Examples of DC Circuit Fault Isolation Procedures

TM 11-5895-878-14&P

isolation procedures at other TCF's which handle the

associated alarm output) to verify that no failure has

defective circuit.

occurred in the loop current distribution system serving

the subscriber loop. If a blown fuse alarm is received,

(2) If the noise is not encountered in (1)

the line should be checked for a short circuit.

above, then the fault is in the subscriber's terminal

equipment or cable. Maintenance should be notified

(2) Open connection at subscriber terminals.

and an alternate cable pair should be sought.

If no failure in the loop battery supply is detected, the

station technical controller should:

(3) If patching was necessary in some leg of

the circuit, the technical controller, upon notification that

(a) Locate the jack appearances of the

the / malfunction has been corrected, coordinates with

subscriber circuit at the DC Patch bay and patch a

technical controllers or subscribers involved to restore

voltmeter into a monitor jack appearance of the

the circuit to the normal-through configuration.

Transmit circuit (Receive circuit of Subscriber) to

measure the loop voltage.

(4) He then completes station records, trouble

reports, and log entries.

(b) If a zero reading is obtained, by

patching a voltmeter into the subscriber circuit "REC

c. Fault Isolation of a VF Subscriber Circuit by the

MON" jack appearance at the DC (Low Level) patch

Loop-Back Method. The technical controller is notified

bay, the technical controller will be checking to

by a local subscriber that the circuit level is not correct.

determine that a steady mark condition is not being

The technical controller decides to patch the circuit out

received from the VFCT. Refer to maintenance.

of service since a spare is available to the originating

subscriber or switch. The technical controller clears his

own station and then proceeds to isolate the fault, step

obtained, he should isolate the subscriber loop from the

by step toward the distant end (toward the end

in-station circuitry by inserting a shorting-type dummy

originating the signal).

plug-in the line transmit jack appearance at the DC

The following procedure could be used:

Patch Bay. A + 6 vdc will be applied to the line.

(1) From the subscriber through the local

(d) If the receiving device continues to

TCF can be cleared as discussed in b above.

run open, request that user maintenance personnel

check the continuity of the subscriber loop.

(2) The next TCF toward the signal source

should be notified to loop back the circuit under test.

(e) Upon verification by maintenance

That distant station patches the 0 dBm Rec to the 0

that the trouble has been located at the subscriber

dBm Send of the same channel under test at the EL

terminal (for example, an open connection), and has

patch bay. When the local technical controller inputs a

been corrected, he should ensure that the normal

1000 Hz test tone on the Send Circuit at a level of 0

interface equipment has been returned to the

dBm, he will receive the same test tone back on that

subscriber's receive channel. Remove the dummy plug

channel's receive circuit. If the level is correct (0 dBm),

from the line jack appearance at the DC Patch bay to re-

that leg of the channel is cleared. If the level is wrong,

install the subscriber loop into the in-station circuitry.

the fault has been isolated between the two TCF's.

(f) Complete station records, trouble

(3) If the tone looped back was received at

reports, and log entries.

the proper level of 0 dBm, the distant TCF should be

(3) Defective cable. The station Technical

requested to remove the loop back patch and the next

Control activity is notified by maintenance that the

TCF further toward the signal source should be

failure in the subscriber loop is caused by a break in the

requested to perform a similar patch. In this way, the

cable between the station and the subscriber location.

trouble can be isolated between any two EL patch bays

The report indicates that the cable will be out of service

in the channel.

for some period of time. The area circuit records have

been reviewed by the station technical controller and

3-13. Examples of DC Circuit Fault

they indicate that an alternate link is not available, or

Isolation Procedures

feasible, between the two locations. He has further

determined that it is not practicable to copy the

a. Open Circuit Condition. The station Technical

incoming traffic at the station for use by the subscriber.

Control activity is notified by a connected telegraph

The station technical controller should:

subscriber that incoming traffic has been interrupted and

that the page printer has started to run open. Circuit

(a) Contact the distant VFCT terminal

layout record cards at the station indicate that the

and request that the technical controller stop traffic to

subscriber circuit is connected by cable to a nearby

the subscriber until the fault has been cleared.

military installation. The technical controller, based on

(b) Insert a dummy plug in the line jack

his knowledge that a break in the DC loop is the most

appearance of the subscriber's receive circuit ("Line

likely reason for the condition reported, reasons that the

Trans" Jack) at the DC Patch bay to isolate the

communications failure could be caused by a failure of

subscriber loop from the in-station circuitry.

the loop battery, an open connection at the subscriber

terminal, a defect in the cable, or an electrical failure of

personnel that the cable has been repaired, he should

the receiving device.

remove the dummy plug from the line jack appearance

(1) Loop battery check.

The technical

at the DC

controller should check the appropriate fuse panel (or

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