experiments show that for pulses in the micro and nanosecond range, a carbon black absorber gives lowest igniting pulse energies (absorption 99 %, combustible, high decomposition temperature) [17][22][24].
Test acceptance criteria and safety factors
Where ignition is considered to have occurred and the absorber is undamaged, these results can be treated as inherently safe data under the following conditions:
• A safety factor as follows is applied to the achieved igniting power:
– For continuous wave radiation and for pulsed wave radiation greater than 1 s duration:
A safety factor of 1,5 shall be applied
– For pulsed wave radiation less than or equal to 1 s and for pulse trains: A safety factor of 3 shall be applied
• After application of this safety factor, the adjusted igniting power is not more than 20 % above the data from Table A.1
In scenarios where no ignition is deemed to have occurred, such as when power or energy levels cannot be further increased during testing, and the absorber remains undamaged, the results can be classified as inherently safe data.
• A safety factor as follows is applied to the highest non incendive beam power as follows: – For continuous wave radiation and for pulsed wave radiation greater than 1 s duration:
A safety factor of 1,5 shall be applied
– For pulsed wave radiation less than or equal to 1 s and for pulse trains: A safety factor of 3 shall be applied
• After application of the above safety factors, the adjusted non-incendive beam power is not more than 20 % above the data from Table A.1
To obtain inherently safe beam strength data with an applied safety factor, an alternative reference gas that is more sensitive to ignition can be utilized For instance, ethene (C₂H₄) can serve as a test gas for continuous wave radiation and pulsed wave radiation exceeding 1 second in IIA/T3 atmospheres, applicable to beam areas of approximately 2 mm² It is essential that ignition is not deemed to have occurred by the end of the test, and the absorber must remain undamaged.
Due to significant statistical variations in ignition caused by small hot surfaces, it is essential to apply a safety factor Additionally, caution is necessary when evaluating experiments as non-incendive, as minor changes in test parameters can greatly affect the outcomes.
All equipment utilizing optical radiation must feature all necessary markings as dictated by applicable equipment protection techniques, such as flameproof enclosures ("d") and intrinsically safe apparatus ("i") Electrical equipment, components, and Ex parts that emit optical radiation and are safeguarded by the protection types outlined in this standard should be marked in compliance with IEC 60079-0, including the specific symbol representing the type of protection employed.
“op is”: for inherently safe optical radiation;
“op pr”: for protected optical radiation;
“op sh”: for optical system with interlock b) the symbol of the temperature class and Group and the suffixes A, B or C as stated in IEC 60079-0, but:
Equipment that emits optical radiation but is not suitable for installation in hazardous areas must be marked as 'Associated Equipment.' Additionally, if Table 2 specifies a temperature class restriction, this should be noted after the type of protection.
Example: [Ex op is IIC T4 Gb]
To ensure compliance with Table 2, it may be necessary to utilize a column from Table 2 that corresponds to optical power or irradiance values linked to a different temperature class than that specified in the Ex marking string for other relevant electrical equipment protection techniques Only the most restrictive temperature class value should be indicated on the equipment, and multiple temperature class markings are prohibited.
• Equipment which conforms to EPL Ga:
Ex op is IIC T6 Ga
• Equipment which conforms to EPL Gb:
Ex op pr IIC T4 Gb
• Equipment, which is installed outside the hazardous area and provides optical radiation to the hazardous area, limit values taken from Table 2 or Table 4:
[Ex op is IIA T3 Ga]
• Equipment with an optical source protected by type of protection encapsulation ‘m’ and type of protection ‘op is’
Ex mb op is IIC T4 Gb
The certificate shall identify the relevant EPL of the equipment (there may be more than one EPL for the different parts of the equipment)
Table A.1 presents reference values for ignition tests conducted with a propane-air mixture at a temperature of 40 °C The absorber was connected to the end of an optical fiber and was continuously irradiated.
Table A.1 – Reference values for ignition tests with a mixture of propane in air at 40 °C mixture temperature
Fibre core diameter Minimum igniting power at
1 064 nm (absorption: 83 %, 5 % propane by volume)
Minimum igniting power at 805 nm (absorption: 93 %, 4 % propane by volume) àm mW mW
NOTE Other reference test data (e.g for 8 àm core diameter, 1 550 nm wavelength) are currently not available
The potential hazard associated with optics in the infrared and visible electromagnetic spectrum depends on:
The hazard of optics in explosive atmospheres is significantly influenced by various factors, particularly the ignition mechanism The most critical scenario occurs when an absorber is present, as ignition can occur at a point when the dimensions of the radiation or absorber are smaller than the quenching distance of the explosive gas Notably, radiation from a fiber optic cable diverges quickly, resulting in an irradiated area that can expand to square centimeters Ignition conditions can be defined by the fundamental parameters of energy, area, and time, with area and time directly impacting the ignition criterion.
Infinite time correlates with continuous wave radiation, as demonstrated in the research findings presented in Table B.1, Figure B.1, and Figure B.2 Ignition occurs through hot surface ignition when the beam interacts with an absorber, with smaller surfaces requiring higher igniting irradiance Consequently, smaller surfaces must reach elevated temperatures to initiate ignition Notably, no ignition was detected below 50 mW optical power across all gas/vapor mixtures, except for carbon disulfide, reinforcing the established maximum permissible power value.
