T-304B Current Meter

OPERATINGINSTRUCTIONS

Wilcom

T-304B Current MeterOperating Instructions

811-434-009March 2007

Copyright (c) 2007 WilcomAll Rights reserved

Wilcom reserves the right to make changes to thematerial contained herein without notice and shall not beresponsible for any damages caused by reliance on thematerial presented.

This document may not be copied or duplicated in partor in whole for any purpose without the express writtenpermission of Wilcom.

PageGeneral .................................................................. 1Specifications ......................................................... 2Panel Features ....................................................... 5Operationg the T304B ............................................. 7

Current Measurements................................. 7Maintenance............................................................ 9

Battery Replacement ................................... 9Applications ......................................................... 8

Measuring Cable Shield Continuity............ 10Cable Testing ............................................ 17Troubleshooting ..................................... 18Pedestal Terminal Testing ...................... 22Evaluating Bond Connections.................... 29

Wilcom Products .................................................. . 32Service and Repair................................................... 34Warranty ..................................................................36

Figures

1. T304B Front Panel............... .............................. 42. Cable Shield Connections.... .............................. 113. Current Values in Connection.............................. 114. Plotting a Current Measurement.......................... 165. Aerial Cable Terminal Testing............................. 206. Calculating Shield Resistance .............................. 267. Bond Measurements............. .............................. 288. Shield Current Measurement Form....................... 33

Contents

i

General

Model T304B

ii

The Model T304B Current Meter is a portable, batterypowered current measurement device that is designed tooperate with a Model T305 Current Probe. This combi-nation allows current measurement on cables up to twoinches in diameter. The Model T272A Current Probeprovides a suitable alternative for use on cables up tofour inches.

Used with any of these probes, the T304B provides accurrent measurement from 10 mA to 100 A. Thus, theT304B is ideal for measuring power system inducedcurrents in cable shields and ground connectors. Aselector switch on the T304B front panel sets the inter-nal circuits for proper correction factors when using theT305 and T272A probes.

The T304B is powered by four standard 9-volt batteries.While 300 hours of intermittent operation is normallypossible between battery changes, prolonged measure-ment of very high currents, particularly with the T272A,shortens the battery life.

General

1

Specifications

Current Range: 7 switch-selectable ranges,100 mA to 100 A full scale

Frequencywith T305 or T272A: 50 Hz to 1 kHz

Accuracy 50 Hz to 1 kHzwith T305: ±10% of reading,

+3% of full scalewith T272A: ±5% of reading,

+2% of full scale

PowerBatteries: four 9-volt batteries, type 216

or similarBattery Life: 300 hours intermittent duty,

depending on battery ratings

EnvironmentalOperating Temp.: 0° to 50°CHumidity: 95% at 35°C, 40% at 50°CAltitude : 15,000 meters (non operating)

PhysicalWidth: 4 3/8 in. (11.1 cm)Height: 6 3/8 in. (16.2 cm)Depth: 4 1/2 in. (16.2 cm)Weight: 3.2 lbs. (1.5 kg)

2

T305 Current ProbeClamp-on current transducer for 2" maximumdiameter conductor.

T272A Current ProbeClamp-on current transducer for 4" maximumdiameter conductor.

T305, T272ATurns: 2400Shunt: Internal resistance shunt for

24.5 mV/ampere output intohigh impedance load

Accuracy, with NMS: ±15% of reading plusmeasuring set accuracy

Specifications

3

Figure 1. The T304B Front Panel

Panel Features

4

Panel Features

The following describes the T304B front panelcontrols and connections for current measurements, asshown in Figure 1. The features are described counter-clockwise from the power switch.

The ON/OFF toggle switch applies power to the unit.

The dual banana jack, labeled CURRENT PROBE,provides a connection for the T305, and T272A

The T305/T272 selector toggle switch selects the cor-rect input networks for the T305 or T272A probe. Selectthe T305 position when using the T272A probe.

The SIG GEN and COMmon ground banana jack pro-vides a 100 mA signal output for testing cable bonds thatdo not have a current flow.

The MTR ADJust feature nulls the meter at a specifiedvalue for bond percentage measurements.

