C L A M P - O N G R O U N D NOTES R E S I S TA N C E M E A S U R E M E N T (Models 3710 and 3730) This measurement method is innovative and quite unique. It of fers the abil- ity to measure the resistance without disconnecting the gr ound. This type of measurement also offers the advantage of including the bonding to gr ound and the overall grounding connection resistances. Principle of Operation Usually, a common distribution line gr ounded system can be simulated as a simple basic cir e 29 or an equivalent cir cuit as shown in Figur cuit, shown in Figure 30. If voltage E is applied to any measur grounding pole Rx ed through a special transformer current I flows thr , ough the circuit, thereby establishing the following equation. 1 E/I = Rx + –––––––––– where, usually n 1 1 –––– Rx >> ––––––––––– ∑ Rk n 1 –––– k=1 ∑ Rk k=1 Therefore, E/I = Rx is established. If I is detected with E kept constant, meas - ured grounding pole resistance can be obtained. Refer again to Figures 29 and 30. Current is fed to a special transformer via a power amplifier fr a 1.7 kHz om constant voltage oscillator. This current is detected by a detection CT. Only the 1.7 kHz signal fr e- quency is amplified by a filter amplifier This occurs befor the . e A/ D conversion and after syn- F IGURE 29 chronous rectification. It is then displayed on the LCD. The filter amplifier is used to cut off both earth current at commer- cial frequency and high-fr equency noise. V oltage is detected by coils wound around the injection CT which is then amplified, r ectified, and compared by a level compara- tor. If the clamp is not closed properly, an “open jaw” annuncia - F IGURE 30 tor appears on the LCD. Examples: Typical In-Field Measur ements NOTES Pole Mounted Transformer Remove any molding covering the gr ound conductor, and provide sufficient room for the Model 3710/3730 jaws, which must be able to close easily around the conductor. The jaws can be placed ar ound the ground rod itself. Note: The clamp must be placed so that the jaws ar in an electrical path e from the system neutral or gr ound wire to the ground rod or rods as the circuit provides. Select the curr ound conductor and measur ent range “A.” Clamp onto the gr e the ground current. The maximum current range is 30 A. If the ground current exceeds 5 A, ground resistance measurements are not possible. Do not proceed further with the measurement. Instead, remove the clamp-on tester from the circuit, noting the location for maintenance, and continue to the next test location. After noting the ground current, select the ground resistance range “Ω” and measure the resistance directly. The reading you measure with the 3710/3730 indicates the resistance not just of the r od, but also of the connection to the system neutral and all bonding connections between the neutral and the r od. Note that in Figur ound rod. In this type e 31 there is both a butt plate and a gr of circuit, the instrument must be placed above the bond so that both grounds are included in the test. For futur e reference note the date, ohms reading, current reading and pole number . Replace any molding you may have removed from the conductor. Note: A high reading indicates one or more of the following: A) poor ground rod B) open ground conductor C) high resistance bonds on the rod or splices on the conductor; watch for buried split butts, clamps, and hammer-on connections. F IGURE 31 Service Entrance or Meter NOTES Follow basically the same pr ocedure as in the first example. Notice that Figure 32 shows the possibility of multiple gr ound rods, and in Figure 33 the ground rods have been replaced with a water pipe gr ound. You may also have both types acting as a gr ound. In these cases, it is necessary to make the measurements between the service neutral and both gr ounded points. F IGURE 32 F IGURE 33 Pad Mounted Transformer NOTES Note: Never open transformer enclosur - es. They are the property of the elec trical utility . . This test is for high voltage experts only Observe all safety r esent. equirements, since dangerously high voltage is pr Locate and number all r (usually only a single od is present). If the ods r ground rods are inside the enclosur e, refer to Figure 34 and if they are out- side the enclosur refer to Figure 35. If a single r is found within the e, od enclosure, the measurement should be taken on the conductor just beforthe e bond on the ground rod. Often, more than one ground conductor is tied to this clamp, looping back to the enclosur e or neutral. In many cases, the best r eading can be obtained by clamping the 3710/3730 onto the ground rod itself, below the point when the gr ound conductors are attached to the rod, so that you are measuring the ground circuit. Care must be taken to find a conductor with only oneeturn path to the neutral. r F IGURE 34 Transmission Towers F IGURE 35 Observe all safety r esent. equirements, since dangerously high voltage is pr NOTES Locate the ground conductor at the base of the towerNote: Many different . configurations exist. Car should be taken when sear for the ground e ching conductor. Fig. 36 shows a single leg mounted on a concr pad with an ete external ground conductor. The point at which you clamp the gr ound tester should be above all splices and connections which allow for