Monitoring a Grounded System

Jeffrey R. Jowett, Megger

Electrical faults cause enormous damage due to fires and loss of equipment. But of far greater importance is the danger to human life through electrocution. Ground faults in particular pose the risk of death because a human is generally at ground potential, thereby providing an alternate current path. It takes only about 5 mA to cause a shock, and as little as 100 mA can be fatal. That makes ground fault monitoring of vital importance to maintaining a safe electrical plant.

Grounding Schemes

Let’s look at various grounding schemes and how they operate. Grounding schemes are coded using a five-letter system:

• T: direct connection to earth (Terra)
• I: isolated from earth
• S: separate
• C: combined

The first letter indicates how the transformer (source) is grounded; the second letter indicates grounding of the load. The next two letters indicate the configuration of the neutral and ground leads, respectively.

There are three basic configurations: TT, TN & IT:

1. TT is a simple and commonly used system where both the transformer and load are tied directly to earth (Figure 1). A residual current protective device (RCD) is required.

Figure 1: TT Configuration

2. In TN, the transformer is still tied directly to earth, but the load is connected to earth via a conductor. There are several ways to do this:

• In TN-S, the load is connected by a separate conductor (protective earth PE) (Figure 2).

Figure 2: TN-S Configuration

• In TN-C, the load is connected via the neutral conductor (Figure 3).

Figure 3: TN-C Configuration

• TN-C-S is a combination system where some sections of the load are grounded by a separate conductor while others use the neutral. The TN system is a low-impedance system; only the impedance of the grounding conductors limits the ground fault. This scheme can be modified by adding a ground resistor or capacitor to make it a high-impedance system (Figure 4). The advantage of this is improvement of equipment fault protection and greater personnel safety. It may also reduce the potential current of the transients by adding impedance. The extent of the protection would depend on the situation. An inductor can also be used to help dampen transients, but the resistor is more common.

Figure 4: TN-C-S Configuration

3. In an IT system, the transformer is not tied to earth but is isolated. The load is   connected by a ground rod (Figure 5). The advantage is that a single ground fault will not trip the breaker, making it the method of choice in critical systems such as operating rooms and railroad signaling.

Figure 5: IT Configuration

TT Systems

This article focuses on TT systems. Advantages of these systems are simplicity and low cost and also that faults on the LV and MV grid do not migrate to other customers on the LV grid. Disadvantages include:

• High voltages can occur between system components and the neutral.
• Possible overvoltages put stress on equipment insulation.
• Customers must maintain their own grounding systems, therefore complete reliability cannot be guaranteed.

In a TT system, current does not normally travel on the ground conductor but returns by the neutral. So the normal flow would be current flowing through the phases and returning on the neutral (Figure 6).

A ground fault has a path back to the source though the ground rod (Figure 7).

This path is in parallel with the normal return through the neutral, therefore the current will be split between these two paths (Figure 8). These parallel paths then are used to trigger a protective device (RCD).

The coil of the protective device is placed around the line and neutral. In an unfaulted system (Figure 9a), the sum of line and return equals zero. But a fault will divide the current, some of it returning through the earth ground, creating a differential between line and neutral. This trips the breaker (Figure 9b).

Protection

This system provides a basis for the operation of protective devices, but it can also be the source of dangerous voltages that develop on equipment while the fault is in progress and before elimination by the operation of the protective device. The core of this problem, and its elimination, is the integrity of the earth ground. This is expressed in the industry-standard term “touch potential,” the voltage to which a person could be subjected due to coming in contact with a piece of equipment during a fault condition. This is calculated by Ohm’s Law and is equal to the resistance of the earth ground path times the prospective fault current (Figure 10). The higher the impedance of the earth ground path, the less current will pass through it, precipitating a rise in voltage to potentially dangerous levels.

Since the human body’s tolerance to the passage of current is minimal, this is a potentially lethal combination. Eliminating the danger is dependent on two factors: maintenance and monitoring.

The grounding system must be maintained at low impedance. Many industries and authorities set their own standards, but a generally accepted standard for commercial and industrial is 5 Ω to remote earth, i.e. measured at a distance beyond the immediate sphere of influence of the electrode under test. More demanding systems, such as substations, computer rooms, telecommunications, data centers, and hospitals, may require lower values such as 1.5 or 1 Ω.

This is best tested using a dedicated three- or four-terminal ground tester and performing a fall of potential test. Since this test may require a lot of space to perform, other methods are available in the literature for addressing various difficult situations. High-current grid testers are employed to test the integrity of a grid under ground, as lightning strokes, fault clearance, and other disturbances can severely disrupt a grounding grid’s structure.

Once tested and indicated to be inadequate, a grounding electrode can be restored to specification by adding more rods or otherwise expanding a grid, driving rods deeper, or installing various systems that provide a constant moist environment, stabilize the soil around the rod(s), improve the ion content to promote conductivity, or improve the electrode’s capabilities in various other ways. Finally, a regular periodic maintenance program of testing and recording data is essential.

The Case for Nine-Channel Measurements

Common monitoring for ground faults tracks the neutral only. This is not sufficient. If the system is balanced, unfaulted, and free of zero-sequence harmonics, the neutral would measure zero amperes, as the sum of A, B, and C current would cancel out on the neutral. Introduction of a ground fault will split the current between the neutral and earth returns, unbalancing the returns on the three phases and leading to a rise in neutral current.

But it’s still not that simple. Other power quality phenomena, including unbalanced loads and zero-sequence harmonics, can also cause a rise in neutral current. However, these phenomena do not cause a rise in earth ground current. Therefore, monitoring both neutral and ground currents provides a means of distinguishing between these phenomena and identifying ground faults.

The most thorough way to monitor a grounding system is with a nine-channel (four voltage; five current) power quality monitor. Nine channels enable the monitoring of all three phase-to-neutral voltages and between neutral and ground, as well as phase, neutral, and ground currents.

Measuring the voltage between ground and neutral is a quick way of assessing the relative integrity of the ground electrode. The poorer the connection (higher impedance), the higher the voltage between neutral and ground conductors will be. If voltage rises above 50 volts during a fault, a serious hazard exists. But taking spot measurements is not enough to detect a ground fault because they can be intermittent. For instance, the fault may only occur when the connected piece of equipment is energized or in the presence of sufficient moisture. The thorough and efficient way to address such issues is with a nine-channel power quality recorder. This will catch intermittent faults that spot checks will miss.

An increase in ground current only when a specific piece of equipment turns on — and this actuation is identifiable through individual phase voltage and current measurements — indicates the ground fault is within that piece of equipment. Similarly, if ground current increases after it rains, the ground fault is likely due to water ingress in an exposed cable. If neutral-to-ground voltage increases to high levels, that would indicate a poor earth ground.

Conclusion

A nine-channel power quality monitor will identify ground faults and distinguish them from other power quality phenomena, identify poor grounding and dangerous safety conditions in the presence of a ground fault, and detect intermittent ground faults and identify their cause.

Jeffrey R. Jowett is a Senior Applications Engineer for Megger in Valley Forge, Pennsylvania, serving the manufacturing lines of Biddle, Megger, and Multi-Amp for electrical test and measurement instrumentation. He holds a BS in biology and chemistry from Ursinus College. He was employed for 22 years with James G. Biddle Co., which became Biddle Instruments and is now Megger.