By Estellito Rangel Jr.
By Estellito Rangel Jr.
April 4, 2018 — The design and installation of electrical equipment for a facility with an explosive atmosphere begins after a detailed area classification is completed. The International Electrotechnical Commission has developed a number of standards that deal with multiple requirements for these areas in the IEC 60079 series of explosive atmosphere standards, many of which have been adopted in North America.
Below are some of the changes in recent editions of the IEC 60079 series, plus some recommendations for safer installations.
CABLE GLANDS FOR EX D ENCLOSURES
The fifth edition of IEC 60079-141 allows the use of Ex d cable glands certified in accordance with IEC 60079-12 as equipment, and combined with appropriate cables with a minimum length of the connected cable of 3 metres. This was a major change compared to the previous edition’s requirement,3 in which Ex d enclosures bigger than 122 in.3 (2 dm3) imposed the use of barrier glands.
The U.K.’s Health and Safety Executive (HSE) took exception to this change, and issued a safety notice in February 20164 cautioning users that there was an increased risk of re and explosion propagating through the cable sheath, they felt, and that this could be avoided through the use of barrier glands. (It should be noted the U.K. was the only country to take this position.)
The HSE endorsed the U.K. Committee proposal to keep the previous edition requirements valid in the U.K.5 regarding the cable glands to Ex d enclosures, considering that:
• It was not shown documented evidence of any tests carried out to justify the removal of the previous requirements.
• The drop pressure test for cables on the standard’s Annex E (Informative) is not related to cable standards, and is not reasonably practicable.
A barrier gland (Figure 1) ensures that gas migration through the cable is prevented, and also provides a flame barrier should an explosion occur, thereby maintaining the explosion protection integrity of the equipment to which it is connected (as per the “Rapidex catalogue”, CMP Products).
IEC 60079-14’s position on initial inspection involves checking whether the equipment has been properly selected in accordance with the standards’ requirements and has been correctly installed before coming into operation.6
Annex C contains schedules for this inspection, related to the type of protection for that equipment. These schedules were originally found in IEC 60079-17,7 but their inclusion in IEC 60079-14 was considered a good change.
Recommendations for safer installations
OPTICAL RADIATION TRANSMITTING EQUIPMENT
The second edition of IEC 60079-28 was issued in 2015.8 It describes requirements and precautions to be taken when using optical radiation transmitting equipment in explosive gas or dust atmospheres.
Today’s more powerful LED technology presents real challenges for the safety of products used in hazardous locations.
At the IECEx meeting in Umhlanga, South Africa (September 2016), a detailed report presented the issues involving LEDs and the possible ignition sources caused by powerful light. It was shown that, out of 1000 certificates issued by IECEx for Ex luminaires, less than 10% take into account the risk of optical radiation based on consistent and standardized evaluation in accordance with IEC 60079-28.9
The number of LED luminaires found in applications where flammable vapours and combustible particles are present is increasing rapidly. A variety of substances can stick to the surface of the luminaires in industrial applications, such as refineries, chemical plants, ships, etc. When these substances cover the transparent surface of the luminaires, they absorb the optical radiation and can start heating up. The risk of ignition is significant when the optical radiation has not been accounted for in the design of these luminaires.
As such, the IECEx certification process for Ex LED luminaires was not considered consistent, and this affects end users who are selecting and accepting products without a careful examination of their certificates.
Figure 2 shows a toaster: P = 750W for two pieces of bread. It provides about 5 mW/mm2 of radiation during the toasting process, which is enough energy to toast the bread after a short while, if not completely burn it.
That same amount of radiation, 5mW/mm2, is the commonly used limit for safe optical radiation in IEC 60079-28. The wavelength of toaster radiation is different, but I use this example to demonstrate the impact of radiation. So, users need to verify whether the certificates of LED luminaires issued include the IEC 60079-28 requirements’ evaluation.
