Reducing personal injuries from arc flash events should be the main goal of any arc flash hazard assessment. Many facilities have locations where the arc flash energy levels are extremely high. These dangerous areas require electricians and technicians to wear heavy layers of personal protective equipment (PPE) when working on energized equipment. Unfortunately, in some areas, this PPE can increase the chances of heat stroke and other heat-related problems.
This article will discuss how arc flash energy levels are calculated and what factors affect these calculations the most. There are several design options, retrofit methods, and work procedures you can implement to reduce the arc flash energy level, including the use of fuses, relays, and circuit breakers. Lowering the arc flash energy level will decrease equipment damage and increase equipment and personnel protection.
Design engineers have a few options to reduce system voltage or fault currents. However, the best and most direct way to reduce arc flash hazards is to reduce fault-clearing times.
Arc flash energy can be estimated or calculated by using tables found in NFPA 70E, equations found in IEEE Standard 1584, or equations developed by Ralph Lee (for use in locations greater than 15kV). Lee's equations are based on his method given in a 1982 IEEE IAS Transactions paper, “The Other Electrical Hazard: Electric Arc Blast Burns.” These methods determine the arc flash hazard incident energy levels and arc flash boundary areas. For this article, we will use the IEEE 1584 equations to calculate energy levels.
The arc flash energy equations found in IEEE 1584 use many variables, including equipment bolted fault current values, arcing fault current values, and upstream protective device clearing times. The calculations are also dependent upon factors such as operating voltage, gap length, and type of grounding. These last three items are very difficult to change in an existing facility. The greatest effect on the calculations is time the upstream device operates, followed by working distance, and then available fault current.
Figures 1, 2, and 3 show just how much time and distance can affect the energy calculations. In Figs. 1 and 2, the working distance is held constant at 18 inches. Only in the last graph (Fig. 3) is the working distance varied. Each graph shows the arc flash energy level on the vertical axis (blue line). The numbers on the red line show the NFPA 70E hazard risk category (HRC) level.
In Fig. 1, you can see the effect of raising the fault current when the device operating time is held constant at 0.025 seconds. This is the typical clearing time of a molded-case circuit breaker tripping instantaneously. As the fault current is raised from 5,000A to 90,000A, the HRC varies from 0 to 1. Therefore, if the device is tripping quickly, raising the fault current does not significantly affect the HRC level. The energy level only rises slightly.
In Fig. 2, you can see the effect of changing the device operating time from 0.025 seconds to 0.5 seconds (0.5 seconds is the longest short time delay setting available for breakers with solid-state trip units). For these calculations, the fault current is held constant at 30,000A. This graph clearly demonstrates that the device operating time has a major impact on the calculations. The HRC changes from 0 to 4, and the energy levels approach 40 calories per centimeter squared.
Figure 3 shows a plot of personnel working distance versus incident energy level. The fault current and device operating time are held constant while the working distance is raised from 18 inches to 225 inches. This graph shows that increasing the distance between the worker and the fault location reduces the energy.
In the real world, it's important to remember that changing fault currents could affect the upstream device operating time. If the fault current is reduced below the breaker's instantaneous setting or fuse current-limiting threshold, then longer delay times can be expected. Usually, the net effect of a lower fault current and longer device operating time is a higher energy level. This occurs quite often when equipment downstream of an automatic transfer switch (ATS) is fed by a generator and normal utility source. The energy levels of this downstream equipment can be higher when on generator power than when fed by utility power. This is because the upstream protective device operates slower due to the generator's reduced fault current. The time current curve in Fig. 4 shows how the reduced fault current can affect the way the protective device operates. This demonstrates the importance of performing the energy calculations and why it's important to know all sources and levels of fault current.
Improved safety through design
Reducing the fault current can have an effect on the energy calculations, but it's extremely hard to reduce at an existing facility. It is usually done during the design phase of a new facility. Several ways to reduce this fault current are the use of several smaller transformers and unit substations versus one large one. This also increases the system reliability by having several substations feeding equipment instead of one. Installing tie breakers will further increase the reliability should one of the substation services fail (the bus tie breakers must not be closed unless one of the main breakers is open).
Another method of reducing the fault current is the use of current-limiting reactors. These devices have been used in many facilities to reduce the fault current to levels below the equipment short-circuit ratings. Although this is a costly method, there may be other reasons for implementing this option.
The best way to reduce arc flash energy is to clear the arcing fault as quickly as possible without sacrificing coordination. Current-limiting fuses operate extremely fast (if operated in the current-limiting range), reducing arcing current and energy levels. They are simple to install and have very little maintenance requirements. However, if the line side is energized, the electrician will need to wear appropriate PPE (based upon the line side energy calculations) to install a fuse that has blown.
When using circuit breakers, there are many options available to reduce the arc flash energy level. Using simple current-limiting breakers that will operate in the instantaneous region and current-limiting region will reduce the energy levels. In addition, using breakers with solid-state trip units that have adjustable long, short, and instantaneous settings is an effective method for reducing arc flash energy. Figure 5 on page 54 shows the adjustable trip functions for a breaker with a solid-state trip unit. This is achieved by setting the short time pickup and delay as low as possible but coordinating with the largest downstream load. This ensures that the breaker will still trip quickly if the arcing fault current falls below the breaker instantaneous setting.
