Environmental Protection of HMI Components
Electromechanical devices, including switches, keypads, keyboards, pointing devices and other elements such as indicators and alarms, are critical aspects of the Human Machine Interface (HMI) for controlling equipment and systems, including a range of appliances.
Full consideration must be given to the environment in which these devices will be deployed before specifying for any advanced HMI system. Where water, fuel, cleaning solutions, fine dust, and other materials may come in contact with control panels it is important to select components with an appropriate rating for environmental protection. Failure to do so might lead to significant danger to people and equipment.
What is the IP Rating System?
It’s important to understand what the IP rating system is and why it matters. Widely used throughout Europe, the IP Rating System describes the ‘degree of protection’ as defined by the International Electrotechnical Commission standard 60529 (IEC60529). IP stands for International Protection, frequently referred to as ‘Ingress Protection.’ The degree of protection as defined by the IEC is used to describe:
- Protection offered to people against hazardous parts inside an enclosure.
- Protection of equipment inside the enclosure against the intrusion of solids.
- Protection of equipment inside the enclosure against the ingress of liquids.
HMI devices need to be rated since they fall within the IEC definition of an enclosure: ‘a part providing protection of equipment against certain external influences and in any direction protection against direct contact’.
The rating proves the device has been tested according to international standards, providing a more detailed measure than generic marketing terms like ‘waterproof’. Understanding the IP system is simple. The IP code designation consists of the letters IP followed by two numerals, and is sometimes followed by optional letters.
The first number is a measure of how well the enclosure can prevent an invasion by solids. This encompasses all foreign bodies - anything from large objects like parts of the body (fingers, hands) or tools, down to the tiniest of dust particles. In this example, the 6 indicates the device the letter ‘x’, e.g. IPx4, which addresses moisture only. The IEC standard defines additional letters that can be appended to classify only the level of protection against access to hazardous parts by persons:
Protected against hazardous parts with
Back of hand
Further letters can be appended to provide additional information related to the protection of the device:
High voltage device
Device moving during water test
Device standing still during water test
The IP rating system covers the degree of protection against the penetration of solids and liquids from various pressures, directions, and volumes making this a necessity when designing devices used in hazardous environments.
In the same example, a switch rated IP65M indicates that it is protected against dust and water to the same IP65 level, but the device was not still during the test.
Three digit IP ratings
A third number was once used to specify the resistance of equipment to impact and is still shown on some products. This was dropped from the third edition of IEC60529 onwards, and has been superseded by the separate IK number according to EN 622023. While newer products are likely to be given an IK rating instead, there is not an exact correspondence of values between the old and the new standards.
Stainless steel designs can provide added protection against harmful chemicals but still must incorporate internal and external sealing mechanisms to meet accepted international requirements.
The IP rating system does not cover condensation, fungus and harmful vapor so assumptions should not be made that a highly rated switch will be protected. Explosion-proof switches are also not included. Switches for use in oil refineries, for instance, must be specified for that use and must be intrinsically safe. Some IP ratings may require further clarification from the manufacturer, especially for those ending IPx7 and IPx8. The number 8 specifies the device is suitable for continuous immersion in water under conditions which shall be specified by the manufacturer.
The depth of the water and the duration of continuous immersion would need to be defined. The reason is that some oceans are so deep virtually no object can withstand the pressures, and would collapse. Also, continuous might be interpreted as eternal – nobody would guarantee something forever. Where there is doubt, seek further clarification.
German standard DIN 40050-9 extends IEC 60529 with an additional rating for environmental sealing - IP69K, for high-pressure, high-temperature wash-down applications. Devices must not only be dust tight (IP6X), but also able to withstand high-pressure and steam cleaning. The test is very strict: 80°C water is sprayed through a nozzle at pressures of 8–10 MPa (80–100 bar) and a flow rate of 14–16 Litres/min. The nozzle must be held 10–15 cm from the tested device at angles of 0°, 30°, 60° and 90° for 30 seconds each. The test device sits on a turntable that rotates once every 12 seconds (5 rpm). The IP69K test specification was initially developed for road vehicles, especially construction vehicles and those that need regular intensive cleaning, but also finds use in other intensely hygienic applications (e.g. food industry).
