by Horst Messerer, Product Manager, Data, Network & Bus Technology, HELUKABEL
Just because it has a UL mark does not necessarily guarantee acceptance; after all, approval can be a tricky business. Before choosing a cable for a device or machine design, it pays to know the difference between these approval types.
The North American market is an important one for selling internationally produced machinery and plant engineering. Underwriters Laboratories (UL) approval is just as important for manufacturers because it facilitates entry into the US and Canadian markets. UL approval can apply either to an entire machine or to all the individual components used, although it’s generally the latter that facilitates acceptance. However, there are major differences between the individual approval types and the way in which they are handled by inspectors, which might cause potential problems during the acceptance process.
There are two types of UL approvals – UL Listed and UL Recognized – and each has its own unique differences. Inspectors are generally more familiar with UL Listed requirements, which means that they more readily accept this marking. Inspectors tend to have the standard memorized since they encounter it repeatedly, and can therefore make faster decisions during the acceptance process. For machinery and plant engineers, this saves a great deal of time and reduces costs because test and acceptance costs are much lower.
The UL Recognized mark, on the other hand, conceals hundreds of Appliance Wiring Material (AWM) Styles, which can be found on the UL identity card that identifies a manufacturer’s existing AWM Styles. Approval is granted after a product for a machine or wired device is submitted to UL with information about the application (such as voltage level, flame retardant properties, temperature range, and so on). UL personnel then test the product’s mechanical properties and issue an existing AWM Style if the required parameters are met. If this is not the case, a new AWM Style is issued.
However, acceptance can be a daunting task. Due to the vast number of AWM Styles, the inspector cannot say off hand which category the product belongs. Additional research is required to determine the correct category and with 50 to 80 different cables for each machine, this can take a long time to verify. Furthermore, during the research process, the inspector might come across a criterion that prevents acceptance. In this case, the customer would have to adjust its equipment before applying for approval again. Obviously this costs valuable time and money.
Generally, if any problems occur it’s not because the product is not approved, but rather because it has the wrong type of approval. The following example explains the complexity of the issue.
A machine engineer sets up several machines in a factory. The machines are wired together with a polyurethane (PUR) jacketed track cable that has a UL Recognized AWM Style or UL Listed CMX approval.
While PUR is an excellent material for permanently moving cables that require a long service life, it does not have a high level of flame retardance due to its chemical composition. So while this cable may be suitable for wiring inside the machine, it is not suitable for connecting individual machines.
Then there’s the following problem: the cables are laid along the floor in open or closed cable trays. Cables installed in this way fall under the generic cabling classification (CM/CMG), which requires a bundled fire test. When cables are installed in trays, UL requires them to have high flame retardant properties. Therefore, the fire marshal conducting the tests will accept the wiring inside the machine, but not the wiring between the machines. As a result, the already installed cables must be completely or partially removed and replaced with versions that have a PVC jacket and UL Listed CMG approval.
Customers often have no idea which requirements their applications must fulfil or what the different installation options offered by North American suppliers actually look like. Since the issue is complex, it is recommended to use UL Listed cables as much as possible.
When in doubt, choose UL Listed
Purchasers or technicians should always opt for UL Listed first because it’s easier to check the standard of the cable, and acceptance takes place more quickly. When a product complies with a UL Listed standard, the inspector will be on the customer’s side as they are spared the need to do any AWM research. Only when you cannot find the appropriate standard should you look into AWM Styles.
AWM Styles are perfectly legitimate since they are tailored to the application, but not all applications can be squeezed to fit the requirements of a UL Listed standard. Case in point: a woodworking machine will have different wiring requirements compared to a metalworking machine or an automated machine.
The following example illustrates the best approach to take when choosing the right cable. A track cable needs to be UL approved. For this, two standard jacket compounds are available to choose from — PUR and PVC. PVC jackets can be CM or CMG listed. PUR jackets can be AWM Style or CMX Listed. However, the latter version only satisfies the individual cable fire test and therefore does not bring any advantages with regard to fire resistance. While the inspector will immediately be aware of the relatively weak fire test of the CMX standard, he will have to look this up for the AWM Style. If the CMG track cable with PVC jacket is used, this cable will not last as long in the track as the PUR cable. However, the cable can also be installed in a cable tray and so the inspector will accept this.
Europe focuses on the materials used in the cable, while the USA concentrates on flame retardant properties.
Unfortunately, North American and European approvals do not always align, since many elements have become involved over time and there are different ways of looking at the situation. With European approvals, great attention is paid primarily to the materials used, such as the number of high-risk plasticizers or lead-free products. Cables installed in buildings must be halogen-free or have low smoke density or low toxicity characteristics.
However, flame retardant properties are the priority for UL certification. Unfortunately, flame retardance is not Europe’s first priority when smoke, combustible or toxic gases are produced in a fire. While regulating agencies may disagree on the right way to go about it, the fact is that in North America fires also break out when extremely flame retardant cables are used due to all the hazardous materials they contain, and fires are no less avoided when European solutions are used.
UL Listed data cables
Data cables are described in UL Standard 444. Depending on the application and flame retardant requirements, the individual chapters list the CMP, CMR, CM, CMG and CMX standard allocations according to the respective data cable criteria.
CM or CMG UL Listed (bundled fire test) as well as AWM Style UL Recognized (individual vertical or horizontal fire test) versions apply to PVC-jacketed cables. The same applies to Flame Retardant Non-Corrosive (FRNC) versions. CMX UL Listed or UL Recognized versions are applicable to PUR jackets; however, both types only exist with individual vertical or horizontal fire tests.
Cables with polyethylene (PE) jackets for outdoor or underground installation have no flame retardant properties and are therefore not UL certified. In terms of flame retardant properties, PUR can never satisfy a bundled fire test, however this material has good chemical and mechanical resistance. In cable track applications, this cable has a significantly longer service life than PVC-based materials.