Another way to reduce the arc flash energy at locations where solid-state trip units are being used is to order the trip units with a zone interlocking feature. This feature adds communication between the main, tie, and feeder breakers. If a fault occurs downstream from the feeder breaker, a “restraint” signal is sent to the main and tie breakers to time out using their normal programmed LSI trip settings. If a fault occurs between the main and feeder breakers, a “no restraint” signal is sent to the main and bus tie breakers. They will then trip at a very low pickup and time delay.
Another similar method is to employ differential protection. Zones of protection are set up using differential relays and current transformers. If a fault occurs within the zone, the relay trips at extremely low pickups and time delays. This again greatly reduces the arc flash energy. Some manufacturers are now offering this option on low-voltage switchgear using breakers with solid-state trip units.
Some switchgear manufacturers have employed arc flash venting methods in their design. Should a fault occur, the flash energy is directed through a vent and outside to a safe area. However, this option can be expensive and should only be employed at main substations where the fault currents (and arc flash energy) can be extremely high.
Another interesting way that arc flash energy has been reduced is by the use of flash sensors. The sensors are installed within the switchgear and will trigger if a light flash from a fault (or other source) occurs. These devices then trip the main and tie breakers.
Relying on retrofit methods
You can reduce the incident energy level by replacing older non-current limiting fuses with modern fast operating fuses with faster clearing times. Even replacing expulsion-type fuses with current-limiting fuses have been shown to be beneficial.
Another simple method is to reduce the fuse ampere size. Many circuits in a distribution system are under utilized. If a 400A fuse is feeding loads that draw only 200A, then the 400A fuse can be reduced, which will increase the chances that the fuse will operate in the current-limiting region for an arcing fault. However, it's important to check the inrush current of motors and transformers and verify that the fuse will not be damaged when these devices are energized.
As previously discussed, device operating time has a great effect upon the energy levels. Therefore, reducing the trip times to as low as possible without sacrificing selective coordination is very important. For double-ended unit substations or switchboards, it's important to reduce the main settings to coordinate with the largest feeder breaker not with the tie breaker. This is because the tie breaker is rarely used and is there only for emergency or maintenance conditions.
One of the least expensive ways to reduce arc flash energy is the modification of work procedures. The best and safest way is to de-energize the equipment. Although this may seem obvious, it's not always implemented. Before working on energized equipment, personnel should consider all possible ways to shut the equipment down. Although this may require working on the equipment during non-traditional working hours, it is definitely the safest method.
If de-energizing the electrical equipment is not an option, then consideration should be given to the three items below, with the last item being the highest priority.
- Lower the fault current
- Increase the work distances
- Reduce device clearing (trip) times
The fault current can sometimes be lowered by the elimination of paralleling of transformers. In many facilities, double-ended substations or network faults are paralleled during normal periods. If the equipment is going to be worked on while energized, these paralleled transformers can sometimes be shut down, thus lowering the available fault current.
A facility should always eliminate personnel working between a transformer secondary and the downstream secondary (main) breaker when energized. This is the most dangerous (and usually the highest arc flash energy area) location in a distribution system. The arc flash energy level is much higher at this location because the primary device is seeing a reduced (when calculated in primary amperes) fault current on the secondary side. This lower fault current seen by the primary device will cause it to trip at a longer delay. This longer delay increases the arc flash energy level.
As the discussions stated previously, time is important when considering arc flash energy calculations. Many modern solid-state trip units and protective relays are now employing group settings or maintenance settings, which enable the owner to program lower pickups and faster trip times. An external switch is used to instruct the relay or trip unit to use the lower settings. Before work is performed, the electrician simply toggles the switch to the maintenance position. Should a fault occur, the protective device will pick up and trip quickly, thus reducing the arc flash energy level.
Putting distance between the hazard and the electrician is also beneficial. One way to reduce the hazards during infrared surveys is to use infrared windows. These windows are mounted at important locations that need to be inspected by infrared equipment. These windows allow the technician to scan the location to look for hot spots (loose connections) without removing covers and doors.
One of the most hazardous tasks to perform is racking in or out breakers. Many documented accidents have occurred during this operation. Recently, several manufacturers have been marketing remote breaker racking devices. These devices allow the operator to stand to the side while the machine racks in the breaker. Increasing this working distance reduces the arc flash energy at the operator's location.
We have looked at several ways to reduce the arc flash energy in a facility. Arc flash energy can be reduced by (in order of effect): decreasing the trip times; reducing fault currents; and increasing the worker distance.
There are many methods and techniques that the technician or engineer can use to reduce the incident energy levels. An electrical engineer familiar with the requirements of NFPA 70E and IEEE 1584 should analyze each location to see which solutions are most cost effective and beneficial. Working on the equipment when de-energized is the safest solution, and it can be implemented at any location.