Locating cables and pipes: Part 1Follow
This section introduces the principals and techniques of locating buried cable and pipe utilities with the RD7100 system. For more information on the theory of cable and pipe location, refer to The theory of buried cable and pipe location, which is available to download from radiodetection.com
Passive frequency detection takes advantage of signals that may be present on buried metallic conductors. Dependent on the model, RD7100 locators support up to three types of passive frequencies: Power, Radio and CPS signals.
You can detect these frequencies without the aid of a transmitter if they are present on the utility you are surveying.
The RD7100PLM locator allow you to take advantage of the harmonic signals found on power networks.
When strong or interfering power signals are present, accurate tracing of a target cable can be challenging. Power Filters™ allow you to establish if a single large power signal comes from one source or from the presence of multiple cables. The different harmonic characteristics of the detected lines can then be used to trace and mark their route.
Once in Power Mode, press the key to switch out of Radiodetection’s sensitive Power Mode and scroll through the five individual Power Filters.
The use of an individual Power Filter harmonic can also allow you to locate power lines in situations where the total signal would otherwise be too large.
Active frequencies are applied to a buried conductor using the transmitter. The transmitter can apply a signal using three methods:
In Direct Connection, you connect the transmitter output directly to the utility. The transmitter will then apply a discrete signal which you can locate using the locator. This is the preferred method of applying a transmitter signal to a utility and in the majority of applications will apply a stronger signal to the utility, which may increase the locate distance.
To directly connect to a non-energized conductive utility:
- Switch the transmitter off.
- Connect the Direct Connection lead into the transmitter accessory socket.
- Clip the red connection lead to the utility ensuring that the area around the connection is clean and that a positive connection is achieved.
- Clip the black connection lead as far away as possible and at 90° to the ground stake or suitable ground point nearby ensuring that a positive connection is achieved.
- The display will show the Direct Connection lead connected icon.
Direct Connection Lead Icon
WARNING! Direct Connection to live conductors is POTENTIALLY LETHAL. Direct connections to live conductors should be attempted by fully qualified personnel only using the relevant products that allow connections to energized lines.
WARNING! The transmitter is capable of outputting potentially lethal voltages. Take care when handling the terminals, connection leads and ground stake, notify other technicians working on the line of the hazard and guard exposed conductors to prevent accidental contact.
In this mode of operation the transmitter is placed on the ground over or near the survey area. If a Direct Connection lead or signal clamp is not plugged into the transmitter, it will automatically go into induction mode. In this mode, only frequencies applicable for induction mode will be made available as the key is pressed.
Once activated the transmitter will induce the signal indiscriminately to any nearby buried conductors.
Please note that these signals will also be airborne and it is advisable to keep the distance between the transmitter and locator at least 10m / 30’ – this distance may need to be increased, particularly if depth measurements are taken.
An optional signal clamp can be connected to the transmitter and clamped around a cable or pipe to apply the transmitter signal. This method of applying the transmitter signal is particularly useful on insulated live wires and removes the need to disconnect the supply to the cable. Clamps are available up to 8.5” / 215mm in diameter.
WARNING! Do not clamp around uninsulated live conductors.
WARNING! Before applying or removing the clamp around a power cable, ensure that the clamp is connected to the transmitter at all times.
Choice of frequency for active location
The choice of signal frequency is an important factor for effective tracing and identification of buried lines, and there is no single frequency that covers all conditions. For simple instruments to be used by relatively non-technical personnel, there is no option but to make a compromise, and choose a single frequency high enough to give good performance in the induction mode, but not so high that it will couple too easily into unwanted lines. Active signals between 8kHz and 33kHz are commonly used for these applications. For more comprehensive equipment for problem-solving by technically competent technicians, a range of frequencies may be provided. Typical examples of these and reasons for their use are illustrated below.
512 Hz active signal
This low frequency is most useful for line tracing and identification over long distances. It does not couple easily to unwanted lines however it is too low for induction, and it falls within the band of power frequency harmonic interference.
8 kHz active signal
This medium frequency is the most useful general-purpose signal, high enough for induction, outside the power frequency interference band, and with limited coupling to wanted lines however it may not be high enough to impose a strong signal on small diameter line like telecom cables.
33 kHz active signal
This higher frequency is easily applied by induction to most lines, so is very useful for initial search. It travels on small diameter line however it couples more easily to unwanted lines, and loses its strength over shorter distances than lower frequencies.
100 kHz and over.
100kHz active signal
This very high frequency range deals with the difficult cases – induction onto small diameter lines in dry sandy soil, and short lengths of cable. It is very easy to apply by induction however it couples very easily to unwanted lines, and does not travel far.
For more information visit the Knowledge Base section in support.radiodetection.com or refer to the application note “The theory of buried cable and pipe location" which is available as a free download from www.radiodetection.com
It is important to select the correct or appropriate frequency for your particular application.
To select a frequency on the locator:
- Press the key to cycle through available frequencies.
- Alternatively, hold down the key and press the or keys to cycle up or down the range of frequencies.
If locating using an active frequency you must also set your transmitter to output the matching frequency. You can change your transmitter’s output frequency manually using your transmitter’s keypad.
To manually select a transmitter output frequency:
- Press the key to cycle through available frequencies.
NOTE: Some frequencies require that you connect an accessory, for example an A-Frame, before the frequency is available.