At Central Offices
When conducting a grounding audit of a central office there are three different measurements required.
Before testing, locate the MGB (Master Ground Bar) within the central office to determine the type of grounding system that exists. As shown on this page, the MGB will have ground leads connecting to the:
- MGN (Multi-Grounded Neutral) or incoming service,
- ground field,
- water pipe, and
- structural or building steel
First, perform the Stakeless test on all the individual grounds coming off of the MGB. The purpose is to ensure that all the grounds are connected, especially the MGN. It is important to note that you are not measuring the individual resistance, rather the loop resistance of what you are clamped around. As shown in Figure 1, connect the Fluke 1625 or 1623 and both the inducing and sensing clamps, which are placed around each connection to measure the loop resistance of the MGN, the ground field, the water pipe, and the building steel.
Second, perform the 3-pole fall-of-potential test of the entire ground system, connecting to the MGB as illustrated on in Figure 2. To get to remote earth, many phone companies utilize unused cable pairs going out as much as a mile. Record the measurement and repeat this test at least annually.
Third, measure the individual resistances of the ground system using the Selective test of the Fluke 1625 or 1623. Connect the Fluke tester, as shown in Figure 3. Measure the resistance of the MGN; the value is the resistance of that particular leg of the MGB. Then measure the ground field. This reading is the actual resistance value of the central office ground field. Now move on to the water pipe, and then repeat for the resistance of the building steel. You can easily verify the accuracy of these measurements through Ohm's Law. The resistance of the individual legs, when calculated, should equal the resistance of the entire system given (allow for reasonable error since all ground elements may not be measured).
These test methods provide the most accurate measure of a central office, because it gives you the individual resistances and their actual behavior in a ground system. Although accurate, the measurements would not show how the system behaves as a network, because in the event of a lightning strike or fault current, everything is connected.
To prove this, you need to perform a few additional tests on individual resistances.
First, perform the 3-pole fall-of-potential test on each leg off the MGB and record each measurement. Using Ohm's Law again, these measurements should be equal to the resistance of the entire system. From the calculations you will see that you are from 20 % to 30 % off the total RE value.
Finally, measure the resistances of the various legs of the MGB using the selective stakeless method. It works like the stakeless method, but it differs in the way we use the two separate clamps. We place the inducing voltage clamp around the cable going to the MGB, and since the MGB is connected to the incoming power, which is parallel to the earth system, we have achieved that requirement. Take the sensing clamp and place it around the ground cable leading out to the ground field. When we measure the resistance, this is the actual resistance of the ground field, plus the parallel path of the MGB. And because it should be very low ohmically, it should have no real effect on the measured reading. This process can be repeated for the other legs of the ground bar i.e. water pipe and structural steel.
To measure the MGB via the stakeless selective method, place the inducing voltage clamp around the line to the water pipe (since the copper water pipe should have very low resistance) and your reading will be the resistance for only the MGN.