Your Questions Answered: The latest trends in arc flash mitigation methods - Consulting

By designing electrical systems with arc flash mitigation in mind, a facility will be a safer environment with improved equipment reliability. The design process might include comprehensive short circuit, coordination and arc flash studies to specify appropriate overcurrent protection devices, or other mitigation strategies as needed.

The learning objectives for the webcast were:

Review the basics of arc flash mitigation and the studies required to ensure appropriate specifications.
Learn about how to design an electrical system to meet or exceed NFPA 70: National Electrical Code requirements.
Discuss how electrical engineers can design mitigation strategies for safety of personnel.
Understand the latest technologies in arc flash mitigation and best practices to minimize risks.

Following the live Consulting-Specifying Engineer webcast, “Learn the latest trends in arc flash mitigation methods,” on Jan. 23, the webcast instructor, Dan Costello, PE, LEED AP, district application engineer for Eaton, answers the questions he was unable to get to during the live event. More than 100 questions were asked! Here, Costello provides answers to some of those questions.

Webcast instructor:

Dan Costello PE, LEED AP, District Application Engineer, Eaton

Question: What standards should I reference for arc flash requirements?

Costello: The standard is IEEE1584.

Question: How often should arc flash studies be performed?

Costello: If what the gear is feeding, additional loads or incoming service changes, the study needs to be updated. Hospitals and industrials often change the electrical system based of renovations and changes. Other than this, a good practice is every five years.

Question: How would you mitigate high arc flash on the incoming breaker line side connections?

Costello: The breaker upstream can be interlocked with the main breaker to open. For example, a substation lineup with a main breaker on the primary side of the transformer is interlocked with a main breaker in the LV lineup.

Question: Once the breaker is racked out and shutters closed, it is not considered a hazard anymore to open the door and physically remove the breaker?

Costello: Shutters are only for incidental contact of the bus, not to contain the arc.

Question: Are there specific code requirements requiring the designer to perform arc flash studies for a new vs existing modified electrical system or is that the strictly the responsibility of the facility owner to get those studies done to meet OSHA and other standards for providing proper labeling for personnel working on that equipment?

Costello: Ultimately it is the owner’s responsibility for full arc flash safety program including the study.

Question: Is arc flash rating with door open or closed?

Costello: All standards are with the doors closed. Typically, there is a LV control cabinet on the front of the gear which can be opened for service and does not compromise the are rating.

Question: For arc flash PPE ratings and the definition of 50% chance of a second-degree burn, are you aware of any guidance about reducing the percent chance of second-degree burns by having additional arc flash PPE rating above the calculated hazard value?

Costello: Equipment downtime is greatly reduced by the arc quenching system as no structures, breakers or busing will need to be replaced. The AQS fires in 2 to 10 ms and the LV main breaker will open within 4 cycles. The incident energy needs to be calculated.

Question: Are cable limiting fuses acceptable by the AHJ?

Costello: Depending on the AHJ. Typically, I see this retrofit applications for industrial facilities.

Question: Using two transformers and reducing the potential energy to 50K: Is this going to keep you alive or just reduce the cost of the gear?

Costello: Splitting the load will lower the available fault current, which lower the maximum incident energy.

Question: Is there a technical reference or program for DC arc flash study?

Costello: The commonly used short circuit and coordination software should include a module for arc flash calculations.

Question: How could a fuse in line with a breaker be considered as current limiting if they are series rated?

Costello: These need to be a tested combination because the dynamic impedance of the breaker. This is why the up-over-and-down method cannot be used.

Question: Are the cable limiters are located on electrical panels or transformer only?

Costello: These can be installed on most cables.

Question: Does the zone selective scheme take the place of instantaneous settings?

Costello: It takes the place of the short-time and longtime part of the time-current curve so every fault is in the instantaneous range.

Question: Can AQD’s be added to existing gear?

Costello: Yes, however this will be expensive because the light and current need to be installed.

Question: What are typical ratings for arc quenching equipment?

Costello: The available withstand ratings are 100mS@85kAIC, 100mS@100kAIC, 500mS@856kAIC.

Question: Talk about switchgear faults arcing to line side of MCB.

Costello: The use of a virtual main accounts got this. For example, a substation with a breaker on the primary side of the transformer uses differential to trip this breaker. If there is an arc relaying system, both the primary and secondary breakers will open.

Question: How much time does it take to recover the site after the CLIP system fires? Is this a huge job or does that manage to largely self-contain the response at the CLIP? (Recognizing the 0.25s fault that started it will still need to be resolved/repaired.)

Costello: The CLIP interrupts in a quarter cycle. Reinstalling a new unit will take several hours. Keep in mind that arc quenching devices can be cable bypassed to get the gear back up and running.

