U.S. occupational safety laws and UL 508A require that a control cabinet be equipped with an interlock under certain conditions. This refers to a safety device that prevents the doors from being opened while the main switch is on.
Relation to UL
In control cabinets built and certified in accordance with UL 508A, the interlock is used to protect against electric shock from live components that are not protected against accidental contact. The interlock function is typically implemented either by a mechanical interlock linkage or by a magnetic door lock.
What is an interlock?
In English, the term “interlock” generally refers to a safety device designed to prevent an undesirable or even dangerous event from occurring during the operation of a machine. In the context of control cabinet construction, this refers to a mechanical, electrical, or electronic safety device that prevents a cabinet door from being opened as long as the components inside are energized. This safety measure is of great importance in control cabinets intended for use in the United States and Canada, as these devices often do not need to be as touch-proof as their counterparts for the European market. However, even in the U.S., not all control cabinets are required to have an interlock.
Which control cabinets must be equipped with an interlock?
UL 508A stipulates that all control cabinets that control an industrial machine or system and operate at a voltage exceeding 50 volts (AC) or 60 volts (DC) must be equipped with an interlock. The UL standard does not specify exactly how this interlock must be designed. As mentioned, both mechanical and electrical solutions—or combinations of the two—have become established in practice. More recent developments combine mechanical and electronic components.
Exceptions: These control cabinets do not require an interlock
In addition to the voltage limits already mentioned—up to which an interlock is not required—there are other exceptions in which this safety mechanism can be omitted.
In theory, some types of control cabinets, such as terminal boxes or distribution cabinets, can also be designed without doors and could therefore, logically, function without an interlock. However, UL 508A imposes additional requirements on control cabinets that often make such a design impractical: If the control cabinets contain resettable components (fuses, etc.) and non-insulated live parts, access to these fuses must be possible without the use of a tool. To meet these requirements, a design with a door must generally be chosen, which must then also be equipped with an interlock.
In practice, therefore, almost no control cabinet can function without a door. But if an interlock is required for this door, what requirements must it meet?
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What are UL’s requirements for interlocks in control cabinets?
We have already discussed the basic requirement for an interlock: As long as current is flowing, the door must not be able to open. In the simplest case, this is achieved mechanically. In addition, electrical and electronic systems are also used today, sometimes in combination with mechanical components. Particularly with mechanical solutions, it is always important to ensure that the protective function of the control cabinet enclosure is not compromised. This is, of course, especially true when the cabinet is located in a potentially explosive atmosphere. However, UL 508A specifies a whole series of additional requirements for the interlock.
It must be possible to outsmart the interlock
UL 508A stipulates that the interlock of a control cabinet must be able to be “bypassed” (the standard uses the term “defeated”). What does it mean to bypass an interlock? Often, troubleshooting in a control cabinet or adjusting certain components is only possible while the system is in operation. For this purpose, it must be possible to open the control cabinet doors without first having to shut off the power supply.
UL recognizes the importance of this capability and therefore mandates that the interlock on a control cabinet must be bypassable in accordance with UL 508A. To this end, solutions are often used in which the interlock can be deactivated using a tool. This is often a standard screwdriver, but there are also solutions involving special tools designed to prevent unauthorized operation.
Turn on with the doors open
In addition to locking the cabinet doors when the main switch is on, it must also be ensured that the power supply, once turned off, cannot be turned back on while the door is open —in other words, the main switch must be locked. There is, however, an exception to this rule as well, to allow for adjustment and maintenance. Qualified personnel must be able to restore power even when the door is open. However, UL 508A requires that this involve a“deliberate action”; this could involve, for example, pressing in or pulling out the switch so that it can be turned.
Snap Lock
The “snap-lock” function refers to the interlock automatically engaging as soon as the control cabinet door is closed. This must occur even if the interlock mechanism has previously been bypassed. The defeat mechanism must not be permanently enabled; rather, it must be reactivated each time the doors are opened.
Live electrical circuits
If a control panel is shut down in the standard manner using the main switch, some absolutely essential circuits may still remain energized. If this is the case and the circuits cannot be covered in a way that prevents contact, they must be clearly marked, and a warning notice must be posted inside the control cabinet.
Lockable Main Switch
On this point, the UL standard is straightforward: It must be possible to lock a main switch on an interlock cabinet in the “Off” position. This requirement is not only about safety when working on the control cabinet itself, but above all about preventing the controlled machine from accidentally restarting.
New Requirements for the Interlock System Effective in 2022
The 2022 amendment to UL 508A also introduces several changes regarding interlocks. The most significant change is that it is now generally possible to design any control cabinet without using an interlock. Previously, exemptions from the requirement to use an interlock applied only under very specific conditions.
Control cabinet without interlock—access only with a key or tool
A key or tool is required to access the control cabinet. Conversely, this means that it must not be possible to simply open the lock by hand. All live components on the inside of the control cabinet door must be designed to be safe from accidental contact if they carry a voltage of more than 50 or 60 volts, and the control cabinet must be labeled with an appropriate warning. “Protected against accidental contact” means that the components must be protected against accidental contact with the palm of the hand or similar.
In practice, only components that operate on 24 V DC are installed in a control cabinet door. This applies, for example, to pushbuttons and displays.
Control cabinet without interlock – access without a key or tools
A control cabinet may be accessible without a key or tools; however, according to the new UL 508A standards from 2022, this is permitted only if all live components operating at 50 or 60 volts, respectively, are enclosed in a separate compartment or shielded against contact. The requirements for protection against accidental contact are particularly high in this case. It must be ensured that the components in question cannot even be touched with a finger. In this context, the term “finger protection” is used.
Access Secured with an Interlock
If the conditions mentioned above are not met, the disconnect device supplying power to the enclosure must interlock mechanically and/or electrically with the enclosure door. This is the actual interlock. If it is used in a control cabinet, it must be ensured that no cabinet door can be opened as long as the power supply is not interrupted. The requirement to use a snap-lock fastener (see above) remains in effect.
What do these changes mean in practice?
The option to forgo an interlock primarily means a significant gain in flexibility when planning and designing a control cabinet. This allows for a much more customized approach to meeting the requirements of the control cabinet’s future user. However, whether a control cabinet without an interlock is safer, more user-friendly, and more cost-effective than one with an interlock always depends on the specific circumstances. To make this assessment effectively, in-depth knowledge of control cabinet construction and UL 508A is required. If in doubt, an expert should be consulted before embarking on costly experiments.
Zusammenfassung
An interlock is a safety device defined in the UL 508A standard that prevents the doors of a control cabinet from being opened while the main switch is on. This safety device can be mechanical, electrical, or electronic. However, this requirement applies only to control cabinets that control industrial equipment or machinery and are operating at a voltage exceeding 50 volts for alternating current or 60 volts for direct current. Furthermore, UL 508A stipulates that it must be possible to bypass the interlock. The simplest method is to use a screwdriver to deactivate the interlock. There are also other requirements, such as the so-called “snap-lock” function, which describes the automatic activation of the interlock as soon as the control cabinet doors are closed.