The CALibrate toggle switch, when in the Down posi-tion, allows the MTR ADJ to null the meter.

5

Panel Features

In the UP position, the meter is calibrated. The CurrentRange switch provides selection of any of the followingseven current ranges: 0-100mA, 0-300mA, 0-1A, 0-3A,0-10A, 0-30A, and 0-100A.

The BATT TEST pushbutton verifies the condition ofthe batteries when the current meter is in use. The BATTOK range of the current meter indicates battery condi-tion.

6

Operating the T304B

Power on the T304B by setting the ON/OFF switch toON. Check the condition of the batteries by holding theBATT TEST pushbutton and observing the current meter.If the dial indicator is in the BATT OK range, thebatteries have sufficient power to operate the set. If not,replace the batteries as described in BatteryReplacement.

Current measurements on accircuits can be made with oneof the clamp-on current probesand a T304B with a measur-

ing range of 100 mA to 100 A full scale. The currentprobe inductively couples induced ac current to a mea-suring set by means of an adapter cable (for directreadings in amperes or milliamperes). While the pri-mary application of the T304B and a current probe is tocheck the presence of shielding current in a telephonecable shield, currents may also be measured on groundconductor and low voltage ac circuits.

The T305 current probes have a shunt resistor built intothe clamp-on transducer. The values of these shunts aresuch that the low dynamic input impedance of theT304B results in the shunts having little effect

CurrentMeasurements

7

except to cause some high frequency loss (3 dB point at4 to 8 kHz) on the T304B most sensitive range. Refer toSpecifications.

CAUTION: The T304B and current probes operate onlow voltage ac circuits. DO NOT use the probe oncircuits with hazardous high voltages. Abide by yourcompany’s practices regarding working in the presenceof hazardous electrical conditions.

Measurements are made by clamping the probe aroundthe cable or conductor under test. The section entitledApplications describes the methods that are used forchecking shielding and ground currents on telephonecable systems.

Operating the T304B

8

1. Remove the four screws from the front panel.2. Slide the panel assembly from the case.3. Unsnap the batteries and replace them with fresh

batteries.4. Set the ON/OFF switch to ON. Check the batteries by

pressing the BATT TEST button and verify that themeter indicator falls within the BATT OK area.

5. Return the test set to its case and replace the fourscrews in the front panel.

In the event of a problemunrelated to the batteries, it isrecommended that the com-plete unit be returned for re-pair to Wilcom, Inc. at the

address at the front of this manual. Information regard-ing the suspected trouble should be enclosed.

If you need technical assistance with the T304B, contactApplications Engineering at the following toll-freenumber in Laconia, NH: (800) 222-1898

Maintenance

TechnicalAsssistance

BatteryReplacement

Use the following procedureto replace the batteries:

9

The following describes the use of the T304B in deter-mining cable shield continuity, testing of faults on cable,troubleshooting, and bond measurement.

Figure 2 depicts a typicalcable shield circuit from cen-tral office to cable or sub-scriber end. The various typesof grounds and connections

that can occur along this current path are shown in thisdiagram. In practice grounds and connections such as apedestal, splice case, or access terminal, occur at asingle point.

The shielding mechanism for voice frequencies on acable is the induced current from overhead or nearbypower lines that flows along the shield circuit. A circuitmust be created and maintained with the lowest practicalimpedance or resistance possible. Therefore, all con-nections to grounds and bonds across cabled shield endsshould be evaluated along the cable route to assess thecondition of the current circuit. The current shown inFigure 2 varies from point to point along the cable route.Figure 3 shows a closer view of a typical connectionpoint that might be found in a pedestal.

Applications

MeasuringCable Shield

Continuity

10

Applications

11

In Figure 3, the current that is measured on the shieldtoward the central office side of the connection islabeled IO and that on the field side IF. The remainingcurrents flow in the ground leads or to some remotepoint via a cable or shielded wire (labeled IG1, IG2, andIG3 for illustrative purposes).

Many variables affect the amount of induced current incable shields; therefore, the amount of current that flowson a particular cable shield cannot always be accuratelypredicted. Instead, use the proven test procedures thatare listed below.

