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To protect persons from injuries caused by electricity, control panels that are used in the U.S. must be designed so that they ensure protection against direct contact. This means that no live parts can be touched under normal operating conditions.


The U.S. standards therefore specify that the enclosure doors must be locked in accordance with specific guidelines, but that they must be accessible for maintenance and service by qualified personnel.

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The following U.S. standards specify in which cases and under which conditions door interlocking must be installed:

  • UL508A (section 66.1.2 and 66.1.3)
  • NFPA79 (section 6.2.4 and 6.2.5)


When an industrial control panel controls an industrial machinery and operates under a voltage more than 50 V AC / 60 V DC, each door of the control panel  must be locked with the disconnecting means (e.g. the main switch) so that the door can only be opened, when the disconnecting means is switched off.


The following methods are available for this:

  1. Purely mechanical, e.g. using the main switch
  2. Electrical
  3. Combination of mechanical and electrical

The door interlocking must also meet the following conditions:

1. Defeat the interlock
It must be possible for a qualified person to defeat the interlock with a tool so that the door can be opened for service and maintenance with the main switch being switched on.


2. Snap lock function
The interlocking must be activated automatically when closing the doors without additional measures (snap lock function).


3. Blocking of disconnecting means
With open control panel door activation of the disconnecting means must be blocked by the interlocking.


Exception: It must be possible for a qualified person to cancel this safety function with a deliberate action.

You can find detailed information in the reference manual "Industrial Control Panels and Electrical Equipment of Industrial Machinery for North America" (section 5.4 Protective measures)

Accessories for door interlocking according to UL 508A and NFPA 79