Testing Earth Ground
To maintain the best protection against damage from surges, and to reduce interference from noise generating circuits and equipment, the system must be connected correctly to earth ground. In addition, the earth ground that the system is connected to must be of a specified value. Failure to connect the system components to earth ground as described here can severely reduce the capacity of the system to absorb surges, and can cause incorrect operation due to noise being induced into the communications wires.
Earth ground is the connection point of your electrical system to ground. This is a necessary path for unwanted damaging energy to flow away from the electrical system and into the earth.
The maximum allowed resistance to earth ground is 25 Ohms. It is better if the reading is 5 Ohms or less, but 25 Ohms is acceptable. Testing of the earth ground should be performed before the installer arrives at the site to begin installation.
The company installing the electrical system for the site, and the site owner, are responsible for this testing. If the site is a retrofit, then the site should be tested by a reputable company beforehand, and any necessary repairs made before the system is installed. All buildings that will have equipment installed (internally or externally) will need to be tested.
When an earth ground test is performed it can be done in one of two ways, depending on if the electrical service at the site has been connected or not.
- Fall-of-potential: This uses a meter and a set of probes that are placed a distance away from the ground point that is being tested. Each ground point in the system needs to be checked. This method must be done with the neutral to earth ground bond broken at the service entrance. If this bond is not disconnected at the time of testing, then the test is invalid.
- Inductive Clamp: This test uses a clamp on meter at the ground point. This method is only used with the neutral connected to ground, as the electrical path through the service feeder is used as part of the test circuit.
A standard multimeter is not capable of checking the earth grounds on a site. Only a specialized Earth Ground test meter is capable of this.
It is recommended that any building with separate electrical services have the building grounds tied together with a large gauge grounding cable.
Earth Ground resistance is the same as the "Soil Resistivity" of a site. This resistivity can change over time and is dependent upon several factors including:
- moisture content.
- salt content.
- Temperature above freezing (In very cold weather, the resistance can be 5 to 6 times higher than when read in warm weather).
More information can be found here