1. Sequentially check the current flow at each connec-tion point along the cable shield route. Using Figure3 as an example, use the T304B to measure currentsIO, IF, IG1, IG2, and IG3. For this example,

IO = 900 mAIF = 500 mAIG1 = 100 mAIG2 = 40 mAIG3 = 210 mA

Therefore, the largest current that is measured is IOat 900 mA. Calculate ten percent of this current: tenpercent of IO (900 mA) is 90 mA.

Applications

12

CAUTION: Hazardous voltages may be in-duced on bunched conductors if an open shieldexists. USE CAUTION when performing alltests.

2. The current flowing into the above connection pointmust equal the current that flows out of the connec-tion. Five currents are measured. Thus, some com-bination of these currents IO, IF, IG1, IG2, and IG3must add up so that one wire current equals the sumof the other four, or the sum in two wires equals thesum in the other three. Similar logic applies forconnections with a different number of wires. Forthis example, the best combination is

IO = IF + IG1 + IG2 + IG3or900 = 500 + 100 + 40 + 210

Note that the total current to the right of the equal signis 850 mA. Some amount of “error” is involved,which can be caused by power line induction that isnot constant in time so that the current along theshield and in the ground connections varies slightly.Operator reading error or rounding off of a readingcould also be a cause of the unequal equation.

Applications

13

3. The currents in the equation must have an error nolarger than the value that is calculated in procedure2, i.e., the maximum error allowed is ten percent ofthe highest reading that is taken (90 mA in the aboveexample). The error in the above equation, 50 mA,is less than 90 mA so the measurements are essen-tially correct. This rule is proven to be effective andis used to be certain that there are no significantcurrents flowing in the connection which have missedbeing measured. Should one be overlooked, themeasurements should be retaken.

The sample form entitled Shield Current Measurementon Page 33 of this manual is useful for recording fielddata. This form can be used to plot the entire current thatflows along the cable shield circuit. A sample entryfollows:

Office Field GroundSite Side Side Type Current

Ped #46 900 mA 500 mA Rod 40 mAPed #46 Subr Drop 100 mAPed #46 Tap Cable 210 mA

Applications

14

Applications

Figure 4 is an example of the plot that you can obtainfrom the above field measurements. It can be used forfuture reference in investigating the status of the shieldcircuit. Note that power line loads vary considerablyfrom season to season, and thus the actual values on thegraph may change accordingly. The shape of the graphshould remain constant, i.e., the entire graph tends toshift upward to downward from season to season,according to power line loads. To recheck the system inthe future, select more than one location for whichmeasurements are to be taken. Then compare the resultswith the graph.

Ped #49 on the graph is a plot of the currents in the aboveexample. Note that the vertical line between the currentvalues of IO and IF equals the total current that flows inthe connecting wires and cables that are attached to thecable shield at this point, i.e., IG1 + IG2 + IG3.

15

Figu

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ent

Applications

16

Major bonding problemshave been found in splicesand on ground connections incable vaults located in central

offices that have a bearing on both noise and protectionproblems. In addition, grounding of cables and cablesupporting strands varies considerably from location tolocation. Consequently, there is no certainty as to howmuch effect each bond has on noise. In some cases acumulative effect may be observed as each defectivebond is repaired. In many cases little effect is observeduntil the last defective bond is corrected and then adramatic reduction in noise may be noted.

As a result of field experiences it is strongly recom-mended that tests on a cable be started in the cable vaultof the central office to be sure that all splices areproperly bonded, all ground connections are in goodcondition, and all cable shields are tied together asrequired for protection purposes. After the vault bond-ing conditions are checked and are in accordance withall requirements, each cable splice should be tested insequence and the necessary corrective action taken torepair all defective bonds. It may also be desirable tocheck bonds between ground conductors or terminalsand cable supporting strands, and between the groundedstrand and cable shield.

Applications

CableTesting

17

Defective ground connections to cables are not uncom-mon due to corrosion of bonding clamps or to looseclamps. Cold flow of materials under mechanical pres-sure can also effect the quality of a bond.

A number of lateral cables that are connected to the maincable should be tested as they can introduce noise in themain cable. Ground connections to the cable shouldalso be checked.

