NFPA 79 - 2024, Chapter 5: Disconnecting Means

Last edit: 05/01/2024

With the 2021 Edition of the Standard, the title of Sub-chapter 5.1 changed from “Machine Supply Circuit Conductor Terminations.” to “Machine Supply Circuit and Disconnecting Means.”. It should now be more clear that this section deals with the disconnection of all power supplies of the machine electrical equipment. It would be a good idea also to change the title of Chapter 5 of IEC 60204-1 “Incoming supply conductor terminations and devices for disconnecting and switching off”. But this will not be discussed before the next 5 years.

Regarding the need for a door interlock, here what the standard requires: Enclosure Interlocking. When required by, each disconnecting means mounted within or adjacent to a control enclosure that contains live parts operating at 50 volts ac (rms value) or 60 volts dc or more shall be mechanically or electrically interlocked, or both, with the control enclosure doors so that none of the doors open unless the power is disconnected. Interlocking shall be reactivated automatically when all the doors are closed.

Still regarding main disconnection of the control panel, it has been clarified that, if the main disconnector is outside the control panel, its position must be indicated. The necessary distances (Chapter 11) in front of a control panel are also aligned with those required by NEC edition 2020.


The issue is that a machine electrical equipment may have several disconnecting means, especially when it is made of a main electrical panel and a few subpanels. Supposing the machine is powered from one feeder source, each control panel may have a Disconnecting means, but only the main panel will have a Machine Supply Circuit Disconnecting Means. That is the reason the chapter has been reorganised in 3 paragraphs.

5.1 Machine Supply Circuit Disconnecting Means


This paragraph gives requirements for the main panel only, not for other subpanels. In case the machine has more than one external disconnect, the main one, the one receiving the power from the premises, has to be properly marked. This is a new requirement, valid from 2021: A machine supply circuit disconnecting  means shall be marked as "Machine Supply Circuit Disconnect" if additional disconnecting means are supplied from the supply circuit disconnecting  means.




Important are the type of components that can be used for this function:

5.1.10 Type.  

The machine supply circuit disconnecting means shall be one of the following types:

  1. A listed motor circuit switch rated in horsepower;
  2. A listed molded case circuit breaker;
  3. A listed molded case switch;
  4. An instantaneous trip circuit breaker that is part of a listed combination motor controller limited to single motor applications;
  5. A listed self-protected combination controller limited to single motor applications;
  6. An attachment plug and receptacle for cord connection.

Please consider that a switch certified to UL 508 is not suitable as machine supply circuit disconnecting means. The reason is the low SCCR that those components usually have.


5.2 Additional Disconnecting Means

This paragraph applies to disconnecting means other than the main one. Hereafter the type of component suitable for this function:

5.5.4   The following devices shall be permitted to fulfil the function of 5.2.1:

  1. Devices described in 5.1.10.
  2. A manual motor controller marked “suitable as motor disconnect” and in compliance with UL 508, where located on the load side of the last short-circuit protective device (in the branch).
  3. System isolation equipment that incorporates control lockout stations and is listed for disconnection purposes where located on the load side of the machine supply circuit disconnecting means and overcurrent protection.

That means even if in a subpanel, a main disconnect cannot be a UL 508 disconnector.


5.3 Means for Removal of Power for Prevention of Unexpected Start-Up

Several standards deal with the prevention of an unexpected startup:

  • At international level: ISO 14118:2017: Safety of machinery — Prevention of unexpected startup;
  • In USA: ANSI/ASSP Z244.12016:The Control Of Hazardous Energy Lockout, Tagout And Alternative Methods;
  • In Canada: Z46013 (R2018): Control of hazardous energy Lockout and other methods.

NFPA 79 wants to state that, despite for isolation, only circuit breakers and disconnector are acceptable, for prevention of unexpected start-up, contactors are suitable for the scope.

5.4.2   Removal of power shall be accomplished by one of the following:

  1. The machine supply circuit disconnecting means.
  2. Additional devices conforming to 5.1.10 or 5.2.2.
  3. Other means (e.g., a contactor switched off by a control circuit) that meet the requirements of 5.3.3 and 5.3.4.