Grounding Earthing Vibration System Bently Nevada – Rotating Machinery Information Systems
Grounding Earthing Vibration System Bently Nevada
General Practice
Connections between system common and earth ground must occur at earth ground points of equal voltage potential. Usually this is most easily accomplished by connecting system common to earth ground at a single point. That point is usually at the instrument rack or, for intrinsically safe systems, at the barrier “earth” ground bus bar.
These guidelines, in conjunction with good engineering judgement, should result in minimization of ground loop generated noise problems.
General Review
In the past, it has been standard practice to earth ground the system at each Proximitor®. Unless special provisions were made, this occurred automatically since the Proximitor case was internally connected to common, and the standard mounting plate was electrically connected to the housing, which, was generally earth grounded.
On small systems, such as a skid-mounted or packaged machinery trains with locally-mounted monitors, or in plants where effective potential equalizing cables are in use, earth grounding at the Proximitor usually is satisfactory. However, on large monitoring systems with long distances between monitored machines, earth grounding of each Proximitor may create ground loop problems. A ground loop occurs when two remote points of a circuit are connected to non-identical grounds, i.e. a potential difference exists between them. There have been many cases where a significant potential difference existed between earth grounds at different machines. In this situation (Figure 1), the system common conductors become potential equalizing conductors. The resulting current passing through the common conductor’s resistance develops a voltage that adds to the signal voltage and results in noise seen by the monitor. The equivalent circuit in Figure 1 and the resulting equation show the noise which can result. For example, if the difference in grounds is 2 V p-p (0.7 Vrms) and the cable length between the Proximitor and monitor is the same for each channel, the indicated reading due entirely to the ground loop on a vibration monitor with a 200 mV/mil input would be 5 mils p-p.
Another potential problem is the situation where the monitor signal common test points can be at excessive voltage levels relative to panel or control room ground. This can cause considerable electrical problems when earth grounded test equipment is used.
Single-point earth grounding at the monitor or safety barrier earth ground bus bar usually eliminates these problems. Figure 2 shows an equivalent circuit which shows how the problem is eliminated.
Caution
A shock hazard may be present in monitoring systems which are allowed to “float” or be left ungrounded. Bently Nevada does not recommend ungrounded systems.
Bently Nevada Rotating Machinery Information Systems
Implementation
Proximity Probes Grounding
In order to facilitate a single-point earth ground, all standard Bently Nevada weatherproof and explosion-proof Proximitor housings are supplied with Proximitor isolation. The mechanical method of achieving this isolation is shown in Figure 3. If a grounded Proximitor installation is desired, remove one or more of the phenolic isolating washers (Figure 3) from each Proximitor. The Proximitor isolation kit provided with the housing is available separately as Bently Nevada Catalog Number 19094-01 for use when a Bently Nevada housing is not utilized. Figure 4 illustrates the field wiring arrangement necessary to achieve the single-point earth ground and maximum noise immunity with 3300, 7200 and 9000 Series monitors. As the figure indicates, it is necessary to electrically insulate the coaxial connector.
Thermocouples Grounding
In addition, it is recommended that where possible, ungrounded thermocouples be used with Bently Nevada monitoring systems. Bently Nevada’s 7200 and 9000 Series thermocouple monitors have differential inputs with high common mode rejection, but common mode voltages of 10 volts peak to peak or greater between the instrument rack’s ground and a grounded tip thermocouple will overload the input circuit.
The 3300 thermocouple monitor contains galvanic isolation (up to 250 Vdc) to ensure high rejection of common mode ripple and noise. Figure 6 illustrates typical field wiring for grounded and ungrounded thermocouple systems.
There is an additional advantage to the individual Proximitor isolation method now provided over the older optional isolation technique. The individual isolation plates preclude a potential fault condition within intrinsically safe systems. Previous isolation designs did not eliminate the conductive path between each Proximitor or interface module within a housing. In this instance, a wiring fault (i.e., five of six signal commons left unconnected) would result in an excessive power return current in the remaining conductor. The new design eliminates this possibility.
It should be noted that with the single-point grounding system, the voltage potentials shown in Figure 1 still exist. However, the potential difference will now exist between the Proximitor common (which is internally connected to the Proximitor case) and the Proximitor housing (Proximitor common is now at the instrument rack earth potential). This is unacceptable in an intrinsically safe system. This problem is usually eliminated by potential equalizing cables between machines and a central ground.
Velocity Transducers Grounding
Figure 5 illustrates the typical field wiring arrangement for velocity Seismoprobes®. Single-point earth ground is achieved by grounding the cables’s shield at the monitor end only. When a Velocity-to-Displacement Converter (VDC) or a Velocity Interface Module is used, single-point earth ground is achieved by connection per Figure 4.
Single-point earth grounding of accelerometer-based monitoring systems is similar to that in Figure 4. Accelerometer interface modules are isolated from earth ground by using the 19094-01 Isolation Kit or one similar to it.
RTDs Grounding
Single-point earth ground systems using Resistance Temperature Detectors (RTDs) are achieved by connection as shown in Figure 7.
