Megger CFL510F Manual de usuario

CABLE FAULT LOCATOR

MEGGER®CFL510F (TDR1000/2)

User Guide

MEGGER®

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SAFETY WARNINGS

Although this tester does not generate any hazardous voltages, circuits to which it can be connected could be dangerous

due to electric shock hazard or due to arcing (initiated by short circuit). While every effort has been made by the

manufacturer to reduce the hazard, the user must assume responsibility for ensuring his, or her own safety.

For use on energised systems rated up to 300V Installation Category III* the fused clip set AVO Part

Number 6111-218, must be used.

* Relates to the transient over voltages likely to be met in ﬁxed wiring installations.

4mm plug to BNC Adaptor:

This 4mm plug to BNC adapter is intended for use with telecomm network cables only, it is not designed or intended for

direct connection to an energised mains supply. However, in normal use it may be subject to telecom network voltages

(TNV) as deﬁned by IEC 60950 3rd edition (1999-04).

The BNC plug and socket are, by necessity, accessible. The outer sheath for this connector is normally at SELV levels,

however under single fault conditions it may carry hazardous voltages. The operator must therefore verify that the

accessible plug or socket is at SELV levels prior to touching, or alternatively wear appropriate insulated gloves.

•The instrument should not be used if any part of it is damaged.

•Test leads, probes and crocodile clips must be in good order, clean and with no broken or cracked insulation.

•Check that all lead connections are correct before making a test.

•A Fused Lead Set must be used to connect to energised live systems. Refer to the accessories section for options.

•Disconnect the test leads before accessing the battery compartment.

•Refer to operating instructions for further explanation and precautions.

•Safety Warnings and Precautions must be read and understood before the instrument is used.

They must be observed during use.

NOTE

THE INSTRUMENTS MUST ONLY BE USED BY SUITABLY TRAINED AND COMPETENT PERSONS.

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Contents

Safety Warnings 3

Introduction 5

General description

Battery replacement

User Controls and Display 8

Layout and functions

Operation 10

General Testing procedure

Line Feed

Connection to cable under test

Measuring distance to fault

TX Null

Velocity Factor

Pulse widths

Techniques to improve accuracy

Care and Maintenance 15

Speciﬁcations 16

Symbols used on the instrument are:

Caution:Referto accompanying notes.

Equipment protected throughout by

double or reinforced nsulation.

Instrument ﬂash tested to 3.7kV rms.

Equipment complies with current

EU directives.

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Introduction

Thank you for purchasing the CFL510F cable fault

locator. Before attempting use of your new

instrument please take the time to read this user

guide, ultimately this will save you time, advise you of

any precautions you need to take and could prevent

damage to yourself and the instrument.

The CFL510F is an advanced instrument capable of

identifying a wide range of cable faults. The

instrument uses a technique called Pulse Echo (also

known as Time Domain Reﬂectometry or TDR). A

pulse is launched into a cable from one end. This can

be on either a pair of conductors, or a conductor and

the screen. The pulse travels down the cable at a

velocity determined by the insulation between the

conductors and this resistance to the ﬂow of the

pulse is characterised as impedance for the cable.

Any changes in cable impedance will cause a

proportion of the pulse to be reﬂected.

The pulse velocity is normally described as a fraction

of the speed of light and is called the Velocity Factor.

By measuring the time between the transmitted pulse

and the reception of the reflected pulse, and

multiplying this by the speed of light and the velocity

factor, the actual distance to the reﬂection point can

be established.

Reﬂections are caused by changes in the cables

characteristic impedance, such as poor joints or

discontinuities. Faults showing an impedance higher

than that of the cables normal impedance will cause

a reﬂection of the same polarity, i.e.positive, whilst

faults with an impedance lower than that of the cable

will cause an inverse negative going reflection.

Matched cable terminations absorb all the pulse

hence no “end of cable” reﬂection will occur, the

cable appearing endless. Open or short circuits will

reﬂect all the pulse and a large reﬂection will be

displayed. At an open or short circuit all the

transmitted energy is reﬂected and the TDR will not

‘see’ the cable beyond that fault.

As a pulse travels down a cable, the size and shape

of that pulse is gradually attenuated by the cable. The

pulse reduces in amplitude and becomes more

elongated or stretched. The level of attenuation (or

losses) is determined by the cable type, the condition

of the cable and any connections along its length.

The limit of how far you can see is determined by the

point beyond which you will not be able to see or

distinguish a reflection. To help identify small

reflections, especially at greater distance the

CFL510F has an adjustable gain setting. By

increasing the gain small reﬂections become more

obvious.

The CFL510F can be used on any cable consisting of

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Introduction

at least two insulated metallic elements, one of which

may be the armouring or screen of the cable. The

CFL510F has internal matching networks to allow

testing of 25, 50, 75 and 100 cables. (These typically

correspond to power, coaxial data and data/telecoms

cable). By selecting the TDR impedance closest to

that of the cable under test, maximum power can be

transmitted into the cable allowing long cables to be

tested.

The velocity factor of the TDR must be adjusted to

match that of the cable under test, allowing an

accurate distance measurement to be read directly

from the instrument. Where the VF of a cable is not

known, but the length is, the cursor can be set to the

end of the cable and the VF on the TDR adjusted until

the correct cable length is displayed.

Other conﬁguration settings include changing the

distance units between metres and feet, changing the

propagation velocity units between a ratio and a

distance per microsecond. Display contrast is fully

adjustable to compensate for all viewing conditions.

A backlight aids viewing in low ambient light

conditions.