WARNING: A full evaluation of the effect ofhigh voltage breakdown on the equipment hasnot been made and it is not known what highvoltage can be tolerated. Consequently, caremust be exercised in avoiding contact with anyhigh voltages during testing.

After taking and recordingcurrent measurements at thecentral office main frame (orvault) between the cableshields and ground bus, pro-

ceed along the cable route.

Applications

Trouble-shooting

18

Applications

Individually measure and record the shield current ateach aerial cable terminal on both sides of the terminalbeing tested. Record the data (in mA) on a form similarto the form in Appendix A. The measurements are madeas follows (Figure 5):

Shield Current + A - (B+C), where AO = Office Side andAF = Field Side.

1. Record the current flow with the T305 clamp aroundthe supporting strand and the shield (A). (The clampmust be completely closed to insure accuracy.)

2. Record the current through the core (B).

3. Record the current through supporting strand (C).

4. Add (B) and (C) and subtract from (A) to obtain theshield current.

By measuring the current as described, the longitudinalcurrent flowing through the conductor bundle and thesupporting strand current is eliminated. The final calcu-lation provides only the shield current.

Aerial Cable Terminal Testing

19

Figu

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. A

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erm

inal

Tes

ting

Applications

20

Measurement (A) should be taken on both sides of theterminal and two calculations recorded, the office sideandthe field side. The core plus the strand current, minus(B+C), is the same for both calculations.

NOTE: The current that flows through themetal bars clamped to the cable shield forelectrical continuity can be used to measureshield current if the bars are accessible to aclamp-on ammeter, excluding the core or sup-porting strand.

Bond Integrity in an Aerial Closure

A technique similar to testing an open shield betweenterminals (described later) can be used to check bondintegrity for aerial facilities. Begin by tightening allscrews and bolts at the aerial closure.

1. Place the bypass probe wire across the aerial cableterminal from cable shield to cable shield whilemonitoring the bypass current through the wire withthe T304B/T305 current meter and clamp-on probe.

2. If any current flows through the bypass conductor,rework all bonding connections.

Applications

21

The current in each possiblepath to ground should also beread and recorded. In a ped-estal the single ground wiremay not be accessible. The

current that flows to ground should then be read byclamping around all the ground wires that lead to thecommon lug. The current in each wire should also berecorded and compared with the total. Other groundconnections include MGN, ground rod, customer pro-tector, shield grounds, etc.

The most current flows toward the ground with the leastresistance (best ground). Interpretation of the databegins by examining why the current flow is largerthrough one path than through another. After measure-ment of the current in all paths at the test location, thepaths with the lower readings usually have high resis-tance or even open bonds or poor ground connections.

A resistance as small as one half an ohm at a connectionis enough to restrict some current flow and should bereworked. At the low voltages usually involved twoohms may block most of the current flow. Since resis-tance cannot be read when a voltage exists across a band,an open shield between terminals can be tested to checkbond integrity (described later).

Applications

PedestalTerminal

Testing

22

Applications

Testing Current in a Cable Shield

Many variables affect the amount of induced current ina cable shield:

• Resistance of shield circuit• Ground impedance at both ends• Proximity of power lines• Length of exposure• Power line loading• Bond integrity at all connections

No accurate method exists for estimating the shieldcurrent at a particular location. Poor bonds are locatedby comparing the amount of current flowing in the shieldcircuit and through various bonded connections. Theprocedure demands a systematic continuity of effort thatincludes correcting each fault before going to the nextsplice.

When the amount of shield current flowing is deter-mined, use the following flowchart describing the shieldcurrent test for selecting the appropriate test procedureto use to locate the problem.

23

Applications

24

Applications

This test is useful for determining the condition of theshield between splices. If current readings are still low(less than 50 mA) after shield continuity was repaired ateach terminal splice, an open shield may exist betweensplices. When this type of shield problem is expected,it is necessary to check the shield by using a digital voltohmeter. The bonds and ground connections must firstbe opened at both ends of the cable section that is beingtested. A working pair of wires within the shield, inaddition to access to the shield itself, are needed to makethis test. The measurements are taken as shown inFigure 6. The shield resistance is calculated as shown instep 3 of Figure 6. (Shield Resistance = Resistance perunit length x length.)

