14.6 Blast Pressure Calculator (LEVEL 2)
Figure 14.19 Blast pressure calculator
The arc blast pressure calculator theoretical formulae developed by Ralph H Lee, published in his paper “Pressures Developed by Arcs” in 1987. There is much debate about how Lee’s theoretical formulae can be interpreted in the real world, but this is still widely cited by engineers when calculating the severity of the blast hazard. More information is given in Chapter 2: What is Arc Flash. This calculator uses his formulae to give an initial blast pressure figure. As can be seen in the screen shot of the calculator, the following details the data entry values that will be required.
Arcing current (kA)
Enter the calculated arcing current in kA. (Not the prospective short circuit current)
Working distance (metres)
Enter the working distance which is the distance from the prospective arcing source to the person’s torso.
The initial blast pressure is given in Kilopascals, lbs/ft2, n/m2 and Atmospheres all of which can be selected via the dropdown list.
14.7 DGUV Box Test Algorithm Calculator (LEVEL 1)
This calculator in based upon the DGUV-I-203-5188 E Guide to the Selection of Personal Protective Equipment for Electrical Work. It is derived from the standard IEC 61482-1-2: Live Working - Protective Clothing Against the Thermal Hazards of an Electric Arc Method 2: Determination of arc protection class of material and clothing by using a constrained and directed arc (box test). Whilst the DGUV guide is used in parts of Europe for low voltage utility applications, I would recommend the use of IEEE 1584 for industrial and commercial purposes. There is more information about this standard and the DGUV Box Test Algorithm in Chapter 7: Protection which gives a detailed comparison to IEEE 1584 and Chapter 16: Rules, Codes and Legislation.
DGUV Calculator Notes
This should be read and applied in accordance with the German publication DGUV -I- 203-5188 E Guide to the Selection of Personal Protective Equipment for Electrical Work.
Requirements for use
Instead of creating an incident energy level in cal/cm2 the calculator will produce an arc power based upon the physics of the arcing current.
Figure 14.20 DGUV Box Test calculator
Voltage – DATA ENTRY
The three-phase line to line voltage in volts.
Maximum & Minimum Short Circuit Current (I”k3max & I”k3min) in kA – DATA ENTRY
A maximum and a minimum short circuit current is required in accordance with German standard VDE 0102:2016 short-circuit currents in three-phase AC systems. This is required in order to calculate a minimum short circuit current in order to determine the minimum arcing current. The standard is based upon DIN EN 60909 Short-circuit currents in three-phase AC systems. The standard DGUV -I-203-078- 5188e states that “calculation software is usually available for this process”. This is to obtain maximum and minimum prospective three phase initial short-circuit AC currents I”k3max and I”k3min. However, for single circuits at low voltage, the minimum and maximum short circuit currents are given in this guide in the prospective short circuit current calculator.
R/X ratio Data Entry
An R/X ratio is required which is not always easy to establish. However, the R/X can be found from the prospective short circuit current calculator as a reciprocal of the X/R value for low voltage circuits fed by a transformer.