Standard errors

Posted on 7 Jan 2014 by The Manufacturer

Staying the right side of the UL standard in the US and North America. Tony Hague, managing director of PP Electrical Systems, explains why compliance confusion can be costly for machinery manufacturers.

Tony Hague, Managing Director, Power Panels Electrical Systems

As a manufacturer of machinery, how confident are you that you are meeting the legal and technical requirements of the US and North American markets?

Confusion in respect to UL508A and NFPA standards can be extremely costly.

As a specialist in electrical control systems and automation – coupled with 15 years’ experience of designing and building systems to meet (Underwriters Laboratory) UL508A standards – at PP Electrical Systems we are amazed at the level of confusion that still shrouds what does and does not constitute a UL certified product.

UL 508A certification is an industrial control panel directive, which is sought by electrical inspectors.

The UL 508A Listing Mark on an industrial control panel provides evidence of third party certification to the municipal inspection authority and to the purchaser of the panel and, in essence, it shows that the panel complies with an acceptable safety standard.

This applies to any company that is supplying control panels or building machinery for export to the United States and North America. This could be firms involved in food processing or packaging equipment, machine tool manufacturers, pharmaceutical, scientific or semiconductor plant. It can carry a far reaching remit.

Some benefits of incorporating an electrical control panel carrying the UL 508A certification include:

  • The UL 508A certification provides the inspection authority and your customer evidence that the control panel complies with nationally recognised safety standards, which ensure public safety and compliance with national and local electrical codes
  • Manufacturers that carry the UL 508A certification are subject to periodic unannounced inspections of their facilities by UL personnel. Through periodic audits, UL makes sure the manufacturer continues to meet the UL requirements for 508A certification
  • The UL Mark on a component means that UL has evaluated and tested samples of this component and has concluded that they meet the necessary requirements, thus protecting the quality and integrity of the control panel.

Vendor beware

We have customers, past and present, who believe that by simply selecting components that are UL listed and integrating them into a control panel that also carried a UL (NEMA) rating, that was the start and end of the exercise.

In fact, a number of machinery manufacturers have adopted this approach and depending on which US states they have been exporting to, may have managed to escape any serious issues.

But that is until they come across a more stringent inspection, whether state specific or just a more detailed and thorough site inspector.

Then the problems then can be huge.

There will be re-working, supplying new components and even the time and financial cost of dispatching engineers on a plane to go and sort out the problem as quickly as they can.

And this isn’t even taking into consideration consequential penalties from the customer due to late installation of machinery and the impact this can have on future business relationships.

All of these issues can be easily avoided by taking a more proactive approach to the subject and working with companies, like ourselves, that can offer a complete UL solution in line with their specific needs.

Prior to manufacturing a UL-approved electrical panel, we will first undertake crucial checks of customer’s submitted designs. Some examples of checks and considerations and the common issues of non-compliance are described below.

It is very important that the design is checked for compliance with the UL 508A standard prior to manufacturing the panel, in order to ensure UL compliance and thus enable us to apply a UL mark to the electrical panel.

Other design considerations include:

  • Specification and selection of UL approved (UL ‘recognised’ or ‘Listed’, as appropriate) cables, components, sub-assemblies and modules with appropriate electrical safety approval ratings
  • Enclosure UL type rating (generally in accordance with the US NEMA (National Electrical Manufacturers Association) type rating numbers. These have some similarity to the European IP (ingress protection) ratings
  • BCP (Branch circuit protection). Appropriate BCP will be built into the panel or it may be provided externally, with suitable panel instructions and end-customer agreement. BCP is implemented as UL 489 Listed circuit breakers or UL 248 series fuses such as Class CC or J type fuses, with appropriate short circuit current ratings
  • Electrical ‘creepage’ and ‘clearance’ checks – to ensure adequate ‘over-the-surface’ and ‘through air’ distances are provided to assure electrical safety barriers are implemented
  • Direct support of live parts electrical safety data checking where appropriate, such as RTI (relative thermal index ), CTI (comparative tracking index) and HWI (hot wire ignition). Typically required for busbar supports and printed circuit boards
  • Insulating materials
  • Grounding (earth bonding)
  • Accessibility of live parts

  • Defining the required SCCR (short circuit current rating) withstand

Electrical panels must be designed to achieve a rated minimum short circuit current rating. A 5000 Amp prospective symmetrical fault current rating will be sufficient in many cases. Some applications require a much higher SCCR and these higher fault current withstand ratings can be achieved with the right design.  The SCCR is one of the most important electrical panel design factors as AHJs (US Authorities Having Jurisdiction) can prevent connection and use of an electrical panel if its SCCR rating is not greater than the symmetrical fault current rating of the power supply that it will be connected to.

  • Design voltage rating and end customer site electrical power network configuration

Electrical panels will normally either be connected to a single-phase 115V or three-phase 460/480Vac 60 Hz public power supply network in the USA. Sometimes system voltages of 575V or even 690Vac three phase may be seen.  Power network connections may be star or delta configuration and the point of connection could for example be a ‘grounded delta’ supply. Further design considerations here will include adequacy of proposed transient voltage surge suppressor modules or components.

In the case of a grounded delta network configuration for example, prospective transient impulse voltages of 6 kV peak can appear between incoming phases and the panel’s earth connection (perhaps for example due to a nearby lightning storm or switching on and off of large electrical loads on the local power network). The electrical panel rated impulse voltage withstand rating requirement will be agreed and the design checked to ensure it complies. Note that standard UL compliant electrical panels will be rated up to 600Vac maximum (normally 115Vac, 230Vac or 480Vac). Connection to a 690Vac power network requires special assessment and design. AHJ’s can prevent connection of electrical panels

PP Electrical Systems employees are well versed in UL508A and NFPA standards for the US market.

that do not have suitable impulse voltage withstand rating.

  • Wire bending space

Electrical panels must be designed to provide the necessary minimum wire bending space in accordance with UL 508A requirements. The bending space is the distance between a field wiring termination point inside the panel and the directly opposite metal wall of the panel. The distance required depends on the ‘ampacity’ (current rating) of the field wiring cables. AHJ’s  (US Authorities Having Jurisdiction) will prevent connection of electrical panels that do not have sufficient wire bending space. Clearly, the cable size calculation is important and it can affect the dimensions of the panel in some cases.

  • US NFPA 70 NEC (National Electrical Code)  and NFPA 79 code compliance

The UL 508A standard incorporates many requirements that are derived from the US NFPA (US National Fire Prevention Association) standard no. 70 – the ‘NEC’ (US National Electrical code). An example is cable ‘ampacities’ (current ratings). Whilst checking an electrical panel design to the many clauses of the UL 508A standard, this does not guarantee that a design will be in accordance with the relevant NEC articles and many of these are application-specific.

This is yet another example of where electrical panels can be prevented from connection to an electrical supply by the authorities, if they are found to be non-compliant with the US NFPA 70 NEC or NFPA 79 machine safety codes.

As you can start to see, when considering all of the implications associated with UL508A and the potential significance of getting it wrong, or indeed simply making the wrong assumptions, we would always recommend that you seek the specialist advice of a professional organisation which has practical experience in this field.

PP will be running a number of UL seminars at its factory in the West Midlands during the course of 2014. To find out more, please contact us on 01922 419109, email [email protected],  visit or follow @pp_electrical on twitter.