Understanding CD

Current Direction recognition is a feature that helps to positively identify a line at points distant from the application of the signal. It is highly desirable, if not essential, for positive identification of long distance lines. These lines can be traced and positively identified through congested areas or when running parallel to other lines.

The CD feature on the RD8200 locators indicate the direction of the current flow on a line. Identity of the target line is established if the locator display indicates that the current is flowing forward and away from the point of application of the transmitter signal.

A signal that has coupled onto adjacent lines finds a return path to the point of the original signal application. This is indicated by the locator arrow pointing back towards the transmitter.

This is in contrast to the forward pointing arrow indicating the target line.

The CD transmitter signal should be directly coupled to the target line with a remote ground connection. Most CD signals are low frequency, making them unsuitable for connection with a normal clamp or induction.  These can however be applied with a transmitter CD clamp.

Using Current Direction (CD) to aid line identification

The RD8200 locators, and Tx-10 and Tx-10B transmitters feature several CD frequencies to help you determine the direction of current through a target line.

NOTE: The transmitter and locator must both have the same CD frequency or frequencies installed to enable the feature to be used. If you have more than one CD frequency installed ensure that the transmitter and locator are both set to the same CD frequency. CD frequencies on both the locator and transmitter will be denoted by the icon on the display.

Current Direction

To select the CD frequency, proceed as follows:

Transmitter

1. Switch the transmitter off.
2. Connect the transmitter to target cable or pipe, either by Direct Connection or by using a CD clamp.
3. Switch the transmitter on.
4. Press the key until the CD frequency is displayed.

CD is a combination of two frequencies, a CD frequency and a locate frequency.

Locator

1. Press the key until the CD frequency is displayed, indicated by the icon. 

4 kHz CD

RD8200 models feature Current Direction using a 4 kHz frequency.  This frequency is designed to enable you to use CD to identify a target line on higher impedance cables such as telephone cables and CATV.

CD reset

About CD reset

When you trace a signal on very long target lines, the transmitter signal gradually bleeds into the ground by capacitance. This means the phase angle of the remaining signal gradually changes. A CD Reset can be used to re-establish correct CD operation. 

To perform a CD reset, press and hold the key.

This is referred to as phase-shift and can occur whenever an alternating current signal is flowing in a system of conductors that have a significant capacitance or inductance. The relative phase angle between the two frequencies will alter, but only over extended distances.

CD reset

The illustration above 'CD Reset' represents a buried line with significant capacitance to ground. It illustrates the gradual phase-shift that occurs along the line. The diagram shows a reference point on each waveform. At first, the reference point occurs right on the Peak of a positive half cycle (A). It gradually moves towards the zero crossing point (D) and ends up on the Peak of a negative half cycle (F). The direction has shifted from forwards to backwards. At the intermediate point (D) the current flow cannot be determined. At this point the CD arrows on the RD8200 locator will flash on and off. In the example shown in the illustration, by returning to (C) which is the last point at which a CD reading was achieved, and performing a CD reset, the response at (C) will be equivalent to that of (A).

In most cases the phase angle shift does not occur until many kilometers from the transmitter and in some cases a CD reset may only be required every 20 kilometers (15 miles).

Performing a CD reset

You should perform a CD reset every time you select a CD frequency. CD reset can only be performed when in CD mode.

To perform a CD reset:

While using any CD frequency,

1. Press and hold the key.

Alternatively

1. Press the key to enter the menu.
2. Scroll to the CDR menu using the or keys.
3. Press the key to enter the CDR menu.
4. Select YES.
5. Press the key to reset CD and exit to the main menu.

CD clamps & stethoscopes

On both the CD / CM clamp and the CD stethoscope there is an arrow that indicates which way the clamp or stethoscope is facing in relation to the transmitter. If the arrow is pointing away from the transmitter when the CD reset is performed the locator display shows 000 and the bar graph reads 100%. If the arrow is pointing towards the transmitter, the locator display shows 111 and there is no bar graph reading.

When using either of these accessories it is important to reset the CD at a known point, near the transmitter, before making any identification measurements