Your questions answered: To Switch or Not to Switch … Key approaches to meet NEC 240.87

The April 24, 2018, “To Switch or Not to Switch … Key approaches to meet NEC 240.87” webcast presenter addressed questions not covered during the live event.


Electrical designers and engineers may view this presentation involving NPFA 70-2017: National Electrical Code (NEC) 240.87 to obtain some of the methods and tools that can be used to select and implement arc flash reducing solutions. The presenters illustrate how electric power affects arcing current and how circuit breakers can operate to reduce the incident energy. Having such an understanding may foster safer and more cost-effective solutions that comply with this code.

Key topics include:

  • An understanding of the intent of NEC 240.87.
  • Arc flash mediation techniques and tools.
  • Determining cost-effective methods that comply.
  • New options for the latest 2018 code.
  • How to calculate incident energy.
  • Arc current versus available fault current.
  • Time-curve software and calculators.

NEC 240.87 is a growing requirement that is spreading across the nation as more states adopt current code. When additional compliance of NEC Articles 700 and 701 come into scope, compliance of 240.87 becomes even more complicated.

There is a perception that a “maintenance switch” is the only solution toward meeting 240.87. Understanding how a maintenance switch impacts the performance of arc-reduction system is critical for effective arc mitigation and cost efficiency. This webcast goes behind the switch to see its impact of reducing incident energy between normal and maintenance modes. Furthermore, there are other techniques that engineers can use to possibly eliminate the maintenance switch and possibly improve arc mitigation.

With or without a maintenance switch, breaker settings must be adjusted to ensure the normal mode is providing equipment protection and the maintenance mode provides equipment and arc-flash protection. Gain knowledge to help determine what “approved equivalent means” might be useful in your next design.

Presenters during the webcast were:

  • Rick Pinchard, business development manager, Siemens
  • Roy Hicks IV, business developer, Siemens
  • Robert W. Jensen, P.E., business development manager, Siemens

Roy Hicks IV, business developer, SiemensPresenter Roy Hicks IV, business developer, Siemens, responded to questions not answered during the live To Switch or Not to Switch…Key approaches to meet NEC 240.87 webcast on April 24, 2018.

Question: Do 1,200-amp breakers in series both require a maintenance switch?

Roy Hicks: The code states all breakers. If a fault occurs between the two breakers, only the upstream breaker will see the fault, so both breakers should have an energy reducing maintenance switch (ERMS).

Q: Can you comply with 240.87 merely by adjusting the instantaneous trip setting during maintenance operations?

Hicks: It’s up to local authority having jurisdiction (AHJ), but if the settings are available without panel removal and a label instructing what to do has sufficed in some areas.

Q: NEC 240.87(A) requires documentation. What should be provided in that documentation? Seems the method from 240.87(B) should be documented and possibly some documentation if items 5, 6, or 7 are selected.

Hicks: It is our recommendation to provide the available arcing current via calculation. If the minimum settings are used, the current should be higher than minimum. If settings are dialed up from minimum, the current should be higher than the settings. This information may suffice for the AHJ.

Q: What is the recommended margin between the instantaneous trip amps and the available arc fault amps?

Hicks: If you’re using a time-current curve (TCC), the tolerance should be in the curve. Plus 10% is pretty typical for electronic trip units. If there is the ability to increase the margin and not run into coordination issues, then increasing the margin (another breaker-type/setting) may improve the incident energy. The code doesn’t specify any values (margins), but the intent is to provide an arc reducing function.

Q: Did the recent arc fault NEC requirements have any effect on the size of the switchgear (i.e., increase or decrease the size of the arc chutes out of the top of the gear)?

Hicks: The NEC 240.87 requirement itself did not. Although manufacturers have added arc resistant features to their gear, which makes the enclosure slightly larger. This reduced the personal protective equipment (PPE) or hazard/risk, but 240.87 requirements still apply.

Q: Does Siemens offer a retrofit option on non-Siemens equipment?

Hicks: Yes, we can add ERMS to our gear and to gear of some other manufacturers.

Q: What are some examples of approved equivalent means that have been used to comply with 240.87?

Hicks: A couple of examples are using relays with arc flash detection, fuses, crowbar devices, and other third-party type devices that may or may not reduce clearing times better than breakers.

Q: What are the labeling requirements when zone selective interlocking (ZSI) or maintenance mode is used? Would two stickers be placed on the equipment?

Hicks: Labeling details haven’t been clear. If ZSI is on the 1,200 amp or larger breakers, then no other actions are required because there is not a maintenance mode or switching required. Therefore, a label indicating ZSI installed for arc reduction should suffice for AHJ and users.

Q: Can I just add a ZSI relay to an existing breaker to comply with NEC 240.87?

Hicks: ZSI only pertains to the short time-delay function. So, if your breaker has that, it should automatically reduce the short time delay if unless it receives a blocking signal from another breaker to not trip on its lowest setting. For example, if you added ZSI to each installed breaker and did not connect the blocking signal wires between them. Each breaker would trip with minimal delay with regard to the short-time pickup. Without the blocking signal one or more breakers may trip (coordination). So, ZSI is one of the code-compliant options by itself.

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