SURGE SUPPRESSORS FOR EX MOTORS ABOVE 1KV
In clause 184.108.40.206, IEC 60079-14 (5th ed.) warns that overvoltage switching can occur when a high-voltage motor is switched off and vacuum circuit breakers or vacuum contactors are being used. The transients depend on various system installation and design factors, such as the arc-extinguishing principle of the contactor or switch, size of the motor, length of the power supply cable and system capacitance, among others.
IEC 60079-14 states that, regardless of the motor size and the arc-extinguishing principle of the switch being used, shutting down the motor during startup can cause overvoltages and, also, when vacuum circuit breakers or vacuum contactors are used for motor switching, the motor installation design should consider using an appropriate surge suppressor, such as a zinc oxide varistor with spark gap.
Under these conditions, the installation of surge suppressors in motor circuits above 1kV should be considered mandatory, especially because there is evidence that ignition can occur during locked rotor tests applied on 4.16kV Ex e induction motors.10
An item not covered in IEC 60079-14 that contributes to a safer plant is safety signage/awareness. Figure 3 shows a safety sign containing the “Ex” (“explosive atmosphere”) inside a yellow triangle against a red background.11 It alerts workers, and shares information about the zone, gas group and permissive temperature class. The hazardous location classification drawing’s number facilitates onsite consultation.
When a new edition of a standard is issued, changes to the previous content (especially the addition of new technical requirements) can result in an existing, certified product not meeting the new requirements. Sometimes, entire equipment design may need to be reviewed by the manufacturer. On the other hand, when modi cations apply to the installation, refresher training for the workforce will become necessary. These can be expensive tasks.
For installers, manufacturers and end users, the implementation of a new standard requires skilled professionals to perform a careful evaluation of the consequences. The current editions of IEC standards regarding installations in hazardous locations involve some additional requirements when compared to earlier editions, and special attention should be paid by installers and manufacturers alike because some of them are controversial.
1. IEC 60079-14 “Explosive atmospheres-Part 14: Electrical installations design, selection and erection”, 2013.
2. IEC 60079-1 “Explosive atmospheres-Part 1: Equipment protection by flameproof enclosures ‘d’.”, 2014.
3. IEC 60079-14 “Explosive atmospheres-Part 14: Electrical installations design, selection and erection (Ed. 4.0)”, 2007.
4. “Use of barrier glands in potentially explosive atmospheres to meet IEC 60079:14 2013 (Edition 5)”, HSE, February 2016, tinyurl.com/y8n2gcaw.
5. BS EN 60079-14, “Explosive atmospheres-Part 14: Electrical installations design, selection and erection”, 2014.
6. Estellito Rangel Jr., Mauricio F. Oliveira and Alan R.S. Queiroz, “The importance of inspections on electrical installations in hazardous locations”, 61st PCIC Conference Record, San Francisco, 2014, p. 155-161.
7. IEC 60079-17 “Explosive atmospheres-Part 17: Electrical installations inspection and maintenance, edition 5.0”, 2013.
8. IEC 60079-28 “Explosive atmospheres-Part 28: Protection of equipment and transmission systems using optical radiation, edition 2.0”, 2015.
9. Tarmo Rintala, “Is certification of LED luminaires causing higher risks on Ex areas?”, Atexor, July 2017.
10. Estellito Rangel Jr. and Carlos Sanguedo, “New requirements for electrical installations in hazardous locations”, 64th PCIC Conference Record, 2017, Calgary, p. 443-450.
11. Estellito Rangel Jr., “Safety in electrical installations in petroleum industry”, 1st IEEE PCIC Mexico Conference Record, Mexico City, 2013.
Estellito Rangel Jr. currently serves as a senior consultant on oil & gas electrical system design, and on workplace electrical safety. Prior to consultancy, he spent 33 years with Petrobras designing electrical systems, performing LV and MV equipment factory acceptance tests, and coordinating hazardous area classification assessments. He is also the first Brazilian member of IEC Technical Committee TC-31 “Equipment for explosive atmospheres”. He can be reached at firstname.lastname@example.org. This article is adapted from a paper presented during the International Technical Session at IEEE PCIC 2017 in Calgary.
This article originally appeared in the March 2018 issue of Electrical Business Magazine.