Ground Rod Resistance Measurement

The ability to properly measure ground resistance is essential in preventing costly downtime due to service interruptions caused by poor grounds.

Ground rod resistance measurement. A current is applied across the outermost pair of stakes and the voltage drop across the inner pair is measured with a ground resistance meter. Nearly all electricians and electrical inspectors are familiar with the national electrical code requirement in sec. Thus if your first ground rod measures 2 5 ohms we would expect the resistance of multiple rods to be the following. Used for measuring resistance to earth of an installed grounding electrode.

The three point method is the most thorough and reliable test method. In facilities with sensitive equipment it should be 5ohms or less. Method 1 using a clamp on ground meter. The lower the number of ohms of resistance a grounding system has the better your protection is.

That is why you have to fully familiarize yourself with your clamp on tester before you use it to measure resistances. Make sure that your clamp on tester s jaws are fully closed as a semi opened clamp will significantly alter the measurement of the ground rod s resistance. If there were 60 similar ground electrodes with a resistance of 10 ω each the measurement of the total loop resistance would be. 250 54 which requires the resistance to ground of a single made electrode e g ground rod to be 25 ohms or less.

Resistance 2 rods 2 5 ohms 2 rods 1 16 1 45 ohms resistance 3 rods 2 5 ohms 3 rods 1 29 1 075 ohms resistance 4 rods 2 5 ohms 4 rods 1 36 0 85 ohms. The nfpa and ieee recommend a ground resistance value of 5 ohms or less while the nec has stated to make sure that system impedance to ground is less than 5 ohms specified in nec 50 56. For example the national electric code requires no greater than 25 ohms of resistance in a ground system. Most of these agencies recommend a ground rod resistance reading or less than 5ohms when using a clamp on meter.

If when taking further readings e you notice an increase of more than 20 from the safe reading of 25ohms or an increase of 20 from your last reading it is highly recommended you investigate the source of the problem. An ideal ground rod resistance should be below 3 ohms. Before measurements of ground resistance are conducted with a ground rod soil or ground resistivity testing is usually conducted with a series of four evenly spaced stakes. The resistance to the flow of electricity is measured by a ground meter and is read in ohms.