Abstract
Due to the high emphasis that has been put on the reliability of electrical systems in the recent past; relaying protection schemes have been designed to coordinate tighter and allow more amounts of energy to fl ow in systems before isolating. Th is has caused a serious problem when we bring up the topic of arc fl ash. If a relay system is set to provide higher then average levels of reliability by decreasing the chance of “nuisance tripping” during system disturbances it increases the likelihood of high incident energy points in the electrical distribution. We are able to reduce the arc fl ash hazard, while still providing a high level of reliability by utilizing light and current sensing relays to detect an equipment failure quickly, thus decreasing the time to trip the upstream device.
I. Introduction
Arc fl ash hazards are an every day occurrence in the life of an electrical worker. From reading the panel meter, to op-erating a low voltage motor starter, to racking out a medium voltage vacuum circuit breaker. Win light of this there is a need to be able to manage the danger in order to limit the risk of exposure to an electrical arc fl ash incident.
Th e NFPA 70E-2004 document gives us the guidelines to providing workers with the proper personnel protective equipment to do specifi c tasks. Th ese guidelines are based on years of research by the IEEE and other private research groups. Th e National Fire Protection Association (NFPA) 70E provides a generic standard for electrical safety require-ment for employee workplaces table.
Th is table outlines the personal protective equipment required to do specifi c tasks. Th e problem with using this table is that not all conditions will be equivalent to the basis of the calculations used in the table.
For example, the tasks listed in the table are based on a specifi c amount of current and a fi xed clearing time. Most conditions in a real life arc fl ash incident may not be equiva-lent to these assumed values. Th erefore it is imperative that facilities undergo an arc fl ash hazard analysis of their system to understand the actual dangers that are present in their systems.
Th e Institute of Electrical and Electronic Engineers (IEEE) have created a standard for performing arc fl ash hazard calculations. Th e IEEE 1584 standard can be used to complete a step by step analysis of a facility to calculate the available incident energy wherever personnel may be exposed.
Th e National Electric Code (NEC) and the Canadian Electric Code (CEC) have recognized the requirement for labeling electrical equipment with respect to arc fl ash hazards. Both governing parties have included sections in their respective codes requiring arc fl ash hazard labeling to be present on electrical equipment.
II. Arc Flash Calculations
Th e IEEE 1584 guide for performing arc fl ash hazard calculations was developed for the accurate calculation of incident energy across a wide range of equipment and voltage ratings. Th e purpose of the guide is to provide the techniques for designers and end users to determine the arc fl ash hazard distance and the incident energy to which workers may be exposed to at their work place.
Th e IEEE 1584 document outlines the following process for determining the incident energy values for a facility.
1) Collect System data
2) Determine Modes of Operation 3) Determine Bolted Fault Currents
4) Determine Arc Fault Currents 5) Determine the Clearing Time 6) Document the System Voltage 7) Select the Working Distances
8) Determine the Flash Protection Boundary
Bolted fault currents are the worst case scenario when it comes to system equipment duty. Since a bolted fault should contain little to no impedance, this will result in the highest amount of current fl owing in the system. Typically in the past, protection systems were designed with bolted fault currents in mind. Instantaneous elements were set to trip at a percentage of the bolted fault current, typically 80% or higher. Th is would protect the electrical equipment from damage during these high current faults.
When it comes to arc fault currents, depending on the voltage, the arcing currents may be as low as 80% of the bolted fault current. Th e formulas below illustrate the conditions.
1gIa = 0.00402 + 0.9831gIbf lg is the log10
Ia is the arcing current (kA)
Ibf is three phase bolted fault current (kA) For system voltages >1000V
In the case of a system whose voltage is greater then 1000 volts, the arcing current is only slightly less then the bolted fault current.
In the case of a low voltage system, which is defi ned as a system <1000 volts. Th e arcing current is dramatically lower then the bolted fault current.
1gIa = K + 0.6621gIbf + 0.966V + 0.000526G + 0.5588V(1gIbf) - 0.00304G(1gIbf) lg is the log10
Ia is the arcing current (kA)
Ibf is three phase bolted fault current (kA) K is -0.153 for open confi gurations
Is -0.097 for box confi gurations V is system voltage (kV)
G is the gap between conductors (mm)
Using the above equation, for a 600 volt system, with a bolted fault current of 20 kA and a typical arc gap of 32 mm.
