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Understanding UL - are you using compliant products

An apparent lack of understanding about the use of UL standards could lead to the loss of major export orders for machine builders warns Ian Gibson, technical director of Flexicon.

Underwriters laboratories (UL) is a major force in the world of standards and you will need to meet its standards if you want to export to an increasing number of large markets, not least the USA.  Any equipment or component that might be incorporated into machines need to meet its standards.

UL operates a very transparent and open system for product approval.  Anyone can log onto www.ul.com and search under “certifications” to find the approvals held by a manufacturer for use in both the USA and Canada. Companies need to beware as anyone can report you if they suspect that equipment or components within equipment do not comply and UL inspectors will then check it.  UL takes policing of its standards very seriously.   

This reporting helps to ensure that the standards are maintained.  It also means that manufacturers need to double check their specification to make sure that they have got it right; and it is easy to inadvertently get it wrong.

The ethos of UL is to work right down the supply chain, so that each component and even the raw material used for the manufacture of that component complies.  You may think that this makes it easier for equipment manufacturers, but sadly this detail and complexity means that mistakes do occur – and this could cost your business dear in lost orders.

Setting the Standard

The first thing to do is to clearly understand how the standard works.  There are two types of UL approvals; ‘UL Listed’ is for any finished product that will be used in its own right and ‘UL Recognised’ applies to a component or a raw material that is used in a piece of equipment that is itself UL Listed.

As with any methodology for applying standards, there are pitfalls that you need to be aware of.

This can best be illustrated by looking at the specification of flexible conduit to protect critical power and data.

Here there is a completely different approach to standards between UL and the European system.  While the European system concentrates on testing a complete conduit system, the UL approach is to test the individual components. 

With UL there are different standards for different types of flexible conduit systems, so the first thing that you need to check is the type of flexible conduit used and then apply the relevant standard.


For a liquid tight, flexible steel PVC sheathed conduit you need to check that it meets UL360.  This standard prescribes inside and outside diameters and focuses on dimensional compatibility.

Because this standard specifies the conduit dimensions, with tolerances, manufacturers can supply just conduit, which can then be used with another manufacturer’s fittings.  This only works because the fittings specified in another standard, UL514B for conduit fittings, are of a compression type that can cope with a wide tolerance.

This can cause problems because the performance of the system is often dependent on the fitting to conduit joint, particularly for the IP rating, pull off strength and the electrical continuity.  The minimum water ingress protection test in UL514B is only similar to IPX4, so the resulting system may not even be liquid tight!  In contrast Flexicon’s liquid tight conduit systems, incorporating both conduit and the fitting, achieve IP66, 67, 68 and 69.

Also if the conduit and fitting are from different suppliers, who do you blame if the cable protection system does not work?

Another issue to be aware of is that UL360 requires a copper bonding wire in the wall of conduit up to 40mm up to a certain length, since the American wiring regulations allow flexible conduit to be used as the earth conductor.  This is in complete contrast to the European standards, which actually prohibit the use of flexible conduit as the earth conductor.


For liquid-tight non-metallic flexible conduit there are similar issues, but the UL1660 standard can be even more confusing.

This standard covers three types of conduit.  Corrugated conduit (known as type C) does not normally use compression fittings, so a supplier will provide both the conduit and the fittings as a complete system.

Integral conduit (known as type B) has a smooth inner and smooth outer tube with internal spiral reinforcement and normally needs metallic or non-metallic liquid tight compression fittings that meet UL514B.  Just like metallic liquid tight systems, different suppliers can provide the conduit and fitting leading to exactly the same issues as already outlined.

Type A flexible conduit in UL1660 is very rarely used, but covers layered non-metallic, braided reinforced conduit for particular applications.


The newest UL standard, UL1696, is actually a system standard that covers both conduits and fittings.  It typically covers corrugated polyamide and spirally reinforced PVC with a corrugated outer flexible conduit.

The UL standards have a laudable aim in that they seek to ensure a minimum quality right down the supply chain.  So for example UL Listed and UL Recognised conduits and fittings are generally manufactured from UL Recognised polymer materials.

Unfortunately adopting this approach is not without it problems.  The onus is on the specifier to check that not only do the components of the system comply to UL, but that the system itself actually meets certain performance criteria.

In the case of liquid tight conduit systems this means that the best approach is to insist on UL approved conduit and fittings that have come from the same manufacturer and have been tested as a system.

As an OEM complying with UL, you are ultimately responsible all the way down the supply chain, so a supplier’s mistake will have a serious impact on your business.  Check the specification of components with care and then double-check them.

And mistakes do occur more frequently than you realise.  Taking flexible conduit as an example, we have come across cases where the flexible conduit supplier is using the wrong standard or an out of date approval.  Another common error is for a supplier to claim blanket approval across an entire product range, whereas UL requires that every sized variant is individually recognised or listed.

Get it wrong and the chances are that UL will find out as they take policing of its standard very seriously.  Anyone can report that equipment or components do not comply and UL inspectors will then check conformity.  This reporting helps ensure that the standards are maintained, but it also places more pressure on OEMs to get it right.

Contact us now for further information on our UL products or to discuss your requirements & application in further detail.



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