to be effective, an earth stake must be used to entice the currents up the stake, through the GDT and around the decoder.
Voltage produced in the earth from a lightning ground strike of 25,000 amps versus distance from the strike and ground resistivity
Current (A) 50,000 V/10ft
Distance (ft) Distance (m) Resistivity
Ohms-c
325 100
3,000 cm 10,000 20,000
Figure 3 illustrates this principle of operation.
Note that the solenoid must be physically near the decoder. Most manufacturers quote a few tens of feet. Where further distances are required, another bypass using GDT and earth stake is required adjacent to the solenoid.
When a manufacturer specifies that earth stakes are only required every ‘x’ feet or not in every valve box, this is a marketing compromise. Please see the spreadsheet later on in this article to note that voltage differences of several thousand volts may be present over just a few feet.
Incidentally, this bypass technique makes high currents more likely in the main cable, thus putting it and its cable joints at greater risk.
An alternative ‘floating’ technique relies on the construction of a voltage barrier within the decoder that can withstand typically 10KV (ten thousand volts) and limit currents through the decoder to an amount that can be withstood by the Printed Circuit Board tracks without damage. Because the voltage surge only lasts a few millionths of a second, this technique is practical to use in a decoder. However, this technique is not easy to implement on decoders with advanced features; for example, systems using solenoid current reduction to allow 10’s of stations to be active at once. This latter type of decoder relies on the previously mentioned earth stake technique.
50,000 100,000 226,299 3,482 754,330 11,605 1,508,660 23,210 3,771,649 58,025 7,543,299 116,051 200,000 15,086,597 232,101
With this floating technique, it is not necessary to limit the distance between decoder and solenoid. In many installations this can be 1000ft with no significant diminution of lightning protection to either decoder or solenoid.
Peak currents in the main cable are usually smaller than with the earth stakes method, thus it is less likely to be damaged.
An illustration of the extent of voltage gradients:
When an earth stake needs to be connected to the decoder for lightning protection, it is important to keep the stake physically near the decoder. The above chart gives figures for what happens when the stake is remote from the decoder.
The following notes explain the various items in the table.
The main table indicates the voltage to which the earth rises at a distance from the ground strike of amperage given in cell B1, here illustrated at 50,000 amps.
The yellow figures are feet from the strike; the green are metres from the strike.
The black figures are volts above normal earth potential during the strike.
The magenta figures are the resistivity of the earth (electrical resistance to currents). Typical resistivity figures are given in the small table below the main table.
The blue figures between the black, represent the voltage difference between two points 10 feet (3m) apart.
Figure 4 illustrates this principle of operation.
With the floating technique, no earth stakes are needed around the cable path or next to each decoder. Nevertheless, it is important to install just one earth, next to the controller, which must be separate from the building earth system.
90
Thus, if a decoder is earthed though its stake and its solenoid is 10ft away in the direction of the strike, this figure will show the voltage difference between the solenoid and its decoder.
Example:
20,000 Ohms-cm, 1300 ft from the strike of 50,000 amps, the earth will rise to a
7,543,299 77,367 3,771,649 38,684 5,028,866 38,684
Resistivity Ohms-c water %wt 2.5% 5%
10% 15% 20% 30%
cm Topsoil
250,000 165,000 53,000 19,000 12,000 6,400
Sandy Loam 150,000 43,000 18,500 10,500 6,300 4,200
potential of 377,165 volts above normal.
A solenoid mounted 10ft away from its decoder will see a voltage difference in its surrounding earth of 2,321V compared to that at the decoder.
It is important to note that the above figures are simplified to assume a constant earth resistivity with soil depth. If the upper layer of soil is of a fairly low resistivity (topsoil) and the underlying is high (e.g. bedrock) the voltage stresses are much higher as the lightning currents are concentrated into the upper layer, producing much higher voltage gradients. This means that a strike much further away will produce destructive voltages than with other geologies.
Testing Lightning Protection:
All practical lightning protection schemes are a compromise, where the manufacturer pitches the degree of protection and its cost to the product against the total number of failures encountered over a large number of installations.
To make a decision on where to be on the curve of cost versus number of failures/year, pre-testing of the product is mandatory. Unfortunately, no worldwide test standards are in place that are applicable to the conditions encountered in buried cable irrigation systems. Moreover, the type of equipment necessary to produce a simulated voltage is rare.
The picture depicts a custom-built test generator used by the author.
This produces 8,000-10,000 volts at between 30,000-40,000 amps.
Several iterations of a new controller and decoder design are usually necessary before the product can be released for
1,885,825 19,342 2,514,433 19,342 3,771,649 23,210 754,330 7,737 1,257,216 9,671 1,885,825 11,605 3,017,319 15,473 377,165 3,868 502,887 3,868 942,912 5,803 1,508,660 7,737
650 200
113,149 1,161 251,443 1,934 377,165 2,321 754,330 3,868 V/10ft
975 300
75,433 580 188,582 1,161 301,732 1,547 V/10ft
1300 400
56,575 348 150,866 774 V/10ft
1625 500
45,260 232 V/10ft
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