Th e arcing current would be equal to 14.07 kA.
If we follow the IEEE 1584 guide, we are required to take two arcing current values and equate the incident energy at both values in order to take into account system and arc impedance variances. Th e guide recommends the second arcing current value be 85% of the calculated arcing current
found above. In the case of the example the lower arcing current used in our incident energy calculations would be 11.96 kA. With this low arcing current it becomes apparent why the traditional approach to protective coordination is fl awed, if the instantaneous setting of the upstream breaker were to be set to 80% of the bolted fault current.
In the case of an arcing fault in this example, the fault would be sustained for as long as the short time delay or long time delay depending on the protection element upstream.
If we analyze the formula for calculating incident energy below we can see the eff ect of time on the total energy released in a fault.
t 610x E = 4.184CFEn (——) (———)
0.2 Dx E is incident energy (j/cm2) Cf is a calculation factor 1.0 for voltages above 1 kV and, 5.5 for voltages at or below 1 kV En is incident energy normalized T is arcing time (seconds) D is distance from the arc (mm) X is the distance exponent
All things equal in the above equation, incident energy is directly proportionate to arcing time. Th is means if we double the arcing time, we double the incident energy. A typical instantaneous element for a low voltage molded case circuit breaker operates in 0.025 seconds, where as the typical short time element operating time is 0.2 seconds.
Th is would mean that the incident energy in a fault cleared by an instantaneous element versus a short time element would be 8 times lower.
III. Reducing Arcing Time
Th ere are multiple ways to reduce the arc fl ash hazard levels of electrical equipment. Standard overcurrent pro-tection can be used for detecting arc fl ash events, a typical overcurrent scheme set to pickup an arcing event on a main service breaker will have the following clearing time associated with it.
Sampling 15-30 ms Contact closure 5 ms
Time delay setting 30-350 ms Breaker operating time 50-80 ms Total fault clearing time typically:
15+5+30+50 = 100 ms minimum 30+5+350+80= 465 ms maximum
In the event of a single phase to ground arcing fault on a high resistance grounded system the total time to clear the fault may be seconds or more. In addition to the longer clearing times, by setting an overcurrent relay system to be
more sensitive for detection of arcing faults, the reliability of the system may be compromised.
Some systems which are used to start large motors or supply current to larger loads in which the load draws large amounts of current for short durations cause problems in respect to protection settings. In these cases the relays cannot be set low enough to detect arcing faults, since the relays may cause nuisance tripping and reduced reliability.
An option of installing a maintenance switch which would change the setpoints of the relays to lower settings, providing an increased level of safety is an option. However this re-quired the relaying to be solid state or numerical protection which means upgrading from induction disk systems. An additional problem of the maintenance switch being left in the wrong position also comes into play in this option.
Another option for reducing the arcing time for a spe-cifi c switchgear lineup is to implement a bus diff erential protection system. In the case of a fault inside of the bus diff erential zone the total time to clear the fault is quite fast as seen below.
Sampling and contact closure 15 ms Breaker operating time 50-80 ms Total fault clearing time typically:
15+50 = 65 ms minimum 15+80 = 95 ms maximum
Th is clearly time is exceptionally fast, which will dramati-cally reduce the arcing time and hence the incident energy released during an arcing fault.
Th e down side to a bus diff erential scheme is it high price tag and complicated design that goes into this type of scheme. Each incoming supply and outgoing feeder must have properly sized current transformers dedicated to the high impedance bus diff erential relays that are typi-cally used in these applications. It should be noted that the protection zone is limited by the placement of the current transformers.
Th e newest form of protection that is presently being employed, utilizes light sensors which are installed in the equipment typically anywhere energized conductors are located. Th is equipment is specifi cally design to pickup the light from an arcing fault and quickly trip the source of power offl ine. Th e typical arc fl ash relay system is comprised of a master relay which may be used to monitor current, for use as a supervisory function, connected to a module which monitors multiple light sensors. Once the light sensor module detects the presence of a light source, which is present during an arcing fault, the master relay will call for a trip of the source supply. Th e total time to isolate the supply is shown below;
Light information 1 ms O/C comparison (if used) 1 ms Contact closure 5 ms
Breaker operating time 50-80 ms
Total fault clearing time typically:
1+1+5+50 = 57 ms minimum 1+1+5+80 = 87 ms maximum
Th e additional benefi t of this system is that it can also detect for phase to ground faults of a low magnitude on a high resistance grounded system.