Advanced HMI components are precisely crafted and designed to include both internal and external sealing mechanisms in order to meet the operability requirements of defined sealing specifications.
In the United States, the National Electrical Manufacturers Association sets the requirements for environmental sealing and its standards are widely used throughout North, Central and South America. The NEMA Standards Publication 250 - 2008 (previously 250 - 2003; 250 - 1997) “Enclosures for Electrical Equipment (1000 Volts Maximum)” defines enclosures according to their type from 1 to 13. NEMA standards meet or exceed the IP ratings listed, but the reverse cannot be assumed. The IP code only address requirements for the protection of people, the ingress of solids and the ingress of water. The NEMA Types consider these and also numerous other requirements including:
- Construction requirements
- Door and cover securement
- Corrosion resistance
- Effects of icing
- Gasket aging and oil resistance
- Coolant effects
For this reason, it is possible to say that a NEMA Type is equivalent to an IP rating but an IP rating is not equivalent to a NEMA Type. IP designations cannot be converted to enclosure numbers and it is the responsibility of the user to verify the enclosure rating necessary for the given application.
The NEMA Type 4x requires tests for corrosion, icing and gasket aging not included in IEC 60529 and that NO water penetrates during the rain test, whereas the IEC permits the penetration of water if there’s no harmful effects and ‘if it does not deposit on insulation parts or reach live parts’. This necessitates NEMA Type 4x enclosures to be hermetically sealed, i.e. they must be airtight to a level which is considered reasonably or completely airtight. So, if an airtight switch is required, a NEMA rated product is preferable. This requirement is not limited to 4x - see NEMA 250 for more details.
The design of food equipment requires sealed devices to maintain hygienic standards for food safety.
NEMA standards are voluntary. It sets the standards but does no compliance testing. Individual manufacturers have to decide whether or not they will design and build their enclosures to meet the requirements. A manufacturer can choose to self-certify or to seek independent third-party verification (testing). This decision is based on the requirements of the marketplace, e.g. a test agency Listing or Mark may be required. Therefore, since NEMA is not a ruling body like UL, the liabilities of results are assumed by the manufacturer.
NEMA and UL
NEMA and UL (Underwriters Laboratories Inc.) work closely to develop standards. UL 50 is harmonized with NEMA 250. The UL standard uses the same numbers but has additional requirements for construction and marking. Importantly, UL requires enclosures to be tested by qualified evaluators so NEMA is not a substitute. Both standards are widely used in North, Central and South America.
Specific industry requirements
Protection ratings can help to make the right decision when investing in components and systems. Internal system vulnerabilities and unnecessary costs can be avoided by choosing components which are appropriate for the specific operating environment. Some common industrial requirements are described here.
Outdoor elements can affect the reliability of remote control devices and impact operator safety.
Factory and machinery
Applications in these areas are commonly exposed to aggressively polluted environments when in use and in clean-down situations. HMI components and systems are frequently exposed to various types of oils, degreasers, sprays, and hazardous chemicals. These types of fluids and the dust particles they produce can be corrosive and have tendencies to harm both the external and internal components. Therefore, IP65 or above devices are recommended to avoid switch failure and, ultimately, costly machine down-time.
Food and drink processing / medical
Equipment and machinery used within these industries may be subject to intense hygiene procedures and aggressive clean-downs. IP65 is recommended as a minimum and in some case IP67 is more appropriate. Consideration should be given to whether particles could be trapped in crevices within the equipment which may later result in bacterial growth. This can be prevented by an additional rubber seal. These types of applications are ideal for flush-mounting components for ease of cleaning during washdowns. The choice of material and resistance to saltwater corrosion is important in marine applications.
Public transportation (external)
Rail vehicles are subject to very strict standards for safety and durability. Externally mounted HMI components and systems are frequently required to meet the IP69K standard (see previous sections). Transport vehicles will be typically cleaned down with high pressure water jets from all angles therefore IP67 switches are recommended at a minimum.
Lifting and moving
Building, maintenance and agricultural machinery will be subject to aggressively polluted environments when in use and in clean-down situations. Typical applications require IP67 or even higher for external use, although IP65 might be suitable for interior applications such as cab controls. HMI components with stainless steel lenses and front bezels may be appropriate where there is high risk of damage.
