DigiTrace DET-3000 Manuel de référence rapide - Page 3

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4.1 Pvf defined:

The speed of light in a vacuum is represented by the number 1 (100%). All other signals are

slower. A cable with a pvf of .85 would transmit a signal at 85% of the speed of light. The pvf of

a cable is determined by the dielectric material that separates the two conductors. By entering

the correct pvf, the instrument is calibrated to the particular cable. Typically, the pvf of the cable

under test will be listed in the cable manufacturer's catalogue. (see table, chapter 7. of this doc-

ument or point your internet browser at: www.tycothermal.com for the most recent version of

this document. In case the pvf of the cable under test is not known it must be determined using

the procedure below.

4.2 Determining the propagation velocity factor of unknown cable types

Simply measure a length of good cable (no faults) and change the DET-3000´s pvf setting until

the display shows the same distance reading as the measured length. If no good cable is avail-

able, the distance to the fault can be measured from both ends of the cable using an arbitrary

pvf (the same must be used at each end). The actual distance to the fault can be calculated as

in the following example:

Total cable length = 250 m (from records or measured)

Distance measured from first end = 90 m

Distance measured from second end = 140 m

_ ~

X 250

97.8 m

Actual distance from first end =

(

140

)

If required, the instrument can then be reconnected at the first end and the pvf adjusted until

the indicated distance is 97.8 m. This value of pvf can then be noted and used for subsequent

fault location on the same type of cable.

It is also important to know that the velocity factor of a cable can change with temperature and

age. It can also vary from one manufacturing run to another. Even new cable can vary as much

as ± 3%.

5. Use common sense

Although a thorough understanding of time domain reflectometry and the measuring instrument

is vital to successful troubleshooting, there is never a substitute for good common sense. If

your DET-3000 indicates a distance of 150 meter to a fault, but there is evidence that a forklift

ran into the pipe at 162 meter, there is a pretty good chance that the fault was caused by the

accident. Familiarity develops versatility. The more you use the DET-3000, the more confident

and comfortable you will become, and the more applications you will find for it.

6. Locating faults on heat tracing systems

using the DET-3000

The DET-3000 comes with 100 OHM test leads preinstalled NEVER USE THE DET-3000 on

cables connected to the mains supply. Besides the live hazard the voltages at mains level

exposed to the inputs will cause severe damage to the unit.

Prior to each measurement verify that the complete installation is isolated from the mains sup-

ply. Disconnect the heating cable from the connection terminals at the power connection. It is

important that all measurements are performed directly at the heating cable to avoid mis-read-

ings due to power cable used to bring the power to the heating cable. Cold leads permanently

joined with the heating cable, as is the case with MI or other types of series heaters, do not

have to be removed. The final distance readout will have to be adjusted by subtracting the

length of the cold lead.

Connect the leads of the DET-3000 to the cable under test. One lead goes to the conductors,

twisted together in a pig tail, and the other to the braid. Select the appropriate pvf (propagation

velocity factor) from the table (see section 7) depending on the heating cable under test (see

table). Refer to section 4.2 of this manual to determine the pvf of unlisted cables or from cables

from which the pvf is not known.

Select a measuring range exceeding the expected length of the circuit under test. The end of the

cable, or a severe cable fault, is seen on the unit display as a deflection from the horizontal line

across the middle of the screen (in case of doubt start off from the Max range of 3000 m).

Examine the trace for any deflections from the horizontal line after the transmit pulse. Examine

the trace from the left to the right, moving the cursor along the horizontal line to the edge of the

first significant deflection seen. Set the cursor at one pixel to the left of the deflection and read