Special note: Before we get too deep into today’s topic, I think I need to state something quite clearly. While we at Danfoss Drives have some solutions to common problems in explosive areas, we don’t hold all of the cards or make any claims that, if you follow the steps below, you can be certain that you’ve accounted for everything. In reality, you should consult a professional who specializes in these areas to validate that you’ve taken the appropriate steps to ensure the safety of all people and equipment in these areas.
With that out of the way, let’s get to the topic at hand.
Up to this point, we’ve covered quite a few different environments that your AC drives could experience. But, for some key industries such as Mining and Oil and Gas, to name a few, there are some unique environments that we haven’t yet touched on that are quite hazardous – literally – and not only to the AC drives.
What makes these areas more dangerous than others is the presence of particulates – gases, dusts and fibers – that have such concentrations or chemical makeups that make them highly combustible at temperatures commonly seen at points on your AC drive or your motor. Additionally, there may be components in your system that create a spark which also offers up a point of ignition of these particulates. So, in order to ensure the safety of the area, we need to take some special precautions.
Comply with the regulations and directives
The first precaution we take is at the regulatory level, which, as a manufacturer, we comply with in order to offer you the right products for the task at hand. There are primarily three regulations that cover most of the world’s hazardous locations and explosive environment areas. The ATEX directives cover the areas of the European Union and are largely accepted in most of the world. As a supplement to ATEX outside of Europe, there’s also the IECEx certification which is based on the IEC requirements. The United States and Canada are covered under the National Electric Code (NEC), though, so they’re handled a bit differently.
Under the cover of the ATEX directives, there are two specific directives that cover protections of equipment and protections of the workplace. As you can imagine, for an AC-drive manufacturer, we’re largely focused on the equipment side, so we’ll focus on that side for this post. ATEX divides the explosive areas into zones: using 0, 1 and 2 to refer to areas of gasses, vapors or mists, and using 20, 21 and 22 to refer to areas of combustible dusts or ignitable fibers. These zones refer to the period of time in these locations that a hazardous concentration of these particulates are present. In Zone 0/20, there is an expectation of the presence of dangerous concentrations continuously, for long periods of time or with a high frequency. In Zone 1/21, the dangerous concentrations only appear occasionally. And in Zone 2/22, these dangerous concentrations are unlikely to occur. But, if they do, will only be present for a short period of time.
The thing about ATEX is it also applies to machines that, while not placed in the explosive atmosphere, could be interfacing/controlling machines such as motors that are placed in these explosive atmospheres. When we’re discussing ATEX requirements, Danfoss Drives is typically looking at AC drives that are mounted in a clean room, but may be monitoring and controlling an explosion-proof motor in an explosive atmosphere. When this is the case, we have to turn to some specific options for AC drives such as the VACON® NXP, VACON® 100 INDUSTRIAL or VLT® AutomationDrive. For these product families, we offer option cards that provide an ATEX-certified input to monitor the temperature of the motors placed in the explosive atmospheres. For these options, we have received certifications for controlling explosion-proof motors located in non-mining equipment in gas and dust environments as long as they are only in Zone 1/21 or Zone 2/22. Our products are not certified, however, to control a motor in Zone 0/20 nor in mining applications.
The IECEx system is largely similar to ATEX in its method of classifying areas. However, while the classification system is quite similar, the certification process is a bit different due to IECEx following only the IEC standards. Where ATEX takes into consideration the connections to other equipment, IECEx is only applied to the device itself. Additionally, the IECEx certification process takes the quality systems of the manufacturer under consideration which provides an extra level of certification requirements. The IECEx certification is largely seen as more of a global certification, so it’s not uncommon to see these product requirements in regions like Australia, for example.
In North America, the NEC is the standard setting the requirements around hazardous locations. There are actually quite a few similarities, but, in a way, the NEC is more generic than the ATEX directives. In another way, though, the NEC is a bit finer in its classifications. Where ATEX uses zones on a high level, NEC defines classes and divisions. Classes are split into Class I, II and III where Class I applies to gasses/mists/vapors, Class II applies to dusts and Class III applies to ignitable fibers/flyings. The divisions are split into Division 1 and Division 2 where Division 1 refers to a high probability of dangerous concentrations (such as ATEX Zone 0/20 and Zone 1/21) and Division 2 refers to normally not hazardous, but can become so for a short period of time (similar to ATEX Zone 2/22). Furthermore, NEC breaks down each class into groups of representative materials. For example, group A is represented by acetylene while group G is represented by non-conductive dusts, such as flour, grain, wood and plastic.
What’s important to note here, however, is that the NEC standards apply only to the direct devices, instead of setting standards for interfacing with devices in hazardous locations as is seen in the ATEX standard. What this means is that, unless a product is offered for uses in these locations, it doesn’t require a specific UL approval for a particular class/division rating. As a matter of fact, only a very few manufacturers have AC drives that work in these areas, and most of them are designed for Class I, Division 1 which means that they feature large, heavy and expensive UL Type 7 enclosures which are overkill for Division 2 areas. Danfoss Drives, however, offers the VACON® X5 HazLo in the USA and Canada which serves Class I, Division 2 areas with sizes, weights and prices that are lower than the Division 1 counterparts.
Check the product label
When it comes to ATEX, determining that a product is certified for a particular area or use is quite straightforward as it’s listed on the product label. For instance, see the label below for the VLT® MCB 112 option.
