Surge protection for industrial control cabinets in accordance with UL508A

Surge protection for use in industrial systems, industrial machinery and control cabinets has been mandatory in the USA since 2018!

Regulations for electrical installations in the USA

The National Electrical Code (NEC) defines the basic rules for electrical installations in the USA. As a safety standard, it describes, among other things, the design of the installation as well as the selection and dimensioning of the relevant devices and materials.

Good to know: The NEC has a similar meaning to the VDE in Germany. The most important difference, however, is that the NEC - unlike the VDE standards - is law and must therefore be complied with. Article 670 of the NEC requirements for industrial plants refers to NFPA 79, Electrical Standard for Industrial Machinery. This reference means that industrial plants that are to be operated in the USA must comply with NFPA 79 as well as the NEC.

NFPA 79 describes all requirements for an industrial installation with regard to its electrical properties. In the area of system protection, the current 2018 version includes the requirements for surge protection. Industrial systems that contain safety circuits must be protected by surge protection. This is intended to protect the functional safety components in the machine from failure.

NEC 2023 Article 409 - Control cabinets for industrial systems

NEC/NFPA 70 Article 409 focuses on industrial control panels and refers to the UL 508A standard of the same name, which must be complied with. It is a safety standard and relates to all aspects of industrial control panels. A UL508A supplement document entitled “REQUIREMENTS FOR COMPONENTS USED IN INDUSTRIAL CONTROL PANELS” helps control panel builders to determine the requirements for components used in industrial control panels. The document replaces Supplement SA and Annex C in the second edition of UL508A and subsequently Annex C in the third edition of UL508A.

Selection of SPD for UL508A - Industrial enclosures

NEC/NFPA 70 requires that surge protective devices (SPDs) be UL1449 listed, making the selection of an appropriate surge protector easier than ever. There are only a few SPDs that are listed for one of the most commonly used surge standards UL1449 5th Edition (AC, DC, DC PV Power), UL497B (Data Signals) and UL497E (RF).

Required properties of surge protection devices

NFPA 79 places two central requirements on surge protective devices (SPDs):

They must be so-called Listed Devices
and the SPD type must correspond to the installation location.

Listed devices are SPDs that meet high safety and failure behavior requirements in accordance with the American product standard for SPDs, UL 1449. They can be installed without any further effort. Recognized devices, on the other hand, are only classified as safe if certain conditions are met during installation. Compliance with these conditions must be verified by the person who plans or installs the SPDs by means of tests and/or calculations. This can involve considerable effort, as certain tests can only be carried out in specialized laboratories.

What is the difference between “UL Listed” and “UL Recognized”?

The main difference between UL Listed and UL Recognized products is that ...

UL Listed products have undergone extensive testing and certification by UL.
while UL Recognized products are only tested as components or materials.

As the “UL Listed” status is associated with more restrictions and is more difficult to obtain than the “UL Recognized” status, many products are not subjected to this expensive test.

Approvals based on the UL SPD type

SPDs are divided into five types according to UL 1449. As in IEC standards, they are numbered consecutively from 1 to 5. However, the meaning of the types according to UL is completely different from that according to IEC: According to IEC, the type indicates the position of the SPD according to the lightning protection zone concept, i.e. its “performance” under normal conditions. According to UL, the type only indicates the maximum short-circuit current above which the SPD can still be safely disconnected in the event of a fault, for example by a fuse. It does not allow any conclusions to be drawn about the “performance” of the SPD. UL Listed Type 1 SPDs may be installed before the main fuse - in the so-called Type 1 location. A UL Type 2 SPD may only be installed downstream of the main fuse (Type 2 Location), and a UL Type 3 SPD must be installed at least 10 m away from the upstream overcurrent protection (Type 3 Location). UL types 4 and 5 are not relevant in this context.

UL- Sign	UL1449 SPD Type	Appropriate Ratings	Described procedure	File update or additional tests	Notes
	1 2	Yes	No	No	The only requirement is that the SPD must meet the electrical and environmental ratings of the UL 508A enclosure. A UL on-site inspection of the switch cabinet with installed SPD is required as part of a standard inspection.
	1CA 2CA 4CA 5	Yes	Yes	Yes	All UL recognized SPDs require a technical evaluation of the existing UL1449-E file of the SPD by UL. At a minimum, this will result in costs for updating the UL file and may also require further testing, which in turn would result in additional costs.

Selection Criteria for Surge Protection Devices (SPDs)

Industrial installations are usually installed inside buildings, downstream of the main circuit breaker, sometimes at a certain distance from it. This corresponds to Type 2 or even Type 3 locations. NFPA 79 takes into account the previously mentioned hierarchy of SPDs, allowing UL Type 2 SPDs to be installed in Type 3 locations, while UL Type 1 SPDs can be installed in any location. The only condition is that the SCCR of the SPD must be higher than the prospective short-circuit current at the point of installation or the machine it is connected to.

Once the UL type has been determined, two rules apply for selecting SPDs:

The service configuration of the power supply network,
The system voltage.

Surge Protection Made Easy

An SPD for an industrial machine or system intended for operation in North America must be:

Suitable for the American power supply network,
A UL Listed device,
The correct type for the installation location.

The DS70-US series from Citel reliably meets the requirements for UL Listed certification and the appropriate type and installation locations.

NEC 2023 Article 409 Section	Location	UL Standard	UL SPD Type	Grid and system voltage	Product
409.70 Surge Protection	Control cabinet	UL 1449	Type 1	120Vac 1-Phase 2W+G Single Phase	DS72US-120S/G
				120-240Vac 1-Phase 2W+G Single Phase	DS72US-240T/NN
				120/240Vac 1-Phase 3W+G Split Phase	DS73US-120T/G
				120/208Vac 3-Phase 4W+G Wye	DS74US-120Y/G
				277/480Vac 3-Phase 4W+G Wye	DS74US-277Y/G
				480Vac 3-Phase 3W+G Delta	DS74US-480D

As UL Listed, Open Type 1 devices, they can be used anywhere in an installation when housed in an enclosure. Installing them in the machine's control cabinet meets this requirement. Additionally, the series complies with the stringent safety requirements of applicable standards. Therefore, only the service configuration and system voltage of the application are relevant for selecting the appropriate SPD.

Select, install, connect – and you're done!