multiple r ods, butt wraps, or butt plates. Central Office Locations The main ground conductor from ground window or ground plane is often too large to clamp around. Due to the wiring practices within the central office, there are many locations at which you can look at the water pipe or counterpoise fr An effective location is usually at the om within the building. ground buss in the power r . oom, or near the backup generator By measuring at several points and comparing the r eadings, both of current flow and resistance, you will be able to identify neutral loops, utility gr ounds and central office grounds. The test is efective and accurate because the ground window is connected to the utility gr ding ound at only one point, accor to standard practices. F IGURE 36 T E L E C O M M U N I C AT I O N S NOTES The clamp-on ground tester developed byAEMC and discussed in the pr e- vious chapter has revolutionized the ability of power companies to measur e their ground resistance values. This same pr oven instrument and technology can be applied to telephone industries to aid in detecting gr and ounding bonding problems. As equipment operates at lower voltages, the system’s ability to remove any manmade or natural overpotentials becomes even more critical. The traditional fall-of-potential tester pr to be labor oved intensive and left a lot of interpr etation to the person making the test. Even more important, the clamp-on gr ound test method allows the user to make this necessary reading without the risky business of r emoving the ground under test from service. In many applications, the gr consists of bonding the two Utilities ound together to avoid any dif ous ference of potentials that could be danger to equipment and personnel alike. The clamp-on “Ohm meter” can be used to test these important bonds. Here are some of the solutions and clamp-on procedures that have applications to the telephone industry . Telephone Cabinets and Enclosur es Grounding plays a very important r in the maintainance of sensitive ole equipment in telephone cabinets and enclosures. In order to protect this equipment, a low resistance path must be maintained in or der for any over- voltage potentials to conduct safely to earth. Thisesistance test is performed r by clamping a ground tester Model 3710/3730 around the driven ground rod, below any common telephone and power company bond connections. F IGURE 37 To avoid any high voltage potentials between the telephone and power com - NOTES panies, a low resistance bond is established. Bonding integrity is performed by clamping around the No. 6 copper wire between the master ground bar (MGB) and the power company’s multigrounded neu- tral (MGN). The r esistance value displayed on the tester will also include loose or poorly landed terminations that may have degraded over time. Additionally the clamp-on , ground tester can be used as a True RMS ammeter . F IGURE 38 Pedestal grounds All cable sheaths ar bonded to a ground bar inside each pedestal. This e ground bar is connected to earth by means of a driven gr rod. The ound ground rod resistance can be found by using the instr ument clamped around the ground rod or the No. 6 cable connecting these two points. See figur39. e F IGURE 39 Note: temporary jumper required only if pedestal does not allow tester to fit. Cable shield bonds to MGN NOTES The cable shields in a buried or above gr e may be ound telephone enclosur grounded by means of the power company’s multigr neutral. The ounded clamp-on ground tester can be utilized to ensur that this connection has e been successfully terminated. The lowesistance return path for the instr r u- ment to make this measur om this bond wir ement will be fr e under test to the MGN back thr ough all other bonds up and/or down stream (theory of parallel resistance). The clamp-on ground tester also is a T . rue RMS ammeter F IGURE 40 Network Interface Device (NID) with a Pr otector Block NOTES The typical customer connection is achieved with the tip and ring dr op cable pair. In order to protect against an overvoltage situation on the telephone wires, a protector block is installed inside the NID. This pr otector has two internal devices that con - duct only when unwant- ed overvoltages are pres- ent. In order for the pro- tector to function pr op- erly, it must have a low resistance path for any fault to conduct to earth. This bonding and ground resistance poten - tial can be verified by using the clamp-on ground resistance tester . F IGURE 41 Simply take a short piece of wire and temporarily jumper the tip side (CO gr otector ound) to the ground connector on the pr block. By clamping ar e, you will now test the gr ound this jumper wir ound resistance potential including all terminations at this location. The r eturn sig- nal path required for the clamp-on ground tester to make this measur ement will be the CO gr ound. Overhead Telephone Distribution Telephone systems deliver ed on overhead poles must also be bonded to the MGN. This is typically performed by supplying a No. 6 copper wire connected to the grounding strand above telephone space. If power is not supplied on these poles, driven ground rods must be installed at required pole intervalsand subsequently tested. Note: Coil wire for attachment to power company MGN F IGURE 42