On this label, the [Ex] icon depicts that this is certified to the ATEX directives. The [II] means that this product is applicable for non-mining applications. The [(2)] refers to the location where the connected device (the motor) is installed. Since this is a 2, it means the motors can be located in Zone 1/21 or Zone 2/22, but not in Zone 0/20. The subsequent letter determines the applications that the product is applicable for. The [G] is for gas areas (Zones 1 and 2) and the [D] for dust areas (Zone 21 and 22). Finally, we have a collection of motor types that this product can interface with, put in brackets. If the product itself would have that type of protection, the brackets would not appear.
For areas with gasses, this product can control not only ‘Ex d’ motors, but also the cheaper, smaller and lighter ‘Ex e’, and ‘Ex n’ type motors, by limiting the motor current to prevent the motor from heating up. The ‘Ex e’ motor type is an increased safety one, which involves design measures (such as wider air and creepage gaps) in order to prevent unacceptably high temperatures and sparks or electrical arcs. The ‘Ex d’ motor type has a flame-proof enclosure that can withstand the pressure of an explosion inside and prevents the transmission of the explosion outside. There are also ‘Ex de’ motors, where the motor itself has a ‘d’ ignition protection class, while the connection box is implemented in compliance with the ‘e’ ignition protection class, which restricts the maximum feeding voltage. Finally, the ‘Ex nA’ motor type is a non-sparking one, with a similar protection approach than the ‘Ex e’ type. ‘Ex e’ and ‘Ex d’ motors can be used in zones 1 and 2, but ‘Ex n’ only in zone 2.
For areas with dusts, this product is compatible with ‘Ex tb’ and ‘Ex tc’ type motors, both of them protected by enclosure type. In this case, the enclosure is so tight (high IP grade) that no combustible dust can enter. Its external surface temperature is also limited to prevent the motors from getting hot enough to ignite the environment. ‘Ex tb’ motors can be used in zones 21 and 22 but ‘Ex tc’ only in zone 22.
The motor label also includes, according to the IEC 60079-0 standard, the explosion group (IIIC, for example, would be conductive dust) and the equipment protection level (Gb, for example, would be high protection level for gases) and, in addition, its maximum surface temperature, as a temperature class from T6 (85° C) to T1 (450° C) for gases or directly for dust, and the IP grade for dust protection.
Don’t forget when using drives interfaced to motors in hazardous zones
For motors in hazardous zones it’s critical to ensure the motor is compatible for use with an AC drive due to the additional stresses that can occur to a motor when operated by a drive. The length and type of cable between the drive and the motor, the motor insulation, the motor frame size, the type of bearings, the power supply voltage and the installation practices need to be considered. Often compatibility can be assured without the need for any additional equipment (for example, without the need for additional filters on the output of the drive) but it’s best practice and especially important to ensure compatibility. The fact that the drive will often be installed in a non-hazardous zone a long way from the hazardous zone, and therefore the length of cable to the motor is often relatively long, adds more reason to check compatibility. You can simply ask your drive and motor suppliers to assist if you don’t have full knowledge on this topic. It should be no problem if these are two different suppliers. More information about the additional motor stresses resulting from using a drive can be found in the post “Is Your Motor Suitable for Frequency Converter Operation?”
Using an ATEX-certified protection device installed in a drive to monitor and prevent the surface temperature of the motor exceeding the allowed limits will ensure the motor operates within its load-ability curve and protect self-ventilated motors when they are operating at reduced speeds, when the cooling capacity of the fan is reduced. Devices such as the VLT® PTC Thermistor Card MCB 112 option for the VLT® AutomationDrive FC 302, VLT® AQUA Drive FC 202 and VLT® HVAC Drive FC 102, OPT-AF card for the VACON® NXP and OPT-BJ card for the VACON® 100 ensure this protection.
Check the hazards present in a facility
The remainder of the steps to protect your workers and the equipment lies largely on the shoulders of the facility where the equipment is to be installed. It’s important to properly identify the given hazards present wherever the equipment is to be located. Identifying the zone is the first step to ensuring the appropriate level of protections exists in the equipment to avoid any negative reactions to the explosive atmospheres that are or may be present. Secondly, utilizing specially designed equipment for explosive atmospheres, such as explosion-proof motors, temperature sensors and encapsulated relays and pairing them with functional safety equipment goes a long way to reducing the risks of operating in these areas.
Although you can never fully eliminate any risk of failures, by taking the appropriate steps to identify hazards and selecting the right equipment, you can at least mitigate most of the risks involved in operating in these special environments.
Check back regularly with us here at FocusOnDrives.com for regular updates on the best ways to ensure that your investments in AC drives are always the safest investments around. Next time, in the final part of the “It’s a harsh world…” series, we’ll look at Drives vs. Noisy environments where we look at how electrical noise and radio frequency emissions impact your AC drives and the systems they’re controlling. Additionally, let us know in the comments what environmental impacts give you the biggest challenges and how we can help you overcome them. Regardless of whose name is on the label, we’re here to help! In the meantime, you can find out more about all our products here.
Authors: Jake Roeder, Global Product Marketing Manager, Danfoss Drives
Abraham González Ponce, Application Knowledge Manager, Application & Service Products, Danfoss Drives