Arc Flash

Your questions answered: When and how to specify arc flash protection relays

Electrical engineers can learn about hardware considerations, signaling and zone selection for higher performance arc flash mitigation systems

By Farhad Moslemi, PEng November 18, 2020
Courtesy: CFE Media and Technology, Schneider Electric

During the Nov. 10, 2020, webcast “When and how to specify arc flash protection relays,” several questions were left unanswered. Learn more here.

Presenter: Farhad Moslemi, PEng, Principal Application Engineer, Energy Automation Business, Schneider Electric

Question: Where do most arc flashes occur?

Moslemi: Cable termination, VT compartment.

Question: What is the difference in operation between ZSI and bus differential? And does it reduce main-breaker line-side arc as well?

Moslemi: ZSI (zone selective interlocking) is based on overcurrent element and blocking signal between downstream (feeder) and upstream (incoming) relays, if the feeder relays picks up for a fault, it will send a blocking signal to main incomer  and main incomer won’t trip.

Differential protection is based on the current summation of amm feeders and incommers, in healthy condition, current summation is zero and for internal fault it will be equal o fault current. ZSI could cover the main incomer but any fault before the CT, won’t be seen by that CT and relay will not operate.

Question: So, if a switchgear bucket were to have an event, does the light sensor activate the specific breaker?

Moslemi: If there is dedicated sensor to that breaker and there is barrier to enclosed the area, yes, it will pick up the fault. Based on the location of fault and switchgear layout, the programmer could set the relay to trip any breaker needed.

Question: Why use light detection?

Moslemi: Detection of light is fastest method to detect a short circuit. It will react sooner than overcurrent.

Question: What about the time from relay to breaker and actual time to open the breaker?

Moslemi: Total clearing time includes: arc detection + relay processing + breaker operation. Arc flash relay will reduce detection + processing (include closing the output) as low as 2 ms. Breaker operation time will add extra delay to entire time.

Question: Why can we not use electromagnetic phenomena f arc current for detection of the arc?

Moslemi: We are looking for fast detection method and increasing coverage area. Light detection is one of the best choices, detection of electromagnetic effect of arc means we will need to do some calculation over a cycle to decide what is the actual waveform and this will take time (a cycle delay).

Question: Why are the fuses in VT not open?

Moslemi: In the field experience, short circuit was before the fuse (VT compartment connectors) and won’t be seen by fuses.

Do you have experience with transfer trip to the serving utility?

Moslemi: It is always a challenge to combine direct tripping the upstream breaker that belongs to another party. In the same plant that used as a site experience example in my presentation, after repair, one output of arc relay allocate to trip upstream relay.

Question: During maintenance, while grounding one of the feeders, protection staff used to isolate arc protection tripping links manually in order not to cause tripping for the entire bus in case flash caused by grounding happened. Is there another way around this?

Moslemi: Basically, we could block entire arc system or part of that (depends on relay capability) by using digital inputs; this input could assert from grounding switch or maintenance mode. By this, arc system will be disabled automatically.

Question: How does the system react to a low level, 1,200 amps and below, arcing ground fault?

Moslemi: Arc sensors will only react to light intensity, in case it supervised with current, the current pick up value should set according to short circuit study, normally current will set above the load and lower than fault value.

Question: Did the fiber optic loop sensor have the ability to detect where on the fiber the flash occurred? If so, is the fiber marked so you know where that location is?

Moslemi: No, I did not see this capability to any of them.

Question: Rather than providing zone protection, could you save money by tripping all incoming and feeder breakers in gear upon detection of light?

Moslemi: The goal of zoning is to isolate the faulty section and keep the healthy part continue to supply the power. If there is no any option to specify fault location and proper fault isolation, then yes, we could open all feeders and incomers.

Question: Do we need to do a load flow study as part of the arc flash study?

Moslemi: We will need a short circuit study.

Question: Will the electromagnetic field of the bus interfere with the functioning of the sensor, if installed in proximity of the bus?

Moslemi: Yes, there are sensors with the shield wire for this application.

Question: What type of sustaining maintenance and inspections are recommended for these sensors?

Moslemi: Since these sensors are self-supervised, then any shortage and open circuit will be seen as an alarm.

