Author: Stephen Frank
September 18th, 2022
Arc flash is one of the most dangerous hazards encountered by electrical workers. This article will look at how current limiting fuses can be used to reduce the severity of arc flash events.
Disclaimer: This article is for educational purposes only. Do not use or reference the information posted in this article as an alternative to official codes and standards.
Definitions
Arc Flash Hazard: A source of possible injury or damage to health associated with the release of energy caused by an electric arc.
Arc Flash Boundary: When an arc flash hazard exists, an approach limit from an arc source at which incident energy equals 1.2 cal/cm^2 (5 J/cm2)*.
Current Limiting Fuses: A specialty fuse designed to clear fault current in less than one half cycle in its current limiting range.
*A significant thermal hazard is one with an incident (thermal) energy of 1.2 cal/cm^2 or more, which is considered to be the energy level necessary for the onset of a second-degree burn.
Case Scenario
In this case scenario we will review an arc flash study that my team performed in the past and discuss how we were able to implement current limiting fuses to successfully reduced the incident energy below the 1.2 cal/cm^2 threashold.
Unfortunately we will not be able to share specific proprietary information such as the detailed arc flash study or the electrical one-line but we can at least review the math behind the calculations before and after the current limiting fuses were implemented.
Determine the Arc Fault Current
This article is not intended to be a detailed explanation on how to perform arc flash calculation, but here are the general steps as outlined in IEEE 1584
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Collect the system and installation data
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Determine the system modes of operation
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Determine the bolted fault currents
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Determine the arc fault currents
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Find the protective device characteristics and the duration of the arcs
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Document the system voltages and classes of equipment
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Select the working distances
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Determine the incident energy for all equipment
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Determine the flash-protection boundary for all equipment
Before we go any further we need to clarify the distinction between available fault current, bolted fault current, and arc fault current.
Available Fault Current (available short-circuit current): This is the fault current available at the supply. You typically obtain this value from the utility, or if necessary you can estimate it based on the size and impedance of the supply transformer.
Bolted Fault Current: The bolted fault current is not the same as the available fault current at the utility connection. The bolted fault current is the result of the available fault current, after taking into account system impedance such as cable runs and the X/R ratio at each point of concern. EEE 1584 only has a basic calculator for estimating bolted fault current, which for the most part will not be sufficient. This is where commercial software such as ETAP can save many many hours of time. These programs can run through hundreds of different possible scenarios to determine the maximum and minimum fault currents at all locations on the distribution network.
Arc Fault Current: The arc fault current is primarily dependent on the bolted fault current, but is different in that it is the actual current flowing through the arc itself. The arc fault current will always be lower than the bolted fault current due to the impedance of the arc (see image below).
In this case scenario we are primarily concerned with the Arc Fault Current. This is because our goal is to mitigate the arc flash incident energy. During an arc flash event it will be the arc fault current flowing through the fuses, not the available fault current, and not the bolted fault current. It is important to make this distinction because the arc fault current will be quite a bit less than the other two fault currents and it’s very important to select a current limiting fuse where the arc fault current is sufficient enough to activate the current limiting range so the fuse melts in the first quarter cycle, and clears the fault completely in the first half cycle.
Resulting Arc Flash Incident Energy (Without Current Limiting Fuses)
From the tables below the resulting faults currents are as follows:
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Available Fault Current: 56.74 kA
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Bolted Fault Current: 39.74 kA
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Arc Fault Current: 19.87 kA
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85% Arc Fault Current*: 16.89 kA
*Informational note: Two arcing currents are used to account for arc current variations on determination of clearing times, from which you use the higher of the two incident energy results.
The resulting incident energy is 4.43 cal/cm^2. This is higher than the 1.2 cal/cm^2 threshold and thus poses a hazard to the electrical workers. Arc flash PPE would be required.
Selection of Current Limiting Fuses
Now we need to identify a current limiting fuse that we can install in our system that will activate the current limiting range at a minimum value of 16.89 kA.
Informational Note: When selecting current limiting fuses, keeping in mind that the fuses still has to meet all the normal rating requirements such as voltage, full load current, interrupt rating, etc.
For our purposes we will be selecting Littelfuse PSR Series, 2000A fuses
For more details on current liming fuse selection refer to the following application notes:
Reducing Arc Energies with Current Limiting Fuses – By mike lang, Principal field engineer
Resulting Arc Flash Incident Energy (With Current Limiting Fuses)
The addition of current limiting fuses servers two important functions:
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It reduces the arc fault current from 19.87 kA down to the RMS let-through current, which for our 2000A PSR series fuses that we selected will about 13 kA.
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It reduces the fault clearing time from 0.065 seconds of the original circuit breaker down to 0.008 seconds (1/2 cycle).
We can now reperform the arc flash calculations using the updated values of 13 kA arcing current and 0.008 seconds fault clearing time.
Summary
The results are considerable. The incident energy reduced from 4.43 cal/cm^2 down to 0.34 cal/cm^2 simply by adding current limiting fuses to the system.
This is possible because these specialty fuses perform two key functions by both reducing the peak let-through current and shortening the fault clearing times.
Informational Note: Incident energy is proportional to time and the square of current (time*current^2).
The most important takeaway from this article is that we were able to mitigate the arc flash hazard for the electrical worker so that they don’t have to rely on just their PPE to protect them.
Hazard elimination shall be the first priority in the implementation of safety-related work practices (NFPA 70E Article 110.1)
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