Electricity plays a vital part in our day-to-day life. Life without electricity is unthinkable today—nearly anywhere in the world. But the same omnipresent electric current, in certain cases, strays away from its intended path and can flow in other unintended circuits and materials. This flow of stray current causes the electrochemical deterioration of metallic surfaces, which is known as stray current corrosion. This process is similar to the reaction involved in electrolysis. The magnitude of the deterioration is directly related to the extent of unintended current flowing in the material on account of the occurrence of faults of stray voltages. Localized faults in insulation and connections resulting in localized stray currents cause localized corrosion damage. Structures in the vicinity of high-voltage equipment and conductors attract stray voltages due to the induction effect, even during normal fault-free operation of a high-voltage circuit.
The onset of stray current corrosion is generally independent of other environmental factors. But mechanisms of other corrosive factors may accelerate the damage caused by stray current corrosion.
Some of the universally occurring sources of stray currents are:
- Grounded direct current (DC) power systems distributing and transmitting power to consumers
- Electrically operated rapid transit systems
- Corrosion protection systems designed with cathodic protection (CP)
- Electrical welding equipment
The minimization of the stray currents and voltages involves:
- Detecting and measuring stray current as well as the source of the voltage and its path
- Mitigation of defects and insulation failures
- Repairing defective connections
- Designing a system such as an impressed current cathodic protection (ICCP) system to offset the effect of stray current
Stray current occurs because of induced or leaked voltage, which is the unintended presence of a voltage difference between two objects, which ideally should have zero voltage difference between them. Even grounded objects at differing locations may have a voltage difference existing between them, and stray current may flow between them. The presence of induced voltage due to the proximity of high-tension cables along with inductance or capacitance in the circuit is one of the causes of the problem. Current leakage due to insulation failure or defective connections can be another cause of stray currents. This can affect equipment enclosures, which are normally connected to ground. Persons coming into contact with the stray voltage may not necessarily sense the current flow, as the current flow may be small.
The Basics of Stray Voltage and Stray Current
Stray voltages and currents can be present in most high-voltage and medium-voltage circuits. The current can be due to an EMF induced on an adjacent structure, a leaked current due to insulation failure; or secondary return current flowing through the impedance of a return path or in a parallel path of a conductive circuit of the power system.
Stray current occurring due to system failures, such as insulation failure, is generally quite dangerous. These system faults result in a buildup of contact voltage in unintended surfaces in close proximity or even in a distant region. Such contact voltage can be dangerous for humans as well as animals, and arcing or fire can result in severe accidents with the loss of lives and property.
Thus, the term "stray current" or "stray voltage" is generally used to denote an unwanted or unintended flow of electricity in any form. Even a small voltage is considered stray voltage. Failure conditions such as these can be due to:
- Cable damage or loose connections
- Inadequate, degraded or deteriorated insulation
- Failed maintenance
- Defective design, installation and commissioning
Some examples of stray voltage generation are capacitance coupling voltages, EMF induced by medium-voltage and high-voltage power lines, EMF observed during lightning, and problems caused by disconnected neutrals.
Stray currents cause metal loss due to electrolysis, which is similar to galvanic corrosion. Severe localized pitting is observed at the site where the stray current leaves the metal toward its ground connection.
Capacitance Coupled Stray Voltages
The capacitance existing between overhead AC power lines and a fluorescent light tube can cause stray current flow in the lamp to make it glow, even when not powered on, due to the capacitive coupling effect. Metallic components in the proximity of neon signs or AC power conductors can display measurable EMF on account of the same capacitive coupling phenomenon. Hence, while working on overhead power transmission systems or in the proximity of medium-voltage and high-voltage lines, safety regulations require that conductors and metallic components be connected to ground effectively.
Induced Stray Voltages
Electromagnetic induction takes place when the power conductors along the length form a loop with another ungrounded conducting system under a parallel transmission system. Stray current is electromagnetically induced in the loop when a person standing on ground comes into contact with it. This stray current can be hazardous. Such stray current may occur on long metallic wire fences constructed under high-voltage transmission lines.
Chemical degradation or any damage caused to insulation can also be the cause of stray currents in nearby objects. This causes a severe fault as the stray current flows to ground through all of the available circuits. Insulation failure of underwater or underground cables could also lead to such faults. Insulation damage due to oil or salty environments and physical damage can occur as well. Overhead insulators can also become damaged or deteriorated.
Neutral Return Currents Due to Unbalanced Machine Loads
When the load on each phase of a three-phase, four-wire system is not equal and balanced, the unbalanced portion of current flows through the neutral wire connected to ground. This unbalanced load could be due to a defective connection or winding defects in the electric machine. As the primary as well as the secondary side of the power supply transformer is connected to grounds, the unbalanced portion of current continuously flows as stray current. This can occur in different industries using three-phase machines.
Typically, an electric-powered railway system uses one of the rails as a return conductor in the circuit, for the flow of current. This return conductor is touching the earth at different points throughout its track length. Thus, some part of the current will, by necessity, flows through ground. Wherever the system uses DC power, this stray current flowing to ground can cause stray voltage in buried metallic pipelines and other buried objects, and cause damage due to electrochemical reaction and corrosion of metal surfaces in contact with the wet soil.
