Three primary categories of low-level currents will be discussed in this paper.
1. Ground Currents are any currents that are flowing on grounds. Grounds can be:
- Chassis grounds,
- Safety grounds,
- Raceways, or
- Reference grounds.
2. Leakage Currents are currents that are derived from normal and abnormal connections of the source to the safety earth. In general, leakage currents are more easily identified that ground currents. Leakage currents often flow back to the source through the associated ground conductor.
However, they may also flow through alternate ground paths, contributing to ground currents throughout the building.
3. Net Currents are non-zero summary currents that flow on conduits, feeders, or branches. They can be related to ground currents, leakage currents that return to the source through parallel ground paths, or neutral currents that flow to the source through parallel ground paths.
These three types of low-level currents, while often related, must be measured and investigated differently – the types of troubleshooting and appropriate solutions vary widely.
Basic ground currents are measured on a ground conductor, typically colored or marked green, or a bare conductor. If reviewing a set of power monitor data, or remotely reviewing site measurements made by another individual, this is the most probable type of low-level current measurement that is made.
As it can be seen above, ground current measurements are being made on the equipment safety ground A as well as on a supplemental ground connection B. Usually, measurement point A is what people refer to when discussing ground currents.
Assuming a normal electrical system (with no attempts made to insulate equipment, raceways, etc.) there are other paths for ground current: raceways, conduit, mounting hardware, mechanical chassis connection, and signal connections. Each of these paths may be a sufficiently low impedance parallel ground path to take a significant portion of the ground current.
As a result, simply measuring the ground current at point A (for instance) provides limited information about the total ground current that may be flowing.
|Title:||Measurement techniques of leakage and ground currents – Judith M. Russell; Consulting Electrical Engineer at PowerLines|
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