Megger CFL535E / TDR2000 is an advanced instrument capable of identifying a wide range of cable faults. The instrument uses a technique called Time Domain Reflectometry (TDR) which in many ways is similar to radar. Narrow pulses of electrical energy are transmitted along a pair of conductors in a cable. The pulse travels through the cable at a velocity determined by the insulation between the conductors and the resistance to the flow of the pulse is characterised as impedance for the cable. Changes in cable impedance will cause a proportion of the pulse to be reflected. The pulse velocity is normally described as a fraction of the speed of light and is called the Velocity Factor. By measuring the time between the transmitted pulse and the reception of the reflected pulse, and multiplying this by the speed of light and the velocity factor, the actual distance to the reflection point can be given.
Faulty cables, poor joints or discontinuities will all cause a change in impedance. Impedance's higher than the cable's cause a normal reflection. Impedance's lower than the cable's cause an inverse reflection. Matched terminations absorb all the pulse hence no reflection will occur, the cable appearing endless. Open or Short circuits will reflect all the pulse energy and the TDR will not see the cable beyond that fault. As a pulse is transmitted down a cable, the size and shape of that pulse is gradually attenuated by losses in the cable: the pulse gets smaller in height and more spread out. The level of attenuation is determined by the cable type, the condition of the cable and any connections along its length. The limit of how far you can see is determined by the point beyond which you will not discern a reflection. To maximise the instruments range, the CFL535E / TDR2000 has an adjustable gain setting on its input that can apply up to 90 dB of gain to the reflected signal to allow you to discern a reflection from farther away. By combining this variable gain with increasing pulse widths, the CFL535E / TDR2000 can discern faults up to 16 km away.
The CFL535E / TDR2000 can be used on any cable consisting of at least two insulated metallic elements, one of which may be the armouring or screen of the cable. The balancing circuit can balance for any cable with a characteristic impedance of 0 - 120Ω. Dual inputs and the large graphic display allow a wide range of comparative tests to be performed between cable pairs or stored results. The instrument has 15 trace memories, enabling previous test results to be displayed and compared with live results. This allows the gradual ageing of a cable to be monitored or characteristic changes to be detected between periodic tests, for example if the cable has suffered water ingress or has been tapped and split.
- Balance Control
- Velocity Factor: ratio of the transmitted pulse speed to the speed of light, or as a distance per microsecond.
- Pulse Widths range from 20ns to 16μs
- 15 Memory Locations