What is grounding?
The term grounding is commonly used in the electrical industry to mean both “equipment grounding” and “system grounding”. Equipment grounding means the connection of earth ground to non-current carrying conductive materials such as conduit, cable trays, junction boxes, enclosures, and motor frames.
System grounding means the connection of earth ground to the neutral points of current carrying conductors such as the neutral point of a circuit, a transformer, rotating machinery, or a system, either solidly or with a current-limiting device.
Figure 1 illustrates the two types of grounding.
What is a grounded system?
It is a system in which at least one conductor or point (usually the middle wire or neutral point of transformer or generator windings) is intentionally grounded, either solidly or through an impedance (IEEE Standard 142-2007 1.2).
The types of system grounding normally used in industrial and commercial power systems are solid grounding, low resistance grounding, high resistance grounding, and ungrounded.
What Is the Purpose of System Grounding?
System grounding, or the intentional connection of a phase or neutral conductor to earth, is for the purpose of controlling the voltage to earth, or ground, within predictable limits. It also provides for a flow of current that will allow detection of an unwanted connection between system conductors and ground [a ground fault].
What is a ground fault?
A ground fault is an unwanted connection between the system conductors and ground. Ground faults often go unnoticed and cause havoc on plant production processes. Shutting down power and damaging equipment, ground faults disrupt the flow of products, leading to hours or even days of lost productivity.
Ground faults cause serious damage to equipment and to your processes. During a fault condition, equipment can be damaged and processes shut down, seriously affecting your bottom line.
Questions and answers
QUESTION #1 – I have overcurrent protection. Do I need additional ground fault protection?
The overcurrent protection will act to interrupt a circuit for currents for which it was designed and set to operate. However, some ground faults, particularly low-level arcing faults, will produce significant damage and create a fire-ignition source without ever reaching the level necessary to activate the overcurrent protective device.
QUESTION #2 – Is there any danger in running a 480-volt ungrounded system in an old manufacturing plant? Should we ground the system?
The main danger in running a 480V ungrounded system is that, when a ground fault occurs, the only indication you will have is the three lights. The voltage on the ungrounded phases will increase to 480V with respect to ground, the voltage on the grounded conductor will be 0V with respect to ground.
With this system, the only way to indicate the presence of a ground fault will be when two lights are of greater brilliance than the faulted phase light. In order to locate the ground fault, you must cycle every feeder breaker until all three lights appear at equal brilliance again.
Once this is done, you continue down that feeder until you find the fault. This sounds very easy to do but proves to be very difficult in the real world.
The plant is normally ungrounded because it is a continuous operational plant, and isolation due to a ground fault should be avoided! This unfortunately translates to locating the ground fault. The only way to locate the ground fault is through cycling of the feeder breakers.
This is what you are trying to avoid. So at the end of the day, the ground fault remains on the system, because there is no easy way to locate it. This is dangerous because any maintenance being performed on the system in a grounded state is subject to full line-to-line potential with respect to ground.
QUESTION #3 – What is the impact, if any, on moving equipment designed for a plant with a floating ground or ungrounded secondary to a plant that has a true grounded system? My thoughts are, it shouldn’t really matter, but I could be mistaken.
In your case (from an ungrounded system to a solidly grounded system), no, it does not matter. However, if you were going the other way (from an SG to a UNG system), then, yes, it would matter. During normal operation, it more than likely will not matter.
However, during a ground fault it will. In an ungrounded system, the faulted phase voltage collapses to ground potential (or ~0V), and the unfaulted phases rise to phase-to-phase voltage with respect to ground.
Since this doesn’t happen on a solidly grounded system, anything rated only 300V phase-to-ground will explode, such as TVSSs, VFDs, meters, etc.
QUESTION #4 What voltage would you read if you placed your leads from L1, L2, or L3 to ground of a 460-volt AC, three-phase power system, Y-connected?
If the Y-connected system is solidly grounded, you will read 266V from line to ground. If the Y-connected system is ungrounded or high-resistance grounded and the system does not have a ground fault, you also read 266V. In the event that there is a fault on one phase, then the faulted phase will show low voltage near 0, and the other two phases will read near 460V.
Reference // Resistance Grounding – Q/A with industry experts by iGard