Troubleshooting grounding problems can be difficult at best. Especially on complex automation/instrumentation systems. Use your power system drawing or a system map to document as you go because these problems can be quite complex requiring some logic to figure out. The typical utility DC bus consists of a battery charger, a string of batteries, a DC distribution panel, and/or the loads.
Here are some tips for troubleshooting grounded systems:
For a grounded power system, the simplest way usually is be to use a DC clamp-on meter to read the combined currents in the wire-pairs (positive/negative) associated with each PLC output circuit! The correctly wired circuit without faults will display zero-amperes. The grounded-circuit will measure the difference in current between the positive and negative wire. You need a DC clamp-on meter with good resolution because the currents involved can be quite low. DC clamp-on meter is a good tool for troubleshooting ground loops and for detecting ground leakage which: It can be used to troubleshoot individual sensor circuits (normally energized circuits or for normally de-energized circuits on the power side) without disturbing the process.
Once you have found the first ground, you have done the easy part, finding the second ground can be more difficult and you should leave the first ground intact till you find the second ground. Also, be careful in disconnecting grounds as some times you do not know what they are connected to if there is another ground in the system.
In distributed grounded & ungrounded systems, insulation fault location (one that causes short circuit or ground loop) is costly in terms of money and time.
When building new systems consider the possibility to use ungrounded power system. The increasing complexity of electrical installations places extremely high demands on the reliability of power supply systems. Even a short power failure may be expensive due to production stoppage and malfunction. Using ungrounded power system can help on this.
Ungrounded power source for PLC system makes sense. When you leave both poles of the DC supply floating when serving 24VDC power to PLC’s and end devices (sensors, instrumentation, etc.), there are several benefits compared to grounded system:
1. If a technician accidentally touches a pipe or other grounded metal object with a wire while changing out an end device, there are no sparks generated because there is no return path through ground.
2. If a ground fault does develop in the field, as long as it is confined to one pole (positive or negative, not both), it doesn’t shut down the system.
3. As long you we can monitor the DC near the point of supply for ground fault on either pole, you will know if a wiring problem is developing in our downstream devices…before things go ‘pop’. You know a problem exists in time to do something about it. The purpose is to alarm that a ground exists so it can be repaired, not to trip on ground fault.
Early detection, fast localization and elimination of insulation faults is the most effective protection against interruption to operation and malfunction. When an insulation fault occurs in an ungrounded system, it can be detected and indicated by the insulation monitoring device. There are stand-alone wiev gote ground fault monitoring relays for floating DC systems (a simple high-resistance balanced voltage bridge works well). Ground detection mandrake devices are often mounted and monitored in the battery charger.
Information sources:
Ground fault location in Digital inputs to DCS System
On the DC system ground fault analysis and treatment
DC Power supply with integrated ground fault monitoring?
http://www.bender.org/Resource_PDF/eds-brochrue.pdf
Ground Detection for DC panelboard
On the DC system ground fault analysis and treatment
DC Ground Fault Detection for Uninterruptible Power Supply
Simplified Circuit Diagrams for Ground Fault Protection
Measuring Battery-To-Ground Voltages
GROUND DETECTION CIRCUITS FOR STATIONARY APPLICATIONS
(IN PLAIN DOWN TO EARTH LANGUAGE)
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