The instrument can be powered by manganese-

alkali, nickel-cadmium or nickel-metal-hydride

batteries. All cells must be of the same type.

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When the low battery symbol appears in the

display window the cells are nearly exhausted and

should be replaced as soon as possible. Use alkaline

cells IEC LR6 (AA) 1.5V or 1.2V rechargeable cells

only.

To install or replace the cells, switch the instrument

off. Disconnect the test leads, loosen the battery

cover retaining screws and remove the cover. Lift out

and disconnect the battery holder.

Replace the cells, ensuring that correct polarity is

observed (shown on the battery holder).

Incorrect battery cell polarity can cause electrolyte

leakage resulting in damage to the instrument

Reﬁtting the battery holder is the reverse of removing

it.

Battery ﬁtting and replacement

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User controls and display

The controls of the TDR have been arranged such that

the instrument is easy to learn and use. The instrument

controls consist of the following

Instrument Display: The display shows the user the

current settings of the instrument and the reﬂected

energy trace from the cable connected.

Tx Null: This rotary control allows the user to

match the internal balance circuit of the TDR to

that of the cable under test. When correctly

adjusted the majority of the displayed

transmitted pulse can be nulled out, allowing

cable features close to the start of the cable to

be identiﬁed. Refer to section TX Null for further

details.

Cursor Left: Moves the ‘distance’ cursor to the

left. Auto repeats if held down.

Cursor Right: Moves the ‘distance’ cursor to

the right. Auto repeats if held down

3000m 1780m 75

0.67R

Ω

Transmission

pulse

Range Distance to cursor Impedance

Gain

Reﬂection

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User controls and display

Find key: Automatically moves the cursor to

the ﬁrst potential fault (change of impedance).

Power: Pressing this button will turn the

instrument on or off depending on its current

state.

Range: A bi-directional button allowing range

adjustment from 10m (30ft) to 3km (10 000ft)

in 6 steps. The bi-directional feature allows

ranges to be selected forward and backward,

without having to scroll through all the ranges

in one direction

Gain: This control is used to increase or

decrease the gain of the instrument. This

helps the user identify faults over the entire

cable length.

Velocity Factor: Adjusts the Velocity Factor

(VF) of the instrument to match that of the

cable under test.

IMPORTANT: The VF value must match the

VF of the cable under test to give an accurate

distance measurement.

Menu: Allows the user to change the units of

measurement for Distance, Velocity Factor,

Impedance and Test Rate. Refer to menu

section later in this text.

Backlight: Pressing this button will toggle the

backlight on and off. The backlight will

automatically switch off after 1 minute

Contrast: Allows the user to manually correct

the display contrast to suit local ambient

lighting conditions.

Output Sockets: 4mm shrouded sockets designed to

accept the leads supplied with the instrument.

Battery cover: Located on the back of the instrument

and provides the user with access to the battery

compartment. The cover must not be removed while

the instrument is switched on or connected to a cable.

The instrument must not be operated with the cover

open.

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Operation

General Testing procedure

Ensure the correct test leads are ﬁrmly ﬁtted into the

sockets of the instrument.

Switch on the instrument. The CFL510F will display

the start screen for a couple of seconds, followed by

a trace. The instrument will be set to the range and

velocity factor last used. If these settings are different

for the cable under test (C.U.T) then use the RANGE

and VF keys to set the correct values. Refer also to

‘Velocity factor’ later in this text.

Connection to Cable Under Test

Connect the test lead to the cable under test.

Connection may be made to a live system with a

voltage to earth (ground) less than 300V with an

installation (over voltage) category of III or lower. This

means that the instrument may be connected to any

ﬁxed wiring of a building installation, but not to

primary supply circuits such as overhead cables.

A Fused Lead Set must be used to connect to

energised live systems. Refer to the accessories

section for options.

When selecting 25 cable impedance, an internal

50Hz/60Hz ﬁlter is automatically switched in.

Measuring distance to fault

Ensure that the whole length of the cable can be seen

on the display, and that the range selected is correct.

Find key

The Find key searches for the furthest major

change of impedance. This may be a fault or the end

of the cable if this is a short or open circuit.

When found the event is displayed and the cursor

placed on the leading edge. The gain, zoom and

cursor position may need to be adjusted for accurate

measurement. Note that Find does not adjust the

TxNull or Velocity Factor.

To enable ‘difficult to see’ faults to be identiﬁed, the

gain of the instrument can be adjusted. With the gain

at minimum small changes of cable impedance may

be overlooked. By increasing the gain the fault

becomes more obvious.

If no faults are obvious, adjust the gain until any major

reﬂections appear. Open and short circuits should

easily be identiﬁed. Partial faults can be less obvious.

If no signiﬁcant reﬂections can be seen use the “TX

Null” to minimise the effect of the output pulse on any

potential “near end” faults (refer to section on TX

Null).

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Operation

In the event of no reﬂections being visible, increase

gain until any reﬂection can be easily identiﬁed.

(If no reﬂections can be seen, try shorting or earthing

the far end of the cable to ensure that you are

“seeing” the whole length of the cable).

The cursor can also be moved using the LEFT and

RIGHT cursor keys. Move the cursor to the beginning

of the reﬂection. The distance to the fault can then be

directly read from the display.

NOTE: The distance calculation is performed using

the velocity factor set in the TDR. If this velocity

factor is not correct for the circuit under test, the

displayed distance will be incorrect.

Opposite are shown two typical trace displays. The

top shows an open circuit at 2000m away; the

second a short circuit at 2000m away.

0.67R

3000m 2000m

100Ω

0.67R

3000m 2000m

100Ω

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