UL Standards: A different conceptual approach altogether

Ian Harwood, EU Regulatory Affairs at Rockwell Automation on North American standards for industrial control panels

Exporting and designing the simplest control cabinet for the United States or Canada can be a challenge. This does not mean that you, or European companies in general, have found it impossible to export to the United States or Canada, but you will probably agree that the process is seldom simple and there are many reasons for this.

The different rules established over the years in different industries or regions have led to radically different approaches. Definitions such as “Branch Circuit” and “Feeder”, or “Supplementary Protection” and “Limited Energy Circuit”, which are cornerstones of the North American system, have no equivalent in the design of industrial installations according to European regulations. The reason for this is not a simple problem of different terminology, but a different conceptual approach.

Even after studying the subject thoroughly, it would be difficult to embrace a logic so different from our own except through basic training that would make it possible for us to understand the origins of these differences.

During this UL awareness and education campaign, one core objective is to bring together two cultures — European and North American — which, despite having developed in radically different ways over the years, share a common goal: a mutual design code capable of establishing rules for operator safety.

When we look into regulations governing industrial equipment and components, there is a particular focus on the needs of panel builders and equipment manufacturers both seeking to understand the effect of UL on selecting the right components.

Below is a breakdown of considerations for typical components:

Relevant UL Standards

The main areas for compliance to standards in the USA are:

• UL508A – Covering North American Standards for Industrial Control Panels, Electrical Installation and Electrical Machinery
• NFPA70 – Is the National Electrical Code for electrical installation in the USA
• NFPA79 – Is the electrical standard for Industrial Machinery in the USA

Power circuits

A clear understanding of the two circuit sections that make up the power circuit is required in order to select and size wires, spacing and components according the UL508A standards:

• Branch Circuits – Includes conductors and components following the last overcurrent protective device protecting a load
• Feeder Circuits – Includes conductors and components on the supply side of the branch circuit overcurrent protective device

Control Circuits

UL508A necessitates a clear understanding of the Control Circuits which carry electric signals directing the performance of a controller in order to define the required conductors and components. UL508A additionally defines these Control Circuits of different Classes as:

• Class 1
  • A Control Circuit on the load side of overcurrent protective device where the voltage does not exceed 600 volts and where the power available is not limited

Or  

• A Control Circuit on the load side of power limiting supply, such as a transformer

• Class 2
  • A Control Circuit supplied from a source having limited voltage (30 Vrms or less) and current capacity, such as from the secondary of a Class 2 transformer, rated for use with Class 2 remote-control or signaling circuits

Stay in the loop

Come back next week for a guest blog by Lutze detailing the important cabling considerations in relation to UL. The series will be completed the following week with Product Approvals delivering a video from home and blog detailing UL inspection and evaluation.

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Author

Stefano Muraro, European Product Manager, Rockwell Automation

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