The effective ignition of n-alkanes requires a minimum power of 200 mW (150 mW with a safety margin), while experiments with reactive absorbers like coal, carbon black, and toner demonstrated that despite their higher absorption rates, they were less effective as ignition sources Additionally, for larger irradiated areas, a permissible power density of 5 mW/mm² is more practical than a stringent power criterion, taking into account the non-ideal grey body absorption of the inert absorber.
In a short time frame within a confined space, a laser pulse can generate an ignition source akin to an electric spark through air breakdown Research indicates that a spark with energy near the minimum ignition energy (MIE) can effectively ignite an explosive mixture when conditions are optimized, such as with picosecond (ps) and nanosecond (ns) pulses.
The effectiveness of this ignition process depends on
• pulse length and repetition rate;
2 The information provided in this annex is taken from [1]
Microsecond and nanosecond pulses with energies near the Minimum Ignition Energy (MIE) can ignite explosive mixtures, with carbon black being the most effective absorber due to its high absorption, elevated decomposition temperature, electron-rich structure, and combustibility In contrast, for millisecond pulses that do not induce breakdown but instead heat the target, the ignition energies exceed the electrical MIE by over an order of magnitude, making inert grey bodies the ideal absorbers Additionally, pulses longer than 1 second should be regarded as continuous wave radiation.
For pulse trains, the ignition criterion for each pulse is based on energy when the pulse duration is less than 1 second At higher repetition rates, previous pulses can affect the behavior of the irradiated area during the current pulse Specifically, for repetition rates exceeding 100 Hz, the average power must not exceed the continuous wave limit, which imposes a maximum repetition rate for a given pulse energy Shorter pulses allow for higher permissible peak power but result in longer duty cycles, providing time for cooling of the target or the decay of any hot material Experiments indicate that for nanosecond pulses within the MIE range (up to 400 µJ), a spark lifetime exceeding 100 µs is unlikely with a beam diameter of 90 µm For longer pulse durations greater than 1 second, peak power should also adhere to the corresponding continuous wave limit.
The remaining combination of fundamental parameters i.e short times over infinite area can be evaluated by the results for the other regimes
Table B.1 – AIT (auto ignition temperature), MESG (maximum experimental safe gap) and measured ignition powers of the chosen combustibles for inert absorbers as the target material (α 1 064 nm %, α 805 nm ) 3
Combustible in brackets: increased mixture temperature
AIT MESG Conc comb at min ignition power
Conc comb at min ignition power
62,5 à m fibre 400 à m fibre 400 à m fibre 600 à m fibre 1 500 à m fibre
PTB* PTB PTB HSL* HSL HSL HSL
(1 064 nm) (1 064 nm) (1 064 nm) (803 nm) (803 nm) (803 nm) (803 nm) °C mm % vol mW mW % vol mW mW mW
200 0,90 4,0 – 658 – – – – tetra- hydrofuran 230 0,87 6,0 267 – – – – – diethyl ether 175 0,87 12,0 89 127 23,0 110 180 380 propanal
(110 °C) 190 0,84 2,0 – 617 – – – – dimethyl ether 240 0,84 8 280 – – – – – ethene 425 0,65 7,0 202 494 7,5 530 – 2 007 methane/ hydrogen 565 0,50 7,0 163 401 – – – –
IIC carbon disulphide 95 0,37 1,5 50/24** 149 – – – – ethyne 305 0,37 25,0 110 167 – – – – hydrogen 560 0,29 10,0 140 331 8,0 340 500 1 620
* HSL = Health and Safety Laboratory of the Health and Safety Executive (UK),
PTB = Physikalisch-Technische Bundesanstalt (Germany)
** 24 mW was obtained for a combustible target (coal)
3 AIT and MESG were taken from [9]
NOTE The given values are for each combustible in its most easily ignitable mixture
Figure B.1 – Minimum radiant igniting power with inert absorber target
(α 1064 nm %, α 805 nm %) and continuous wave-radiation of 1064 nm
M ini m um ig ni tin g po w er , m W
35 mW non-ignition hydrogen ethine carbon disulfide Adler CS 2 [7] methane n-pentane iso-propylic alcohol propane ethene THF diethyl ether dimethyl ether
Figure B.2 – Minimum radiant igniting power with inert absorber target
(α 1 064 nm %, α 805 nm %) and continuous wave-radiation
(PTB: 1064 nm, HSL: 805 nm, [8]: 803 nm) for some n-alkanes
Table B.2 presents a comparison of the measured minimum igniting optical pulse energy (\$Q_{e,p i,min}\$) at a beam diameter of 90 µm, alongside auto ignition temperatures (AIT) and minimum ignition energies (MIE) sourced from literature [9] The data is analyzed at various concentrations expressed in percent by volume (\$ϕ\$).
Fuel Q e,p i,min ϕ AIT MIE ϕ MIE Q e,p i,min /MIE àJ % °C àJ %
Nanosecond Pulses (20 ns to 200 ns) propane 499 4,0 470 240 5,2 2,1 ethene 179 5,5 425 82 6,5 2,2 hydrogen 44 12 560 17 28 2,6
NOTE The target material was carbon black
M ini m um ig ni tin g po w er , m W methane HSL n-pentane HSL
The ignition hazard assessment is the crucial first step when evaluating optical radiation If the assessment indicates that ignition is unlikely, there is no need for further application of this standard.