1. Using the digital volt ohmeter measure the loopresistance R1 of an idle pair between two pedestalsor splices. Record the resistance. Take the measure-ment with the pair shorted at the far end (Figure 6,step 1).

2. Connect the shorted pair at the far end to the cableshield (Figure 6, step 2).

Open Shield Between Terminals

25

Applications

Figu

re 6

. C

alcu

latin

g Sh

ield

Res

ista

nce

26

3. The tip and ring of the pair at the measuring end areconnected to one terminal of the bridge. The shieldis connected to the other terminal of the bridge,(Figure 6, step 2).

If the shield resistance per unit length is known and themeasured shield resistance exceeds the estimated shieldresistance by 25% (where shield resistance equals theresistance per unit length x length), a shield problem islikely to exist. If the shield resistance is well above 25%of the estimated resistance, an open shield probablyexists.

Basis of Bond Measurement & Evaluation

This bond measurement is based upon Ohm’s law,current division in a parallel branch resistive circuit.Refer to the Inset of Figure 7. Current I flows normallythrough the bond and is measured by the T305 probe.Under these conditions, current I2 equals current I. Nowplace the bypass probe between points A and B. Somecurrent I1 now flows along this lead, leaving I2, less thanI, flowing in the original circuit. When I1 equals I2, thenthe total resistance between points A and B along thepath that includes the bond under check is the same asthe total resistance of the bypass probe lead.

Applications

27

Applications

Figure 7. Bond Measurements

28

Thus, the bond circuit is better than the bypass circuitand the bond does not need any further work or attention.If more current flows in the bypass than in the bondcircuit, then the bond circuit has more resistance than thebypass probe and the bond should be reworked orrebuilt.

The standard of measure for bond resistance for this testis the total resistance of the bypass probe. WilcomProducts supplies bypass probes according to thecustomer’s desire for a specific resistance standard.

The points A and B are described below.

Assuming that current ispresent and flowing in theconductor under test, includestep 7 if the current is less than15 mA or zero.

1. Place the clamp-on current probe around the conduc-tor for which the bond is to be evaluated (refer toInset, Figure 7).

2. Set the current range switch on the T304B to read thecurrent flow in the conductor within the clamp-onprobe.

Applications

EvaluatingBond

Connections

29

3. Place the CAL toggle switch on the T304B to theMTR ADJ (down) position.

4. Adjust the control knob closest to the CAL/MTR ADJswitch so the meter reading of the T304B is now fullscale; the upper scale reads "1."

5. Take the bypass probe and connect points A and B oneach side of the bond you wish to check. The clamp-on probe must be between points A and B; if not, thetest is invalid. The clamp-on probe is always placedaround the conductor under test; the only exceptionis in step 7 below.

6. When the bypass probe is connected, the T304Bmeter reading should drop. Be certain to place theCAL toggle switch to the CAL(up) position whenreturning to normal meter operation. The MTR ADJposition is only to be used in the bond checkingmode.

7. When step 2 is initiated and the current is less than 15mA or not flowing, remove the clamp-on probe fromthe conductor and place it around the bypass probewire. Then connect the bypass probe across the bondto be tested.

Applications

30

If the current now exceeds 15 mA, then the bond isdefective or open, and must be reworked or rebuilt.If the current value in the bypass probe wire remainsless than 15 mA, then go to step 8.

8. The current is too low for normal measurement. TheT304B has a built-in current generator that suppliesample current through any bond in order to check thebond quality. With currents below 15 mA or zero,the internal generator should be used to supplycurrent through the bond. Take the double bananaplug with two separate leads from the T304B lid andplace the banana plug into banana jacks on the leftside of the meter labeled SIG GEN and COM. Con-nect these leads to points A' and B' as shown in theInset of Figure 7. That is, place the T304B generatorleads so as to cause current to flow through the bondof interest. No switch is necessary to turn thegenerator on, connecting it across the bond is all thatis required. Now return to step 1 and when in step 5,referring to the Inset of Figure 7, note that thepoints of connection A and B for the Bypass Probe liebetween points A' and B' for the Generator connec-tion.