Courtesy: CFE Media and Technology, Schneider Electric

Courtesy: CFE Media and Technology, Schneider Electric

Question: What is your opinion on using digital input and digital output arc protection scheme? Should be used or must only use faster protection scheme line communication?

Moslemi: Performance of digital inputs are dependent on the relay scan time 9 commonly we will need to assume half a cycle, 8 ms, and operation of output will also delayed by about 7 to 9 ms. In this case, we should expect about 10 to 15 ms delay while using normal I/Os, that is the reason arc flash relays does have a dedicated fast IIO for signaling.

Question: Is connecting an arc flash light sensor to a relay similar to adding a PT/CT input to a relay?

Moslemi: It depends on manufacturers, some only accept fiber connection to relay (special connectors) some will have a twisted pair wire which will connect relay same as digital inputs.

Question: Are there any other type of protection similar to arc flash?

Moslemi: Refer to NFPA 70: National Electrical Code 240, there are different methods of arc mitigation methods. Arc flash detection with light sensor is among the fastest method, there are some device called arc terminator, which will intentionally short circuit the bus during fault to allow breakers to trip, these systems will also use light sensor for activation.

Question: Are these relays UL-listed?

Moslemi: Schneider arc relays are UL-listed.

Question: So, we can use the arc flash sensors in lieu of higher AIC ratings on the electrical gear? Or are the sensors an added safety feature?

Moslemi: Sensors should be used as a fast detection system only. They won’t change any panel rating.

Question: What software will you recommend for the study of arc flash, SKM power tools, Etap or Easy power?

Moslemi: Easy power and SKM.

Question: Is it mandatory to have undervoltage coil in MV breakers to ensure reliable tripping?

Moslemi: It is not necessary, but we could add second trip coil for increasing reliability.

Question: Would this detection system be necessary in the Premset Gear?

Moslemi: It is depending on the calculated incident energy at the location of switchgear. If incident energy is too high then they could add arc system.

Question: Is an arc flash relay different from a protection relay? For example, is an arc flash relay the same as a 50/51 protection relay? Or, are they different?

Moslemi: Arc protection could be and option to some protection relays, these relays will take care of all 50/51, plus arc detection. There other arc relay that are dedicated to arc detection without any other protection elements.

Question: Does the detection sensors clear line-side arc flash?

Moslemi: Line side faults at the incomers will be cleared by upstream breaker, arc sensor could only detect the fault and provide trip signal (to be sent to remote relay).

Question: What are the cost differentials of using light versus pressure systems?

Moslemi: Don’t have the market price, but light detection is cheaper than pressure method.

Question: What is the life expectancy of these sensors (for planned change outs of components), replace before failure? Or is it set and forget for the life of the installation?

Moslemi: It depends on manufacturer spec. Mostly did not mention any life expectancy, but they required regular visual inspection for pollution (max every three years) and testing sensors every year.

Question: On a wide motor control center, what is the best way to determine where to place the light sensors when there are many closed bucket compartments?

Moslemi: If you need to pinpoint the fault location by arc system, there are some arc flash relays that could accept large number of point sensors (up to 170) bay adding some peripheral modules. There are simpler modules with a smaller number of sensors, which could cover few cells (look for Schneider V321 and V125) as examples.

Question: Which power systems study/arc flash study software do you recommend using?

Moslemi: Easy power and SKM are two samples.

Question: Where can I find courses or resources available to learn more on arc flash and coordination study?

Moslemi: There are lots of article in the internet, one of the pioneers in arc protection was VAMP (Finnish company) which is part of Schneider Electric.

Question: Can you please explain again why a 120-volt system might have a higher incident energy than a 208-volt system?

Moslemi: We were comparing an MV system to LV system, 13.8 kV and 480 V. LV system are dealing with higher current.

Question: Does the arc flash relay detect a possible event or the event itself and operate the feeder breaker?

Moslemi: Arc flash relay will detect a light (or light + current) and then will trip the breaker, design engineers should decide which breaker should trip based on their design and study to have proper fault clearing action.

Question: Why the focus always on 3-phase faults and not much on single phase faults?

Moslemi: 3-phase fault current is higher than single phase and more critical.


Farhad Moslemi, PEng
Author Bio: Farhad Moslemi, PEng, principal application engineer, energy automation business, Schneider Electric