Detecting Stray Voltage
A number of electricity distributors and major users conduct regular tests for stray voltage to ensure public safety and prevent corrosion. The instruments used to sense and detect stray voltage can vary, but some of the common devices are electrical voltage tester pens and electric field sensors. Confirmatory testing is done with a low-impedance voltage meter. Electrical volt tester pens are handheld devices that visually indicate a contact with an energized surface.
Then the verification of existence of voltage is done by a low-impedance voltmeter. Electric field detectors sense the electric field in relation to the person’s body. It is done by sensing and detecting the gradient of electric field at a distance, without having to make a direct contact.
Stray current cannot be heard, seen or smelled; there is no easy method to know when a significant stray current exists. Regular system inspection and testing is important, but a serious condition or failure can develop suddenly without any discernible warning.
The corrosion of the stray current is observed as localized pits at points where the current leaves the pipes and structures. Initially, this effect is not visible to the naked eye. The detection is done by measuring metal-structure-to-soil-potential difference. Current-mapping devices use radio detectors. Different instruments are used to detect DC and AC stray currents.
Stray Current Corrosion in Boats
Apart from the normal galvanic corrosion, boats may also suffer corrosion damage on account of stray currents. A healthy boat, for example, may be floating between another boat that has a leak of DC stray current and the path to ground for that current. Instead of moving directly through the water toward the ground, the healthy boat could provide a path of lower resistance for the stray current. Thus, the stray current could enter the healthy boat through the fitting of the hull and move across the bonding system, while leaving the boat via the drive to the water toward the ground. The corrosion would occur at the surface where the current leaves the metallic structure to enter the water.
Internal stray current can be generated by a short in the boat’s electrical wiring system. An external cause is the connection with the shore power source. Any boat with an internal fault can cause stray current corrosion in other healthy boats connected to the common shore power line.
The Connection Shielding Transformers
With a shielding transformer isolating the power source from the boat, stray current will not be able to flow in the boat circuit and thus the circuit is safe and free from stray current. But these transformers can be expensive. Also, other boats without such isolating transformers should not be connected to a boat with such isolation protection.
Corrosion Due to Rapid Transit Systems
Stray current corrosion resulting from electric rapid transit systems is estimated to cost the U.S. economy a sum as high as half a billion dollars annually. This estimate includes the damage that the stray current causes to infrastructure, such as pipelines and cables, in close proximity.
A solidly earthed system lets stray current flow without restriction between the power supply (rectifier) negative bus bar and the nearest underground metal structure. The corrosion due to stray current would predictably take place on structures and fitments of transit rails, tunnels, fasteners, bridges and other transit structures.
An ungrounded transit system has no metallic connection between ground and the power rectifier bus bar. Rail-to-earth insulation resistance is maintained by insulating rail fasteners. Stray currents are expected to be low; however, because of the existence of a huge number of fasteners in parallel, an earth ground can exist, causing significant stray currents to flow.
Diode-connected grounding power systems are a compromise between the ungrounded and the solidly grounded system. They limit the stray current flowing in a solidly grounded system, while also keeping stray electric voltages at a safe level. Diode-connected grounding systems have the power rectifier bus bar connected to grounding mats in series to a diode circuit. The diode allows stray current to flow from grounding mats to the negative bus bar, when a certain minimum threshold voltage is exceeded. The minimum threshold can be set based on the conditions of the substation. Thus, the electric potential above threshold level is allowed to dissipate rather than accumulate in the circuit. Corrosion due to stray current can still occur in this system, on the rails and insulated rail fasteners. The return rails also periodically conduct stray current when a threshold voltage is exceeded. On a diode-connected grounding of rapid transit systems, rails may require early replacement due to stray current corrosion damage.
In electrified rapid transit systems, current is returned through the running rails. The structures connected to ground surrounding the rails also work as a parallel conductor in a parallel circuit connected to the rails. Any structure buried in ground within the proximity will collect stray current, and some current will also leak through the low resistance.
The following aspects of the transit system determine the severity of stray currents:
- Substation design and spacing
- Propulsion current and voltage of traction
- Design of system grounding
- Electric resistance of the rail
- Resistance between track and ground
The older DC-powered rapid transit systems used to suffer from very severe stray current damage. Hence, in modern high-tech rapid transit systems, the stray current damages are minimized by:
- Reducing the designed electrical resistance of the rail in the return path
- Efficient increase in the electrical insulation resistance between the ground and the rails
Prevention of Stray Current Corrosion
The minimization of stray current corrosion is achieved by manipulating the design parameters. The goal is to reduce the flow of stray current, which is accomplished by increasing the overall circuit resistance in various ways.
Cathodic protection with an impressed current system can also be used to offset the effect of stray current. Good structural material can minimize pitting corrosion. Multilayer coatings can minimize stray current corrosion for a limited period.
Whenever the dominant source of DC stray current is readily accessible, the method of current drainage can be used to reduce the stray current flow. In this method, the metallic structure that becomes damaged due to stray current is efficiently connected to the negative terminal of the DC source with a low-resistance connector. The connection is designed as unidirectional, so that stray current can flow from the buried metallic structure to the negative terminal of the power source.
Monitor & Control
Stray current corrosion must be monitored and controlled in a systematic way. The source of power for stray current needs to be detected and measures are needed to minimize failures at the source. In the areas that are prone to stray currents, steels that are less prone to pitting may be used. Effective multilayer coatings with cathodic protection or current drainage systems can also minimize the damage to structures.