Design Settings
Overview
Design Settings is generally the first step in the design process. In this module, you will set the following.
Soil model - enter it explicitly or specify to use the Soil Model module.
Grid energisation - enter the earth fault current level or specify to use the Fault Current module.
Define the Conductor types used for the design.
Soil characteristics
The multilayer soil model can either be determined from field measurements using the soil modelling module or explicitly specified.
Specifying the soil model
This approach is useful for when the soil resistivity model is already known. Select the option Enter below from the Design Settings screen. Enter the Number of layers. To specify uniform soil make the Number of layers equal to 1.
The computation time of grid resistance and soil voltages under Display Results increases with a greater number of soil layers. It is common to use from 3 up to 7 soil layers.
Use Soil Model module
When this selection is made the multilayer soil model is taken from the Soil Modelling module as calculated from field resistivity measurements.
If the Soil Modelling model is not entered correctly or if the model is invalid then SafeGrid will revert to using the data in the manual entry fields.
Frequency dependency method (hf) (Professional edition)
Soil electrical resistivity is one of the most important parameters to consider when designing safe earthing systems especially when analysing behaviour during lightning strikes.
As frequency increases soil effective conductivity increases (resistivity decreases) while effective permittivity decreases, caused by soil ionisation at the high frequencies.
For more information refer to this technical article about soil electrical resistivity frequency dependency.
Grid energisation
Use Grid Energisation to specify the current injected (real and imaginary) into the earth grid or the voltage (GPR) of the earth grid.
Sinusoidal mode
Current or Voltage energisation
Current (A): Specifies the current(s) injected into the ground electrode, in amperes. The value can be specified explicitly by selecting the Enter below option, or the program deduce the amount of fault current discharged in the main grounding grid from the fault current distribution calculation (the option called Use Fault Current module).
Multiple faults can be defined and applied in Build Grid. For an example of where applying multiple faults can be useful, refer to the tutotial on simulating a fall of potential test.
If the results of the fault current calculation are not available for any reason, the program reverts to using the specified value.
Voltage (V): Specifies the GPR of the Main-Ground electrode, in volts.
Voltage as grid energisation setting: In certain circumstances (typically for weak LV or MV network connections) it may be appropriate to use voltage energisation rather than current and to specify that voltage energisation as equal to the r.m.s. phase-to-neutral or phase-to-earth supply voltage. Using voltage as the grid energisation setting ensures the maximum voltage of the grid (Grid Potential Rise) is equal to that voltage energisation setting.
Maximum grid current: The maximum grid current is equal to the maximum grid current multiplied by the decrement factor (d.f.) for the entire duration of the fault. Obtain the maximum prospective grid current (Ig) then calculate the decrement factor (d.f.) using the Safety Criteria module under Advanced finally multiply Ig and d.f. and enter into Grid Energisation under Design Settings.
Reducing maximum grid current: Reducing the maximum grid current can be a simple and effective way at improving the performance and hence the safety of an earthing system. Reduce the maximum grid current by considering that the impedance of the earthing system itself will be in series with the prospective earth fault. Calculate the system (source) impedance add to this the grid impedance and re-calculate a new earth fault current to be used for determining touch and step voltages.
System frequency: Is a variable input, limited up to 400 Hz for Basic or Standard editions, up to 10 MHz for the Professional edition.
Transient mode (Professional edition)
Use the Grid Energisation frame to input a waveform of which the response will be calculated.
Current (A) or Voltage (V): Specify the unit of input waveform.
Waveform: Select one of the built-in input waveforms or upload custom waveform from spreadsheet. The response of the input waveform will be calculated later.
• IEC 62305 function: The function defined in the IEC standard is same as the Heidler function. The difference is that the steepness factor is a constant in the IEC standard. The correction factor, the peak current, the frontal and tail time constants are defined with respect to the following impulse shapes and lightning protection level.
a. Impulse:
i. First positive impulse (10/350 μs)
ii. First negative impulse (1/200 μs)
iii. Subsequent negative impulses (0.25/100 μs)
b. Lightning protection level:
i. LPL I
ii. LPL II
iii. LPL III - IV
• CIGRE function: The CIGRE function has been introduced in section 3.9 of CIGRE technical brochure No. 063. The inputs to this method are the peak current (kA), the frontal time (μs), the decay time (μs) and the steepness of the wave (kA/μs). There is a lower limit to the value of the decay time which is calculated from the frontal time.
• Heidler function: The inputs to this method are the peak current (kA), the frontal time (μs), the decay time (μs) and the steepness factor. The decay time must be greater than 1.135 times the frontal time.
• Double-exponential function: The inputs to this method are the peak current (kA), the frontal time (μs) and the decay time (μs).
• Custom from spreadsheet: The data in a tab delimited file can be imported into SafeGrid and used for calculations. The file formats can be any of the following:
a. Only current values: The file will have only 1 column which contains the values of current. The additional input required in the user interface for this type of file will be the sampling time (μs).
b. Time and current values: This file will have 2 columns, time and current. The time may or may not be uniformly sampled.
Conductor types
Multiple conductor types can be defined here and can be selected in the Build Grid module. The conductor types and their respective minimum size calculations are also added in the reports under Display Results.
By default there are two conductor types defined and these are the same size (mm2) and material. The Description and Size can be changed by clicking the appropriate cell inside the table.
The Material can be selected by pressing the Calculate Minimum Size button which opens a popup to the Grid Conductor Size Calculation window.
The conductor material type will affect the final grid performance results.
Minimum size and fault rating is calculated in accordance with the IEEE Standard 80 method and used for confirming the specified sizes meets the fault rating requirements.
Measurement units
Select your preferred measurement units and they can be saved as default for new projects.
Decrement factor
The decrement factor is used to increase the fault current to account for the asymmetrical current component associated with short duration faults. The effect of the decrement factor is significant for a fault clearing time of ≤ 0.5 seconds.
The decrement factor used can be a computed value based on the system X/R ratio.