NOTE: Prolonged use of the built in currentgenerator greatly reduces the battery life.

Applications

31

32

Wilcom Fiber Optic Products

FM8510 30851001FM8515B 30851511FM8515C 30851514FM8520 30852001FM1317 30131705

ST 04419812-01-04SC 04419716-01-4FC 04419857-01-2Universal Adapter Cap 30031992LC 30032070-01-4

Stabilized Fiber Sources

Optical Power Meters

FS8513A (850/1310nm LED Source w/ST) 30851310FS8514A (850/1310nm Laser Source w/ST) 30851407FS1316 (1310/1550nm Laser Source w/ST) 30131602FS1317 (1550/1625nm Laser Source w/ST) 30131702FS1318 (1310,1490,1550nm Laser w/SC) 30131801

Optical Fiber IdentifierF6121A 30612130F6222 30622210F6222C 30622230Optional 2mm Head 04419965-01-RC-1Optional 1.6mm Head 04420715-01-RC-1

Visual Fault LocatorF6230A - 650nm Laser 306230852.5mm to 1.25mm adapter 30032232-01

Optical Power Meter Small Adapter CapsFM8510, FM8515B, FM8520, FM1317, FM1318

Model Number Wilcom Part Number

Optical Power Meter Large Adapter CapsFM8515C, FM1318C

3079/ST 30030792-01-13175/SC 30031752-01-43019/FC 30030191-01-23011/D4 30030118-01-33013/Biconic 30030134-01-1LC 30032052-01-5

Figure 8. Shield Current Measurement Form

33

For service or repair follow the procedure below:

1. Call Wilcom Customer Service. Support personnelwill determine if the equipment requires service, repairor calibration.

2. If the equipment must be returned to Wilcom forservice, Wilcom Customer Service will issue a ReturnMaterial Authorization (RMA) number and the follow-ing address for return:

Wilcom 73 Daniel Webster Highway Belmont, NH 03220 TEL (800) 222-1898 (USA only) or (603) 524-2622 FAX (603) 524-3735 www.wilcominc.com

IMPORTANTNever send any equipment back to Wilcomwithout a Return Material Authorization (RMA)number.

34

Service and Repair

3. Pack the equipment in its original shipping ma-terial. Be sure to include a statement or report fullydetailing the defect and conditions under which itwas observed. Also be sure to include a contactname and telephone number.

4. Return the equipment, prepaid, to the aboveaddress. Be sure to write the RMA on the shippingslip. Wilcom will refuse and return any package thatdoes not bear the RMA.

Ordering InformationOrders for any of the Wilcom products and any oftheir optional accessories should be directed to theaddress shown above.

Service and Repair

35

All products are warranted against defects in materials and workman-ship. This warranty applies for a period of two (2) years from date ofdelivery, except for Fiber Optic instrumentation and equipmentwhich have a one (1) year warranty on parts and two (2) years on labor.(The only exceptions in the digital testing equipment is the D550Shark, which has a warranty of one (1) year from date of delivery.)Wilcom's obligation under this warranty is limited to servicing oradjusting each instrument returned to its factory within the warrantyperiod, and to replace any components found to be defective. Thiswarranty does not apply to instruments that have been repaired oraltered by unauthorized person, or which have been subject to misuse,negligence or accident.

LIMITATION OF WARRANTYThe foregoing warranties are the exclusive warranties provided byWilcom. Wilcom will not be liable for any special, indirect, incidentalor consequential damages whatsoever resulting from loss of use, lossof data or loss of profits arising out of or in connection with the useor performance of the product, even if Wilcom has been informed ofthe possibility of such damages in advance. All implied warranties,including without limitation warranties of merchantability and fit-ness for a particular purpose, as well as warranties arising from acourse of dealing or usage of trade are expressly disclaimed.

PROPRIETARY INFORMATIONThe information contained in this manual is the proprietary materialof Wilcom, and may not be reproduced, used for manufacturingpurposes, or disclosed to others for any use without written permis-sion from Wilcom.

WARRANTY

36