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
I’m doing study about Grounding of Power System for Industrial Plant with a capacity of 2MVA, 13.8kV/ 230V primary and secondary, 60Hz, 3Phase.
May I request for a reference related Grounding Design.
Thank you for your kind assistance,
Alfonso Palad Jr.
I would like to test effectiveness of a grounding & bonding installation in a complex chemical manufacturing plant. I assume that a multi-measurements between equipment components is best for bonding in addition to verifying earthing. Any experience or comments? Which meter is best. I was thinking that continuity would be sufficient for the bonding part.
In our project we have common grounding grid & in contract design all Low Voltage Panels, Low Current Cabinets & Equipment are connected to the Grounding Grid. But I want to make separate grounding for those Panels/Equipment for more safety. Please provide your comments.
do we need LV power panel system grounding
Controlling the voltage with respect to earth, or ground, within predictable limits. What does that exactly means?
I would really appreciate a detail explanation of the above statement found on the IEEE standard Recommended Practice for Grounding of Industrial and Commercial Power Systems
In the diagram given above system ground and equipment ground are connected with each other. Normally in industry every alternator and transformer have its own and separate grounding pit for neutral grounding. The grounding pits for power equipment are kept isolated than the grounding system used for instrumentation and control system.
Can you give me a document or standard specification? I happen to be doing grounding design. Our original design is medium voltage system, low voltage system, surge system connected together
Thanks alot for helpful article…
Because ground system is very important for safe the system and human accident.
Exelent article. In Argentina we use the TT system conexion for grounding system, as uses in LV 220 volts AC and 50 Hz. We requirement to get the ground system lower than 10 ohm for protecttion, and lower than 3 ohm for ground system at (DY11/13,2kv-0,4kv-0,220kv) transformer services at LV.
I WANT TO START MY CAREER IN ELECTRICAL STREAM AND I DONT KNOW FROM WHERE I GET STARTUP COMPANY FOR IT OR SHOULD I GO FOR PLC OR SCADA