Electric/electronic door interlocking (U.S. standards)

When a control panel supplies industrial machinery with electrical power and operates under a voltage ≥ 50 V AC/60 V DC, the panel can be opened only subject to at least one of the following three conditions. You are advised to discuss these three options with the operator of the control panel or industrial machinery.

Option #1: Door interlocking with only a tool or key
In this case, the following three conditions must be met:

• The doors can be opened only with a key or tool (e.g. double-bit key).
• All live parts must be protected against unintentional direct contact by 50 mm sphere minimum (back of hand proof).
• The accompanying warning must be affixed to the door so that it is visible on the outside of the control panel.

Option #2: Door interlocking by the main disconnecting device (main switch)
The industrial control panel's main disconnecting device is interlocked so that the doors can only be opened when the electrical power has been switched off. This door interlocking can be mechanical, electrical, or a combination of the two.
Interlocking (=snap lock function) must be automatically activated without additional actions being taken when the doors are closed under voltage.

Option #3: Overridable interlocking is also optionally possible
Interlocking can be defeated by a qualified person using a tool so that the door can be opened for service, maintenance, and troubleshooting even with the main switch switched on. If you decide in favor of an interlocking which is able to be defeated, all of the following points must be taken into account:

• Disconnection of the main disconnecting device: Regardless of the door position, it must be possible to open the main disconnecting device without a tool and lock it in the OFF position at any time with a lock.
• Protection against unintentional switching on: When the door is open and the mains disconnecting device is switched off, an unintentional switching on of the main disconnecting device must be prevented. Exception: A qualified person can also switch it on when the door is open by means of a deliberate action.
• Basic protection (protection against direct contact): All live parts must at least be back-of-hand-proof (50 mm sphere).

All relevant information on door interlocking and the override functions must be contained in the documentation for the electrical equipment.

Covering the live parts
Alternatively to the other two options, the control panel can be opened and operated by any person unqualified in electrical technology. In this case, all live components must be physically enclosed or covered so that no bodily contact with the parts of the electrical equipment is possible. In this case, the doors require neither locks (see #1) nor interlocking with the main disconnecting devices (see #2). A corresponding warning must be affixed to the control panel doors according to NFPA 79.