UL 508 A: Industrial Control Panels
Standard for Safety, Industrial Control Panels
UL 508A is the technical standard for the Industrial Control Panels in USA. Starting from the basics of electrical safety, stated by the NEC and looking at the prescriptions stated in NFPA 79, it gives indications for the design of industrial control panels.
While the NFPA 79 provides prescriptions for industrial machinery control panels, UL508A has a bruader scope and it is focused on the criteria in order to design the control panel.
The lastest release of this standard was in April 2018: UL 508A Ed. 3 has been designated as an American National Standard (ANSI). This designation indicates that the standard has been developed under a fair and open process designed to protect the rights and interests of every participant.
In August 2020 an updated version was released with a few changes summarised hereafter.
- Reference to Table 28.1 and/or 38.1; 41.1, 42.3.1: in Section 41.1 and 42.3.1 references to Table 28.1 and Table 38.1 have been adjusted.
- Emergency-Stop in Standard Panels: even though some panels were not Control Panels for Industrial Machinery, but they were panels of a different type, they were supposed to follow the rules stated in the section for Control Panel for Industrial Machinery just because an E-stop was provided. This point has been very confusing. Therefore requirement f) has been added:
f) A emergency stop device, consisting of an actuator (providing mechanically held latching means) and contact block (s), and an emergency stop unit (that receives inputs from multiple emergency stop devices) shall comply with the requirements in the Standard for Low-voltage Switchgear and Controlgear – Part 5-5: Control Circuit Devices and Switching Elements – Electrical Emergency Stop Device with Mechanical Latching Function, UL 60947-5-5.
- Update requirements for 1000 V: the scope of UL 508A permits industrial control panels rated up to 1000 V; however, no other requirements in UL 508A such as electrical spacings, overcurrent protection, etc., have been revised and updated to address circuit rated up to 1000 V. Therefore 10.2, Table 10.2, 31.4.1 (a) , 50.1, 66.6.1, 75.6.2, 78.1, 98.3.1, Table SB4.1 have been updated to cover requirements needed for 1000 V. Furthermore new Table 10.1 A "Minimum required spacings in branch circuits rated 601 - 1000 V" has been added.
- Revise spacing requirements applicable to the receptacles, attachment plugs, and inlets: some UL Listed plug and receptacles type products, inlets and attachmnet plugs evaluated to UL498 and cable assemblies evaluated to UL 2237 and UL 2238 are evaluated to spacing requirements smaller than the ones alllowed by Tables 10.1 and 10.2 for industrial control panels. This leads to a problem meeting the UL508 A spacings between live parts and grounded metal such as when they mounted through the panel enclosure as these products are not designed to meet the larger spacings of UL508 A. Therefore revisions to 28.2.5 have been included and new requirement 10.9 has been added to permit the spacing requirements of UL 498, UL 2237 and UL 2238 to be used for all connector products when they are installed in industrial control panels:
10.9 The spacings for connectors, receptacles, attachment plugs, inlets and cable assemblies described in 28.2.5, 28,6.1, 28.6.6 and 28.7.1 provided in a feeder circuit, branch circuit or control circuit and rated 600 V or less shall be a minimum 3/64 inch (1.2 mm) for a device rated 250 V or less, and a minimum 1/8 inch (3.2 mm) for a device rated more than 250 V, between the following:
a) Uninsulated live parts of opposite polarity;
b) An uninsulated live part and a dead-metal part that is likely to be grounded or exposed to contact by persons when the device is installed as intended, including a metal surface on which the device is mounted in the intended manner or a metal face plate used with a flush receptacle.
- Revision to 30.1.8 for fused disconnect switches: the current requirement in 30.1.8 for disconnect switches rated more than 600 A refers to 30.1.4, which is a requirement for manual motor controllers, instead of referring to UL 98 which is included in 30.1.3. Therefore 30.1.8 has been revised to refer to 30.1.3 instead of 30.1.4.
- Revised definition for low-voltage limited energy circuit: the requirements in 43.1.2 and Table 43.1 were recently revised to increase the maximum DC voltage to 60 V DC. However, the voltages included in the definiton of low-voltage limited energy circuit in 2.33 were not changed. Therefore the voltages referenced in 2.33 have been revised to be consistent with the requirements in Section 43 of UL508 A.
UL508A covers Industrial Control Panels, while for Switchboards (in Europe we would also call them Distribution Panels) UL891 is the reference standard.
Every component, in order to be used in an Industrial Control Panel, must be approved by UL ("listed" or "recognised"). However, this is not enough to guarantee that the Industrial Control Panel is compliant to the UL508A.
The sizing of motor startings and of the main protection have different rules from the ones listed in EN 60204-1. The main problems that can be encountered while approving Industrial Control Panels in USA are:
• Circuit breakers not compliant to the UL standards or installed in the wrong way. In addition to sizing, their use should be considered very carefully because, depending on their certification (UL1077, UL489 and so on), they shall have definite applications in a control panel.
• Non-compliances regarding the spacing between cables or devices.
• Absence of the control panel plate or plate with incomplete data.