By comparing the total clearing time of these three op-tions, we can see that the arc fl ash relay operates the fastest, and also provides the most fl exibility and reliability.
IV. Arc Flash Protection
Th ere are only two known manufacturers of arc fl ash pro-tection equipment currently in the world. One manufacturer of this equipment is Vamp from Finland and the other is ABB. Th e equipment listed below is manufactured by Vamp since I have a thorough knowledge of the designs.
Dedicated arc fl ash systems are designed to trip only for arcing events in the protected equipment. Th is is accom-plished using light sensing probes which are mounted inside of the protected equipment. Currently there are two styles of sensors utilized. Th e fi rst being a fi ber optic sensor which detects light on its entire length minus the fi rst 2.5 meters.
Th e fi ber is designed to be installed inside of the supervised compartments. Since the sensors are made of a glass fi ber it is non-conductive and very cost eff ective. When protecting a low voltage motor control center, the fi ber sensor is the best option. Th e fi ber sensor is connected at both ends to a slave device which communicates its information back to the master device which operates its output contacts based on the data it gathers from the fi eld. From the slave device the fi ber sensor is “weaved” through the motor control center’s compartments anywhere a potential fl ash over may occur.
Th e fi ber sensor is continually self supervised.
Th e second style of light sensor uses a single point to monitor for light. It takes the light input and transforms it into a current signal which is relayed back to the slave unit.
Each compartment in which a fl ash over may occur would have its own point style sensor installed.
Th is style of sensor gives the user the ability to know which compartment the fl ash over took place in, since the eff ects of a fl ash over are sometimes not obvious when the interruption time is very short. Th is sensor, like the fi ber sen-sor, is continually supervised by the relay for error detection and reports if a sensor has failed it diagnostic check.
Th e sensitivity of the point sensor can be seen below. Th e sensor is capable of viewing the light produced by an arc in a full 180 degrees from the face of the sensor. Th is allows it to see the entire compartment for which it is installed.
Th e master relay used in this application is used to de-termine whether or not the output contact should operate based on the information it is receiving from the slave devices which are usually mounted nearest the equipment being protected. Th e master unit is also capable of monitor-ing the current fl owmonitor-ing in the system and it can use this in-formation to supervise the operation of the output contacts.
Th is current supervision is used to limit the chances of a nuisance trip due to errant light being picked up by the light sensors. Since the light sensors will operate if they pickup an intense enough amount of light, they may call for a false trip if a bright fl ashlight, camera fl ash or direct sunlight is shone onto the pickup point of the sensor.
When current supervision is used, the relay will restrain the output contacts from operating until the minimum cur-rent pickup setting of the relay has been satisfi ed.
Th e output contacts of the Vamp arc fl ash relay are ca-pable of operating in 7 ms or less after the inception of a fl ash over. Since the relationship between incident energy and time is directly proportionate, the sooner the arc can be extinguished the lower the total energy released would be.
Real world installations of this equipment have been able to lower the available incident energy from several hundred calories per square centimeter, down to less then 10 calories per square centimeter. Th is allows for safer operation of equipment for switching, maintenance or troubleshooting.
V. Conclusions
Fast clearing time of arcing faults is extremely important due to the relationship of time and incident energy. By using arc fl ash relay systems, reliability is maintained while lower-ing the arc fl ash hazard levels of equipment. Th is optical arc fl ash technology has been in service for upwards of 10 years globally with a proven track record.
Th e use of this equipment has made servicing and op-erating of equipment a safer task. Without it, people may not be able to safely operate the equipment due to high arc fl ash hazard values.
VI. References
[1] IEEE 1584-2002, guide for Arc Flash Hazard Calcu-lations.
[2] NFPA 70E-2004, Standard for Electrical Safety in the Workplace.
[3] VAMP 221 arc protection system operation and con-fi guration instructions.
Chris Gingras has seven years of experience in start-up, commission-ing, maintenance, and design of electrical power and controls equipment rated 120 volts up to 365 kV. He is a NETA Certifi ed Level III Test Technician. Chris is currently the Technical Services Manager for a division of Magna Electric Corporation.