Cable Length Meter CCTV Test Equipment

The Cable Length Meter CLM1 will allow the accurate measurement of severed or open/short-circuit CCTV coaxial cable. For use with RG59 (75) and RG58 (50) 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.

Additional Info

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.

• Measurement range 2.5 - 400 metres


• Measurement method
  TDR
  
• Accuracy
  ? 1.5 metres
  
• Resolution
  2.5 metres
  
• Power source
  PP3 Alkaline battery
  
• Drain
  37mA (off load)
  
• Dimensions (overall)
  118 x 60 x 29mm
  
• Weight
  95g (with battery 140g)
  
• Optional equipment
  Leather-board case with belt clip
  

Cable length measurement and fault location

• Switch on the CLM1 and select the appropriate impedance (SW2 in for 75 and SW2 out for 50)


• 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