Detailed information for grounding specifications can be found on the ASIC website:
                                                 http://www.asic.org/Design_Guides.aspx                

All of Rain Master controllers come equipped with lightning protection built into each product. In some parts of the world however, the occurrence of lightning and its effects are much more prevalent and severe. The -T option (transient series) of Rain Master controllers incorporate additional components to provide increased protection to lightning-induced transients. The -T option is strongly recommended for lightning prone areas to avoid transient related down time to the equipment. The following discussion explains these capabilities as well as the proper installation of the controller to take advantage of the added protection.

Characterization of a lightning strike. The typical lightning strike is comprised of three strokes, with the first stroke surging up to 20,000 amps lasting approximately 30 millionths of a second. Subsequent strokes are about half the intensity. Although theoretically possible, it is economically unfeasible to protect a controller against a direct strike. The protection would cost many times the value of the equipment it is protecting. Lightning-induced transients are electrical impulses induced in a conductor simply by being in the vicinity of a lightning strike. These transients may travel considerable distances from the physical strike location especially when conductors are present. It is these potentially harmful voltages and currents that we attempt to suppress. The closer the strike to the equipment, and the longer the attached conductors (AC line and field valve wiring), the larger the induced transient.

The voltage from a lightning strike rises very fast, typically to its peak in a few millionths of a second. This energy must be returned to earth as quickly as possible, through a low impedance path, otherwise damage to the electronics will occur. At this speed, the inductance of the ground system is much more important than its DC resistance. Increasing the surface area of the conductors and decreasing their length reduces inductance in the ground system. The build-up of electrons on the ground system also must be dissipated to prevent dangerous voltages in the equipment. The ground system must therefore be of sufficient size to absorb the energy.

Necessity of a ground system. In order to neutralize the harmful transient energy, it must be returned to earth ground before the harmful energy can enter the equipment. The protection components incorporated in any manufacturers' controllers rely completely on the external ground path provided during the installation of the controller. Therefore, it is absolutely imperative that a good grounding system be installed. The ground system will consist of one or more properly installed ground rods connected through appropriate means to the controller.

The safety (green wire) ground from the electrical service is NOT suitable for a lightning ground. The length of this wire, its relatively small diameter, and its questionable path and connection to earth ground make it unsuitable for protection from lightning induced transients. The only acceptable grounding for lightning protection is a properly installed and connected ground rod system.

Characterization of the ground system. A single 5/8th inch diameter by 8-foot copper clad ground rod, properly installed, is usually sufficient. The rod must extend beyond the frost line, and into the water table, if possible. The rod should be driven in, not installed in an augured hole. You may core through a concrete pad or foundation. It should be located as close as possible to the controller, so that the #8 (or better) ground wire is as short and direct as possible. The ground clamp inside the controller should be used as the ground point. Be sure to remove the oxide from the copper clad rod before attaching the ground clamp to the rod.

The conductivity of the soil varies greatly with soil type and moisture content. Sand may have 100 times the resistance of clay. In low conductivity soil, such as sand, two or three interconnected ground rods should be used. Spacing should be greater than the sum of their lengths apart. Connection between the rods should be made with copper strap, with a width of at least one percent of its length, e.g. with two 8 foot rods spaced 20 feet apart, the width of the strap should be about 2-1/2 inches. The straps should be connected to the rods with approved clamps and buried in the soil. Where extremely rocky soil prevents the proper installation of driven rods, horizontal radials make an excellent lightning ground. Using #10 gauge non-insulated wire, install four or more radials in different directions buried at least 6 inches or more. Each radial should extend for least 30 feet. The deeper and longer the radial the better the ground. The ground system should be inspected every year for corrosion and loose connections, preferably before the beginning of the storm season.

The resistance measurement of the ground system shall be no more than 10 ohms in order for the electronic protection of the controller to be effective. The lower the reading the better. These measurements can be performed using ground resistance test instrumentation. In the event that the resistance readings are greater than 10 ohms, additional measures may be taken such as the introduction of chemical salts to improve the soil conductivity. The salts however require renewal on a yearly basis to retain their effectiveness.       (ASIC Link to Guidelines)

         http://www.asic.org/uploads/assets/011007_121320_ASIC_GROUNDING_GUIDELINES.doc

Rain Master triple protection. The Rain Master controllers with the -T transient option employ three-stage protection. Looking from the field wiring side, where transients may enter the equipment, the first device is a gas tube suppressor with a breakdown voltage of 90 volts DC, and a surge current rating of 20,000 Amps. Next, a series inductor of 100 microhenries provides high impedance to the fast risetime pulses. Finally, a high-speed solid state transient suppressor with a breakdown rating of 73 volts at 1.5 kilowatts absorbs the remaining transients. Both of the suppressors are returned to earth ground through the circuit board mounting arrangement. The output triacs are rated at 400 volts and are protected against transient turn-on with a RC snubber network.

The common understanding that lightning never strikes twice in the same place is totally incorrect. Lightning follows physical laws, namely, the path of least resistance whenever it hits. Tall conductive structures are the primary targets. The Rain Master protective electronics are designed to handle repeated induced transients by protecting the sensitive electronics without suffering catastrophic failure. The controllers have been laboratory tested at special facilities designed to simulate high intensity lightning transients. When properly installed, the Rain Master controllers with the (-T) heavy-duty lightning and surge option provide years of maintenance free protection.