Question: Is the arc flash incident energy considered to be lowered in a switchboard where the main breaker >1200A has an ERMS/ARMS activated? For example, when working on or racking out a distribution breaker?

Costello: Yes, this will be substantially lower given that the majority of faults and not bolted faults and will be cleared much faster the coordinated trip settings.

Question: Regarding the NEC 2023 revision requiring maintenance on electrical equipment per MFR instruction. Do OEMs such as Eaton provide a required maintenance interval/scope of work? For example, Eaton Pow-R-Line switchboard instruction manual indicates “The switchboard should be given a thorough maintenance check annually.” Is this saying that the Switchboard maintenance is REQUIRED by NEC on an annual basis?

Costello: There is not a lot of maintenance on switchboards. Exercising the breakers every 12 to 18 months and the cable need to be checked for tightness.

Question: Any thoughts on including a vacuum fault interrupter (VFI) with a pad-mounted transformer (mainly for differential protection)?

Costello: This is a great option as you get all of the functionality of an MV breaker at a smaller footprint and less costly.

Question: Is there a product that can remotely monitor the health of the equipment components including terminations, etc.?

Costello: Yes – that is the thermal monitoring, such as Exertherm, which was covered in the presentation.

Question: Are there any recommendations or rules of thumb of re-using arc-resistant switchgear after an arc flash event?

Costello: Yes, this should be recommissioned and tested preferable by the manufacturer even if there is no apparent damage.

Question: What’s the best Arc Reducing method for Emergency Systems where Selective Coordination is required?

Costello: Because the calculation requires the maximum available fault current, a properly sized fuse will clear the fault the fasted at that level.

Question: In industrial plants, changes are made to the load levels handled by electrical equipment. How significant must the change be to consider updating the arc flash label?

Costello: The facility is required to update the fault flash study any time the system changes. Industrials and hospitals redesign and change loads often.

Question: Where do you start if you want to include arc mitigation solutions in your design?

Costello: I would look at the estimated fault current levels early on and consider the type of facility. Hospitals and industrials are more critical, and recommending higher-end solutions is appropriate.

Question: What is the industry standard for utilizing a two-second maximum arc flash duration for arc flash studies?

Costello: I believe that is the NFPA/IEEE standard for studies. This is because without ARMS engaged (or another method), there will be a longer clearing time.

Question: What is the cost difference between a breaker with ZSI and one without?

Costello: Roughly 10% to 15%.

Question: How many engineers/technicians have NETA/NICET certification?

Costello: Doing a quick search, it looks to be more than 155,000 have NICET certification. Eaton has many throughout the country.

Question: When an ARMS switch is specified, what energy calculation value is printed on the arc flash label — with or without ARMS activated? Would you print more than one label?

Costello: Usually, it includes both because the arc mitigation through ARMS can be disengaged.

Question: With ZSI and a fault downstream of the FCB, are the FCB trip settings overridden (reduced) to cause the FCB to trip quicker? Or would it only be in the bus fault condition, as shown in your example, that the MCB would trip quicker?

Costello: ZSI will override the coordination settings of the breaker closest to the fault while the upstream breakers use their coordination settings. This sequence is active regardless if the fault is a load or a bus fault.

Question: How does the cost of the AQD compare to other methods?

Costello: AQD is a 40% to 50% adder.

Question: Does current limiting MCCB help mitigate arc flash energy?

Costello: Yes, it will because the let through current is lower.

Question: The breaker has to be open while remote racking out. Can the arc flash incident still happen in this case?

Costello: Yes, definitely. These are mechanical devices and can fail, especially if not maintained or exercised. The safety is the breaker is racked out where the operator is far from the gear.

Question: Are generic labels still acceptable, or does it have to have a category on them?

Costello: The labels include the calculated incident energy, PPE and boundary distances unique to that specific equipment.

Question: Does a minimum SCCR rating on equipment/panel mitigate arc flash? If so, how do you establish the minimum SCCR rating?

Costello: The SCCR/interrupting rating is separate from the arc flash.

Question: Is PPE required for switchgear interactions such as reading a meter?

Costello: PPE is not required because the doors are closed.

Question: What is the cost premium for the IR windows?

Costello: These are $2,000 to $4,000 installed.

Question: Can ZSI be used between the molded case and power breakers?

Costello: Yes, they can.

Question: Is the AQD a one-size-fits-all solution, or does it require some sizing and settings to be provided for the specific system?

Costello: The AQD is a one-size, static device.

Question: Health care allows the overcurrent protection to not coordinate below 0.1 seconds. Normal emergency must be fully coordinated, often requiring greater arc flash time. Is anyone pushing the NEC to set a time limit for fully coordinated overcurrent protection for safer operation?

Costello: The standard for health care is 0.1 seconds, and I am not aware of this proposed change. Note the 2023 NEC will require the normal side to be selectively coordinated.

