Electrical safety testing is crucial to making sure the operating standards for electrical equipment are up to date and in compliance with safety standards. There’s a reason why there are agencies and parts of government that have put in the work to develop stringent requirements for electrical products that are sold Australia wide. There are various different tests being conducted to ensure the safety of products, and one of them is earth resistance testing. Earthed appliances are potentially the most dangerous ones, and they’re classified as Class I. This class of appliances includes microwaves, ovens, bench grinders, extension leads, etc. All of these appliances have a connection to the ground through an earth conductor, and the goal of this connection is to ensure a low resistance path from an electric current to pass in order to protect the equipment, but more importantly, people using the equipment. If this conductor is damaged or faulty, there may be virulent consequences.
That being said, the measurement of earth resistance for earth electrode systems should be done using an Earth resistance tester when the electrode is initially installed, and then at specific time intervals to ensure the resistance-to-ground doesn’t increase over time. According to the International Electrical Testing Association, the ground electrode should be tested every three years if the earth electrode system is in good shape. Not only does poor grounding increase the risk of equipment failure, but it’s quite dangerous as well. Every facility needs to have an adequately grounded electrical system so that if overvoltage occurs, or a lighting strike hits, the current can safely be redirected to the earth instead. Even though grounding systems have low earth ground resistance values when initially installed, their resistance can increase as a result of the ground rods being eaten away by high temperatures, corrosive soils that have high salt and moisture content.
There are several other factors that can change the earth resistance, including the electrical capacity of your facility expanding in size, which can change the needs in the earth electrode. What was a suitable, low earth resistance will then become obsolete. Further, as your facility adds more sensitive equipment, the problems of electrical noise are significantly increased. This noise wouldn’t affect older, cruder equipment, but can be quite problematic with modern equipment. The earth resistance of your grounding system can also change as more non-metallic conduits and pipes are installed, and where the water table is gradually falling.
Ground resistance testing can be performed using different methods. All of these methods are quite different in nature and require different Earth resistance tester types. The most common ones are clamp-on Earth resistance tester models. These testers allow the operator to perform effective measurements based on Ohm’s Law. The process includes voltage being applied to a complete circuit, measuring the resulting current flow, and getting a result of the resistance of the circuit. A clamp-on Earth resistance tester applies a signal and measures the current without the need for a direct electrical connection. The clamp features a transmit coil which applies the voltage and a receive coil which measures the current.
Another method that used to be extremely popular in the past but has now fallen in favour of clamp-on resistance testing is the fall-of-potential method. The reason why this method isn’t as practiced anymore is that it’s very labor-intensive and time-consuming. This method includes connecting the P1 and C1 terminals of the instrument to the earth’s electrodes if the tester features 4 terminals, or connecting the X terminal if the terminal features 3 terminals. 3-terminal earth resistance tester models are more convenient as they require only one lead to be connected. However, there’s a downside to using a 3-terminal tester over a 4-terminal one, and that is the resistance of the common lead being included in the measurement. The effect of this can be kept to a minimum if you use a short lead, but if you’re performing a complex earth resistance test that requires meeting stringent requirements, you should definitely use all 4 terminals from a 4-terminal tester for added accuracy.
If you’ve found that your earth electrode resistance isn’t low enough, there are a couple of different ways you can improve it, including by using multiple rods, treating the soil or lengthening the earth electrode in the ground. Driving a longer rod deeper into the ground will decrease its resistance. As a general rule of thumb, doubling the length of the rod will reduce the resistance by about 40%. Alternatively, you can use two properly-spaced rods to provide parallel paths, effectively making them two resistances in parallel. If there’s no way to drive the rod deeper into the ground, and if you can’t use two properly spaced rods to create a parallel path, you should resort to chemically treating the soil. Copper sulfate, magnesium sulfate and rock salt are the best non-corrosive materials used to treat the soil. However, there are some exceptions where you can’t use these materials. For example, don’t use soluble sulfate around building foundations, as it can damage concrete.
And although there isn’t much information about the effects of temperature on earth resistivity, there’s one logical conclusion that’s backed up by facts and states that when the temperatures increase, the resistivity decreases, and vice versa. This is due to the fact that water present in the soil determines its resistivity, so naturally, the increase in temperature decreases the resistivity of water. That being said, there are many variables in the determination of resistivity of the ground, which is why Earth resistance testers are essential tools to help you maintain uptime. All ground connections should be checked at least once a year as part of your predictive maintenance schedule. If there’s a 20% or more increase in resistance, there’s probably a problem that needs to be investigated and corrected to lower the resistance to optimum levels.