UL 508A - 66.1 Enclosure Access

Last edit: 19/07/2023

In order to protect operators and maintenance people from the risk of direct contacts (contacts with energised live parts) the standard gives three ways, to the panel manufacturer, how to design a safe product: we call them "Mode a, b and c" referencing to the 3 paragraphs in the standard 2022 edition.


“Mode a”: A key is necessary to open any of the doors: it is allowed, provided all electrical components mounted  on the door have at least IP 1X protection.

[UL 508A: 2022] 66.1 Enclosures
[…] 66.1.2 Opening of an enclosure (e.g. opening doors, lids, covers, and the like) containing live parts operating at or above 50 volts rms ac or 60 volts dc, shall be possible only under one or more of the following conditions:
a)    The use of a key or tool is necessary for access. AII live parts mounted on the inside of doors or covers that are operating at or above 50 volts rms ac or 60 volts dc shall be protected from unintentional direct contact by the inherent design of components or by the application of barriers or obstacles such that a 50 mm (2 in.) sphere cannot contact any live parts. Cautionary marking shall be provided in accordance with 67.4.3.

“Mode b”: it is not possible to open any of the doors with energy inside the panel. No conditions for the components installed inside the panel.

b)    The disconnecting means supplying the enclosure is interlocked (mechanically, electrically or both) with the enclosure door(s) in accordance with 66.1.3.

“Mode c”: It is possible to open at least one of the doors without the use of a key or a tool: The inside of the panel have to be totally protected from direct contacts.

c)    Opening without the use of a key or a tool and without disconnection of live parts shall be possible only when all live parts that are operating at or above 50 volts rms ac or 60 volts dc are separately enclosed or guarded such that there cannot be any contact with live parts.