Cable Length Meter CLM1
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The Cable Length Meter CLM1 will allow the accurate measurement of severed or open/short-circuit CCTV coaxial cable. For use with RG59 (75Ω) and RG58 (50Ω) or equivalent coaxial cable, this meter provides a direct readout in metres of the length of an unterminated run of coaxial cable with a resolution of 2-3 metres. A single range of 2.5 to 400 metres on a 4 digit LCD display is provided with automatic indication of open circuit and short circuit conductors. By this means, the location of a short circuit or open circuit in cable runs of up to 800 metres may be found.
Cable length is measured by injecting a high speed pulse into one end of the cable and timing the return of any reflected signal (time domain reflectometer). This reflection occurs only when the cable is not correctly terminated and so will produce correct results whenever the cable is either completely open circuit or short circuit.
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Only unterminated cables (completely open circuit or short circuit) will produce a correct reading on the LCD display, correctly, or partially terminated cables will produce an indeterminate reading. It is important, therefore, to confirm that any unknown cable to be measured is either short or open circuit by using an ohmmeter if necessary.
Cable length measurement and fault location
- Switch on the CLM1 and select the appropriate impedance (SW2 in for 75Ω and SW2 out for 50Ω)
- Connect the cable under test to the BNC input of the CLM1
- The readout will show the length of the cable in metres preceded by a letter ‘H’ or ‘L’. (‘H’ indicates high impedance or open circuit; ‘L’ indicates low impedance or short circuit). Cable lengths greater than 410 metres will produce an over-range reading of three dashes.
Finding a short circuit or open circuit in cable lengths greater than 400 metres
Although the maximum range of the CLM1 is 400 metres – as long as both ends of the suspect cable are accessible, the location of the fault will produce a reading from at least one end in cable runs of up to 800 metres.
Locating faulty connectors in BNC – BNC leads
The identification of a faulty BNC plug is often difficult to determine, especially if open circuit. With BNC-BNC cables greater than 2.5 metres, the faulty connector will produce a reading of H 0 if open circuit or L 0 if short circuit.
Length Measurement of Non-Standard Speed Cables
Although designed for direct reading of cable lengths in standard (Vp=200Mm/s) cable, the CLM1 may usefully be employed in the length measurement of cables of other speeds by the use of a conversion table.
Download Literature - clm1.pdf (58kb)
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The Cable Length Meter CLM1 is just part of the innovative and portable Ranger series of CCTV test equipment. Click here to discover the whole range |
