The live aluminium or copper conductors are continuously monitored for short-circuit and open circuit faults using ground-breaking Spread Spectrum Time Domain Reflectometry (SSTDR).
To understand SSTDR we will first explain the traditional Time-Domain Reflectometry technique (TDR). TDR is comparable to radar, whereby a pulse is sent out and a reflection is received and interpreted. TDR uses transmission line theory and pulse reflection principles to detect changes in electrical cables. TDR transmits high-energy electrical pulses along the cable which reflect off changes of characteristic impedance. By analysing the magnitude and shape of the reflected pulse the system features and potential faults can be determined. One of the biggest drawbacks of traditional TDR is that its accuracy is impacted by other signals on the line, requiring the system to be powered down to carry out the test. Most TDR testers require someone with the skill, experience and system knowledge to interpret the results.
SSTDR uses the same physics as traditional TDR but incorporates the use of pseudorandom noise and spread spectrum frequencies. Pseudorandom noise signals are specific digital signals that can co-exist with other signals on the line without interference issues. The signal voltages are so low that they sit within the noise floor of the system being monitored. Like TDR, the signals are sent and reflected, however, SSTDR correlates the reflection against the transmitted signal to generate a waveform. The complex SSTDR algorithms are then applied to automatically detect the fault type and provide distance to fault information to the user without the need for interpretation.
TDR will always have its place for carrying out spot tests on failed systems and periodic testing. SSTDR, however, provides an important alternative to this regime as its continuous monitoring of live systems enables the user to respond to emerging and critical faults as they happen while eliminating the requirement for powering down the system to carry out intrusive tests.