Although efficient and small, heavy use of this kind of power control makes life difficult for computers and measurement equipment with needs for reasonably clean power.
Although it might seem to be a separate issue, non-sinusoidal voltage and current waveforms as used by the above mentioned SCR's and pulse width modulated motor controllers, cause similar waveforms of ground current. The resulting IR voltages (where I is current and R is ground line resistance) generated on the ground lines is disturbing to the zero volt reference that the ground line is meant to provide.
A technique that our technicians use to determine the degree that our equipment is effected by poor grounding is as follows:
First make sure that the Lear equipment ground wires are properly connected, that is, each Lear cabinet is connected to it's neighbor using the lugs mounted on the back of each cabinet and this string of cabinets is then connected to the large cabinet housing the entire system. This large cabinet is then connected to your best guess of a decent plant ground. We say "best guess" since few if any of the plant people can tell you anything about the plant grounding in the context of this discussion. Sometimes a discussion with the persons who have a computer in their office and who are located on the plant floor will provide useful information.
After the above is accomplished, start a line where the wire has been isolated from the detector. With the line running, lower the fault current setting in the SET-UP screen to one micro amp below the spec value that was set into the SET-UP TABLE. Observe whether any faults occur. Watch the detail screen to indicate this. If no faults occur after a few seconds, lower the current setting by another micro amp and watch for faults, continue to do this until false faults start to occur. If you get down to one or two micro amps with no faults occurring, the plant grounding is satisfactory for continued operations.
If this is not the case, talk to the plant electrician about what might be done to improve the situation. Perhaps, running the cabinet ground to a different location in the plant may be the best and easiest solution.
Another problem that we encounter when commissioning a new system is the case where the Lear data cables connecting the individual detectors to the system console are sharing a wire trough with plant power wires. A similar exposure is when the Lear data cables are bundled or tie wrapped together with the plant power wiring.
Although the Lear data lines are shielded, this is a severe exposure and can affect the performance of the Lear system.
UPS PROTECTION: We do recommend the use of this type equipment for two reasons. First, a momentary loss of power due to some interruption in the plant power will cause current spool files to be lost. Finished spool files are stored on the Lear hard disk and are not lost. Second, in some cases during the outage event, severe voltage spikes occur which can damage the equipment and/or cause the software to be corrupted.
At the minimum, we strongly suggest some form of voltage transient suppression to protect the Lear equipment from local thunderstorm activity and the turn on-turn-off voltage transients generated by nearby heavy equipment.
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