Aggressive environments can subject operator controls to oils, sprays, fine dust and harmful cleaning agents.
Most marine applications demand a minimum IP67 for external HMI components due to the risk of immersion. Consideration of the choice of material and its resistance to saltwater corrosion is very important; an additional rubber seal might be desirable.
Due to the risk of explosion, components must be specified for that use and must be intrinsically safe. They should be tested as explosion proof, which is not classified within IP ratings.
Overcoming problems of sealing HMI components
Using inappropriately rated switches poses a danger to machines and their operators.
Liquids can act as a conductor of electricity, as can fine dust particles. Short-circuiting is a common problem when ingress penetrates the seal and travels to the PCBs. Dust and liquids in the switching element can affect how the contacts behave.
Solids larger than dust can damage a switch when it gets into the contacts, chiefly by preventing the switch from opening and closing. Liquids can corrode both the contacts and the wiring. Abrasion and corrosion to the contact material can lead to excessive arcing and welding. Arcing is a discharge of electricity—a spark—that can occur when contacts make or break. Welding occurs when contact material melts and fuses, causing contacts to stick. Both of these will reduce a switch’s service life. Intermittent faults may develop as these particles move around inside the enclosure. Particularly difficult and costly to diagnose, they result in increased equipment downtime or even the complete disruption of a service. For instance, a switch that fails on a passenger train can take the entire vehicle out of service until the fault is diagnosed and fixed. Good product design and the correct materials can overcome such problems.
Today’s advanced switches, keypads and other HMI components are precisely crafted from high-grade plastics, metals, and carefully calibrated springs. They are engineered like fine watches to achieve reliable, long service lives and to minimize the need for replacement. High-quality switches are expected to have a mechanical life of between 1 to 10 million operations.
Environmental protection is extremely important to maintain such a long service life. Manufacturers guarantee this by combining precision design with the highest grade materials. The lens, front ring, actuator housing, seal, pressure ring and mounting flange are critical to the actuator’s IP rating. All elements must be precisely engineered so they lock together tightly around the panel and prevent ingress from the front. Unless otherwise stated, switches are normally only tested for protection from the front; switch elements are usually unrated. Devices that possess an integrated switch element are environmentally sealed at the front and the rear - IP67 and IP65 respectively.
Additional protection of switch elements is also possible by using accessories. For example, an EAO series 84 can achieve IP67 at the rear when fitted with rubber plugs. Where front and rear sealing is important, always check with the manufacturer. Although switch elements are not sealed, self-cleaning contacts can help them maintain optimal performance if they are ever penetrated by dirt. Self-cleaning contacts slide against each other when making or breaking a circuit, removing contamination to keep contact resistance low. Switches for higher currents use gold-plated silver or all silver contacts that resist the effects of arcing. Minor periodic arcing with silver contacts is actually beneficial because it keeps contacts free from dirt and corrosion.
Most appropriate materials
To address the ergonomic risks associated with environmentally harsh applications it is wise to consider stainless steel and durable polymer-based products that operate reliably under demanding conditions. Where required, control products fabricated from these materials offer protection up to IP67 as well as resistance to heat, shock, vibration, and vandalism.
Correct technical data
The IP ratings system is detailed, but on closer examination there are some subjective values, like low pressure and high pressure, that might require further clarification from the manufacturer. For example, EAO publishes the rate of water flow for IPx5 as 12.5 liters per minute and IPx6 as 100 liters per minute without any harmful ingress.
Published data should be a minimum standard based on thorough testing in environ mental test chambers. Further information should be freely available where required, as is the case for IP68 where the duration and depth of immersion might be critical to the application’s use.
Unless otherwise stated, the ratings should be published for the standard product without any modification or use of additional seals and covers.
Understanding how the NEMA and IP Ratings system work is critical to avoiding danger to people and internal system vulnerabilities. A system is only as strong as its weakest link.
Selection of the correct component for the application is crucial to avoid failure and costly downtime. Protection ratings are very helpful for making cost-effective decisions when investing in components - why specify for an HMI device that will resist high-pressure jet washing if it will never be used in dirty environments? It should be noted that neither system is intended to be a product standard.
Product standards may encompass other environmental tests for heat, vibration, etc.
Individual HMI devices will be subject to the end product standard.