Your questions answered: To Switch or Not to Switch … Key approaches to meet NEC 240.87
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
Presenter 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.
Q: Have you considered using a secondary main relay versus a secondary main breaker? The relay can shunt trip an upstream medium-voltage (MV) breaker. The relay is set to minimize the arc flash of the switchgear.
Hicks: I’m not familiar with this scheme. I would need to look into it with more detail and understand if the clearing times using the relay is faster than other methods.
Q: Could you verify that the arc flash reduction maintenance switch is a circuit breaker option? If so, what breakers are likely to have this option?
Hicks: It is an option to most manufactures that have electronic trip units (ETU) available for their breakers. The input to the ETU is your maintenance mode sensor that might use a switch, contact, or communication to put the ETU into the maintenance mode condition.
Q: Will an arc flash study allow lower PPE requirements when incorporating an ERMS despite relying on maintenance personnel to activate the ERMS?
Hicks: Yes, a study should allow you or the manufacturer’s rep to determine what breakers offer the best protection based on their TCC. Some solutions, such as ZSI, may not require any activation from personnel. Instantaneous lower than the arcing current is another example.
Q: How do you calculate the arcing fault value?
Hicks: IEEE 1584 provides a method of calculation. Other software calculators are available.
Q: Can arc faults on 120/208 V systems keep going for a long time? I thought that that happened on 277/480 V systems.
Hicks: The available fault current is most important factor. If I used a breaker that was suitable for both voltages, I might have a longer clearing time than if I used a breaker that was limited to 120/208 V only. That breaker might have a faster clearing time of the same available arcing current. The system voltage and available fault current impacts your available arcing current value, not the duration.
Q: What is the difference between fault current amps and arc fault current amps?
Hicks: The available fault current is the total current from all sources and typically has no resistance or impedance to factor into the fault. An available arcing current calculator has impedance and other computations to determine what the current in the system would be based on the inputs into the calculator. For those reasons, the arcing current is typically much lower than the available fault current. Because the available fault current is the higher value of the two, we use that to determine the ratings of the equipment and the protective devices within. For example: If the available fault current is 46 kAIC, then the equipment is going to be 50 kA or more rated to handle the total current. This is considered the bolted fault current or rating.
Q: As asked previously, why is ZSI a better balance than the other approaches, given that it shows the highest Cal rating?
Hicks: The example using the ZSI was slightly higher because ZSI only pertains to short time function (GF too but not in the example). The instantaneous clearing time of the breaker was 50 milliseconds. The minimum short time-delay setting is 80 milliseconds. If you turned up the instantaneous pickup to a value higher than the arcing current, then it will trip at around 80 milliseconds (fastest short time). Using 80 milliseconds instead of 50 milliseconds to determine the arcing current/incident energy will make the values slightly higher. This is the best ZSI can do for clearing the fault in time. I used the dual trip ETU to have the downstream breaker trip on instantaneous and my main trip on short-time (ZSI or no-ZSI). Therefore, my main had around an 80-millisecond IE outcome and branch had a lower 50-millisecond IE outcome. I used that technique to have some level of coordination if there was a fault in the system. The code doesn’t specify any value I need to reach, but from a system design perspective, I would want to lower the IE as much as possible without risk of losing total power on a faulted branch. I met code by simply providing an arc reducing mode, “maintenance mode.”
Q: System voltage tolerances in relation to arcing current calculations and 10% manufacturer’s TCC tolerances also should be considered, correct?
Hicks: Yes, if you want to optimize or understand what value the ERMS is providing. Many installs may have an ERMS and not setup the ETU or simply have the settings dialed to their lowest setting, which really defeats the purpose of the ERMS.