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AN INTRODUCTION TO CABLE LENGTH CONSIDERATIONS
While it is tempting to think of wire as an ideal conductor that gets a signal from point A to point B without alteration, this is often not the case – particularly for long cable runs. In an ideal world, the cable connecting a sensor to its associated monitoring system would be a perfectly lossless conductor that would not alter the signal whatsoever. In the real world, however, the cable exhibits, resistance, inductance, and capacitance ((Figure 1). These values for R, L, and C are typically expressed on a per-meter or per-kilometer basis when SI units are used. These distributed parameters act to alter and distort the sensor signal in four undesirable ways (Figure2).
Low-pass filtering effects: The distributed cable parameters form a low-pass filter that allows low frequencies to pass with little or no attenuation but effectively blocks high-frequency components of the signal from passing. This results in an altered waveform that may be unsuitable for conducting proper diagnostics. The signal may be so degraded that it is unsuitable not just for diagnostics, but protection as well.
Slew-rate effects: For speed measurements, the signal often originates as a rectangular pulse train but distorts due to the impact of wiring capacitance (known as slew-rate). The pulse train shape with abrupt vertical changes originates because the speed sensor is often observing a toothed surface, a key, a keyway, or some other physical discontinuity in the surface observed by the sensor. If there were no limitations on slew rate, such vertical discontinuities could be reproduced. But in real wiring, the capacitance limits the ability for instantaneous changes in signal voltage and the crisp, rectangular pulse train begins to resemble a series of smoothed pulses where the abrupt edges are rounded. While slew-rate rarely affects vibration and dynamic pressure signals, it can frequently affect speed signals.
Resistance effects: Cable resistance forms a voltage divider that drops some of the original signal voltage produced by the sensor across the wiring instead of entirely across the input impedance of the monitor. This has the undesirable effect of decreasing the signal-to-noise ratio.
FURTHER WIRING INFLUENCES
Voltage and current must also be considered. When the signal is transmitted as a varying voltage, it is referred to as voltage-mode signal transmission. Although voltage-mode is considered the conventional method for transmitting vibration signals, and is commonplace, it is the most vulnerable to the four effects discussed above. As such, voltage-mode transmission tends to constrain the length of field wiring more than alternative modes. Many manufacturers, however, do not support anything other than voltage-mode transmission with their sensors.
When the signal is instead transmitted as a varying current, known as current-mode signal transmission, it mitigates many of these effects and allows much longer field wiring lengths than with conventional voltage-mode transmission.
Note, though, that there are certain situations in which neither voltage-mode nor current-mode transmission are adequate over long distances. By using frequency modulation to transmit the signal, certain combination of longer field wiring distances and frequency content can be supported, as an alternative to conventional 4-20mA scheme.
While signal degradation is always a constraint due to field wiring length, another constraint occurs when the installation is in a hazardous area andintrinsically safe (IS) methods must be used to achieve compliance: stored energy. Both cable inductance and cable capacitance have the effect of storing energy. When a fault in the wiring or device takes place, IS constrains a resulting spark from releasing sufficient energy to ignite the surrounding flammable atmosphere. It does not prevent a spark, but instead limits the energy in the spark. Consequently, approvals for IS provide strict limits on the total amount of inductance and capacitance that can exist in the hazardous area, and thus the amount of energy that can be stored and released as a spark.
Signal degradation is not the only constraint that exists on field wiring length when the installation involves a hazardous area and IS wiring. In such installations, the user must compute the wiring length constraint under both signal degradation and stored energy criteria and then use the more restrictive of the two separate constraints.
Dr. Luc Fromaigeat, Technical Support Engineer Energy at Meggitt SA Switzerland. He has 35+ years of field experience in Vibration, acoustics analysis, and diagnostics. He teaches Vibrations for the Swiss “Brevet Federal de Maintenance” and for Meggitt worldwide. He authored an extensive application note with worked example calculations and a downloadable calculator for field wiring calculations https://meggittsensing. com/energy/app-note-cable-length/