Question: Do you need an AQD for each breaker or just one for a switchgear lineup?

Costello: AQD is required for each main breaker.

Question: What is considered as qualified personnel?

Costello: They need to have proper training, use of PPE, interpret the equipment arc flash labels, the ability to read one-line diagrams to open all sources and the ability to ground equipment — typically, a facility electrician.

Question: Are three phases eventually all involved once an arc is established of sufficient energy nearby?

Costello: They can be, however, a three-bolted fault is rare and this “worst case” fault is the value used to calculate the incident energy.

Question: On average, what is the manufacturers’ expected life for SWBDs and SWGR (i.e., 30 years or less)?

Costello: In general, the industry standard is 20 years. However, with proper maintenance, the gear will last far longer.

Question: From experience, how dependable is the listed breaker clearing time? What is the time limit for last time tested? With older gear or less than 1200A, is it acceptable to turn down the instantaneous, or is this somewhere against UL or other practices?

Costello: The breakers are tested per UL standards and must comply. Because breakers are mechanical devices, there will be some hystesis, which affects time. However, the UL test has a maximum allowed opening time.

Question: Is Eaton seeing more people using zone selective Interlocking to reduce arc flash? I used this in non-Eaton gear in 2005 in the semiconductors. I have not seen it in data centers.

Costello: I see more people using this, not so much in critical applications such as data centers and hospitals. I have seen more interest in ZSI in wastewater plants because it is active all the time.

Question: Arc flash resistant switchgear is expensive. Should we promote the arc flash resistant switchgear for LV switchgear or for MV switchgear or for both? How do we determine during the design phase of the project if we need to design with arc resistant switchgear?

Costello: The arc resistant gear is less expensive than the ant light/current sensor system. Also, ARMS can be applied to LV switchgear and programmed into MV relays.

Question: What are the arc resistance switchgear advantages vs. a light sensor option?

Costello: Arc resistant gear is less expensive and does not require maintenance. The light and current sensors need periodic maintenance. Typically, there is less equipment damage when using the sensor system.

Question: Will arc resistant switchgear will keep the operator safe when the door is open?

Costello: It will not. The arc rating test is with the doors closed. The first big takeaway in the webinar is deenergize the gear.

Question: NFPA 70B requires annual IR scanning. Does continuous thermal monitoring replace this requirement?

Costello: Yes, this does comply. It offers an advantage over IR scans because the gear is continuously monitored and is independent of when the gear is scanned.

Question: At what point should we consider arc flash mitigation (i.e., at what level of available fault current?

Costello: At a level above a PPE of 1, I would start looking at arc mitigation methods. ARMS is an inexpensive option and can be programmed into MV relays.

Question: Can ZSI be implemented on any manufacturer’s breakers, or is it limited to Eaton?

Costello: All the manufacturers offer this. However, ZSI will not work between different manufacturers.

Question: I don’t typically see labels with incident energy on both line and load sides given as in your example. Is this something that is required in some localities?

Costello: Typically, it is included on the line side.

Question: Do you recommend using the 2 second threshold for clearing times more than 2 seconds?

Costello: If it takes longer than 2 seconds, I would calculate using the longer time. However, I would address why the clear time is so long.

Question: For switchgear that vents arc flash incident energy away from the user, does that typically require replacement of the entire switchgear after an arc flash event, or is the damage typically limited to the affected section of the switchgear? Understanding that every incident can be different, but I would like to understand from previous documented incidents.

Costello: Depending on the level of arc flash, the gear will need repair. At minimum, the blast gates at the top of the gear needs repair/replacement.

Question: Does Eaton use pressure sensors together with light and current for AQD activation? Or just light and current?

Costello: The Eaton system uses only light and current sensing.

Question: Does it make sense to have ZSI and ARMS in a lineup?

Costello: It would be added safety. Remember the breaker two levels up from the fault will use its normal settings. If the breaker closest to the fault doesn’t open the breaker above, that breaker times out using the coordination settings. ZSI has the advantage of always being engaged.

Question: What other preventive maintenance services are required by codes for electrical building systems?

Costello: This is open-ended for now. NFPA dictates “per the manufacturer’s recommendations.” I’m sure in the future there will be more specific requirements.

Question: What would be a good example of a three-phase fault?

Costello: A three-phase bolt fault is just that — all three phases are bolted together, and the gear needs to be rated for this withstand. In practicality, this can only happen on start-up or if the gear is deenergized and all three phases are tied together. If a tool is dropped across all three phases, this a three-phase arcing fault.

Question: Can you discuss common methods used to work on equipment while energized? To what extent can equipment be worked on while energized?

Costello: Use proper PPE and temporary barriers, such as heavy blankets, to prevent incidental contact.

Question: Is only one arc quenching device cubicle required for a large lineup of switchgear?