In UL 508A, Low-Voltage Limited Energy Circuits are a specific type of low-voltage control circuit that allows circuits to be built using components without UL certification, without requiring a separate UL evaluation of the parts used.
Low-Voltage Limited Energy Circuits Solve a Common Problem
A UL-certified control cabinet must be constructed from components that are also UL-certified, i.e., UL Recognized Components. However, it is sometimes necessary—or at least desirable— to custom-manufacture components for certain circuits in order to meet the user’s requirements. The problem: Normally, in such cases, UL evaluation of the component is required. However, UL 508A makes exceptions for two classes of control circuits in the low-voltage range.
Classes of Control Circuits in the Low-Voltage Range
UL 508A defines control circuits as circuits that control a control element— such as a contactor—but do not power electrical loads such as machines, lighting fixtures, transformers, etc. Accordingly, the following regulations apply only to control circuits.
UL 508A and Low-Voltage Limited Energy Circuits
The National Electrical Code (NEC) distinguishes between three classes of control circuits in the low-voltage range up to 600 volts. UL 508A employs a similar classification, with the circuit classes in UL 508A essentially following those of the NEC. However, there is one significant difference: the Low-Voltage Limited Energy Circuit.
Control Circuits
The Control Circuits class roughly corresponds to Class 1 of the NEC. These circuits and their power supplies are constructed from UL-certified components and are not subject to any power or voltage limitations, as long as the voltage remains within the low-voltage range (i.e., up to 600 V in the U.S.).
Class 2
Class 2 control circuits may only be powered by power sources rated up to 100 VA. However, only UL-certified power sources may be used.
Low-Voltage Limited-Energy Circuit
Low-Voltage Limited Energy Circuits are defined only in UL 508A (and not in the NEC) and constitute a separate class of circuits.
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What are low-voltage limited-energy circuits?
UL 508A imposes the same requirements on low-voltage limited-energy circuits—regarding internal and external wiring, overcurrent protection devices, and so on—as it does on Class 2 circuits. Of course, this also means that components without UL certification may be used in both classes of control circuits. The key difference between the two classes lies in the power supply.
In the case of Low-Voltage Limited Energy Circuits, the power supply may consist of components that are also not UL-listed. This makes them easier to implement in a control panel than a control circuit of either of the other two classes.
What are the advantages of using low-voltage limited-energy circuits in UL-certified control panels?
In principle, it is always possible to use parts and components without UL certification when designing a control cabinet. However, in such cases, a new evaluation of the part by Underwriters Laboratories is typically required—otherwise, the control cabinet may not bear the UL label.
This effort is not required for Class 2 control circuits and low-voltage limited-energy circuits. This is particularly important when a control cabinet manufacturer uses components that have been custom-built to meet specific customer requirements or modified from off-the-shelf parts. Without the special regulation for low-voltage control circuits, the use of such customized solutions would generally not be possible, as testing the components in question would entail an effort that cannot be justified in terms of time and cost.
Since the power supply components in low-voltage limited-energy circuits may also consist of non-certified components, these control circuits represent the simplest method of installing non-UL-certified components in a UL-certified control panel.
What should be kept in mind when using low-voltage limited-energy circuits?
As mentioned, low-voltage limited-energy circuits are subject to the same requirements as those specified by UL 508A for Class 2 control circuits. In addition, it should be noted that the maximum permissible voltage for alternating current must not exceed 30 V; for direct current, a maximum of 60 V is permitted. These maximum values already take voltage spikes into account.
It should also be noted that, while power sources without UL certification may be used, there must be galvanic isolation between them and the control circuit; in other words, the two circuits must be electrically isolated from each other. Surge protection is mandatory for low-voltage limited-energy circuits.
Exceptions: Low-voltage limited-energy circuits may not be used here
Unfortunately, however, the use of low-voltage limited-energy circuits is not permitted in some areas. This applies to safety devices (such as control circuits in light barriers or safety limit switches) as well as the control circuits in all electric motors.
Zusammenfassung
The UL 508A standard makes exceptions for two classes of control circuits regarding the requirement that only UL-certified components may be installed in UL-certified control panels. These are Control Circuits and Low-Voltage Limited Energy Circuits, as defined in UL 508A. Both types of control circuits fall within the low-voltage range (up to 600 volts). For Low-Voltage Limited Energy Circuits, the power supply may additionally consist of components that do not have UL certification, which further simplifies implementation. Low-Voltage Limited Energy Circuits solve a common problem, as they allow the use of components that have been custom-manufactured to meet specific customer requirements or consist of modified off-the-shelf parts. However, both the maximum permissible voltage of 30 V for AC and 60 V for DC, as well as the galvanic isolation between the control circuit and the power source, must be observed.
