May, 2020 Continued from previous page
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 rat-
ings listed, but the reverse cannot be assumed. The IP code only addresses requirements for the protec- tion of people, the ingress of solids and the ingress of water. 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. 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 market- place, 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 (Underwriters
Laboratories, Inc.) work closely to develop standards. UL 50 is harmo- nized with NEMA 250. The UL stan- dard uses the same numbers but has additional requirements for construc- tion and marking. Importantly, UL requires enclosures to be tested by qualified evaluators so NEMA is not a substitute. Both standards are widely used in the Americas.
Specific Industry Requirements Protection ratings can help engi-
neers make the right decision when investing in components and sys- tems. Internal system vulnerabilities and unnecessary costs can be avoided by choosing components which are appropriate for the specific operating environment. Applications in factory and
machinery are commonly exposed to aggressively polluted environments when in use and in clean-down situa- tions. 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 downtime. Equipment and machinery used
in the food/drink processing and med- ical industries may be subject to intense hygiene procedures and aggressive clean-downs. IP65 is rec- ommended as a minimum and in some cases IP67 is more appropriate. Considerations 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.
Overcoming Sealing Problems 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 pene- trates the seal and travels to PCBs. Dust and liquids in the switching ele-
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Page 65 Environmental Sealing on Electromechanical Devices
ment can affect how the contacts behave. Solids larger than dust can damage a switch when it gets into the contacts, chiefly by prevent-
ing 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
Switches are IP-rated to prove their resistance to dirt and moisture ingress.
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 equip- ment downtime or even the complete disruption of a service. For instance, a switch that fails on a machine can impact production until the fault is diagnosed and fixed. Good product design and the correct materials can overcome such problems. Today’s advanced switches, keypads and
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