UL 508 A: Industrial Control Panels
Standard for Safety, Industrial Control Panels
Last update 27/07/2021
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.
On the 13th July 2021 a revision was published. Hereafter the main changes in requirements
- It is made clear that "Self-protected Combination Motor Controllers " can be used to protect motor circuits only. For all other Branch Circuit Protective Devices (or BCPD), UL 489 circuit breakers and Fuses in compliance with UL 248 shall be used. Section 31.1.4 was modified.
- Since a few years we are discussing whether the Nameplate Data must be on the outside of the panel or it can be on the inside. We finally reached a compromise: the Label (that contains important data like the panel FLC and the Panel SCCR) can be on the inside; however, if the panel is for Industrial Machineries, the label much be on the outside. Therefore, the standard remains alligned with NFPA 79 that requires Nameplate Data "shall be attached to the outside of the enclosure". At the same time, it is alligned with several UL standards that allows the label to be on the inside, like UL 508. Table 52.1 was modified accordingly.
- It was finally clarified NOTE "b" in Table 38.1: Ampacities of control circuit conductors. The note stated that wire sizes of 20 to 26 AWG could be used "only for connection of control circuits for electronic programmable input/output and static control (having no moving parts)". That "and" should have been an "or" in reality. Now the new note clarifies that wires up to AWG 26 can be used "for connection of electronic control input/output and control devices". We are now fully alligned with NFPA 79, articole 126.96.36.199: "Conductors installed within control enclosures shall not be smaller than 26 AWG".
- Air conditioners should be provided with an SCCR. That is the meaning of the statement in 26.1.1 that they "shall comply with the requirements for general construction and power circuits". If they do not have one, they must be installed on an isolated secondary of a control transformer. The control transformer reduces the fault current to a value that will not cause any safety issue with the enclosure air conditioner in case a short circuit occurs. The new languange extends the exception, in section 26.1.1, to Air Conditioners.
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.