A lab instrument that drifts every time a subway train accelerates nearby. A buried water main that corrodes through decades ahead of its design life. A control room that logs communication errors nobody can trace. These look like unrelated problems, but on projects near electrified transit and utility infrastructure, they often share a root cause: electrical energy ending up where it was never meant to be.
Two risks sit behind most of these issues: electromagnetic interference (EMI) and electrolysis, the stray current corrosion that can attack buried and embedded metal. They are often grouped together in project requirements, and sometimes in the same study, but they work through different mechanisms, affect different assets, and call for different solutions.
This guide explains what each risk is, how it develops around infrastructure, where it tends to appear, and how engineers assess, mitigate, and verify it.
What Is EMI?
Electromagnetic interference (EMI) is unwanted electromagnetic energy that disrupts, degrades, or changes how electrical or electronic equipment performs.
So, what is EMI in practical terms? It is any electromagnetic disturbance that pushes a system away from its intended behaviour, whether that shows up as noise in a signal, an error in a measurement, or a control system responding to something it should ignore.
EMI reaches equipment in two main ways:
- Conducted EMI travels through physical connections such as power wiring, signal cables, and grounding paths.
- Radiated EMI couples through space as electric, magnetic, or radio frequency (RF) fields.
Which path matters depends on the source and on the system being affected. Where existing conditions are uncertain, electromagnetic field surveys measure actual field levels on-site, so the discussion starts from data rather than assumption.
Common Infrastructure Sources of EMI
Most infrastructure-related EMI starts with equipment that carries or converts large amounts of power:
- Transit traction power systems, including rails, overhead lines, and traction substations
- Transformers, switchgear, and utility substations
- High-current feeders and bus ducts
- Large motors and variable frequency drives (VFDs)
- Communication and RF transmitting equipment
A source on this list does not automatically create a problem. Whether it causes practical interference depends on frequency, field strength, distance, the coupling path, equipment sensitivity, grounding, shielding, and how the source operates over time. Each of those factors needs project-specific review.
What Are Electrolysis Risks?
So, what is an electrolysis risk? In chemistry, electrolysis is simply a reaction driven by electric current. In infrastructure, the term usually means something narrower and more costly: electrolytic corrosion, also called stray current corrosion.
It occurs when unintended direct current (DC) travels through soil or conductive structures and leaves a metallic asset through an electrolyte, such as moist soil or the pore water in concrete. Where the current leaves the metal, the metal corrodes.
So when project teams talk about electrolysis risks, the practical concern is not the textbook reaction. It is accelerated corrosion of buried or embedded metal when stray current paths are not adequately controlled. On developments near Toronto Transit Commission (TTC) infrastructure, this risk is assessed through a dedicated TTC study that reviews EMI, electrolysis, and stray current together.
Infrastructure That May Be Exposed
Conductive assets that may need evaluation near electrified transit or power infrastructure include:
- Buried water, gas, and utility piping
- Reinforcing steel in foundations, slabs, and tunnel structures
- Structural steel
- Grounding systems
- Metallic conduits and cable sheaths
- Rails and other track components
How EMI and Electrolysis Risks Develop Around Infrastructure
Electrified transit and utility systems can produce both electromagnetic fields and unintended current paths. The two often share a source, which is why they are frequently studied together, but they are different mechanisms and should not be treated as one problem.
- EMI is About Coupling: The concern is how electromagnetic energy reaches equipment or systems and changes how they perform.
- Electrolysis is About Current Flow: The concern is electrical current entering and leaving conductive infrastructure in a way that may accelerate corrosion.
A streetcar line shows the difference. The same traction current that produces a changing magnetic field near a building can also leak from the rails into the ground. The magnetic field may affect a sensitive instrument inside, while the leaked current may find its way onto a buried pipe outside. Same source, two separate risks, two separate analyses.
Before choosing mitigation, project teams should identify five things: the actual source, the pathway, the affected asset, the operating state, and the performance criterion that must be met. On the EMI side, magnetic field analysis helps answer these questions by predicting field levels from specific sources before construction.
Where These Risks Commonly Appear
Transit and Rail Corridors
Electrified rail, subway, and streetcar systems are among the most common sources of both risks. Many urban transit systems use DC traction power, with the running rails serving as part of the return circuit. Because rails are never perfectly insulated from the ground, some return current leaks into the surrounding soil. Nearby developments, utilities, and sensitive equipment may face stray current, electrolysis, and EMI considerations at the same time.
Utility and Electrical Infrastructure
Substations, transformers, feeders, grounding systems, and high-current equipment can create magnetic fields or conductive coupling paths that require engineering review. Many of these sources sit inside the project itself, one wall or one slab away from sensitive spaces.
Sensitive Buildings and Equipment
Some occupancies need a much quieter electromagnetic environment than others. Hospitals, laboratories, data centres, control rooms, research facilities, and semiconductor spaces may require interference levels well below what an ordinary office would tolerate. When one of these facilities sits near strong sources, the margin for error narrows quickly.
How EMI Can Affect Sensitive Equipment
When EMI reaches sensitive equipment, possible effects include:
- Unstable or drifting measurements
- Image artifacts in magnetic resonance imaging (MRI) and other imaging systems
- Signal noise and communication errors
- Control system disturbances
- Calibration drift
- Intermittent or unreliable equipment performance
None of these outcomes is universal. Whether a given field affects a given device depends on the manufacturer’s limits, the system design, the frequency range involved, and the actual field conditions on-site. A field level that is irrelevant to one piece of equipment may exceed the siting criteria of another.
This is why modelling during design matters. Our guide on what magnetic field analysis is and why it matters explains how predicting field levels early helps prevent EMI before equipment is installed.
How Electrolysis Can Affect Infrastructure
Stray current corrosion tends to follow a predictable sequence:
- DC current leaves its intended return path, typically the running rails, and enters the soil.
- It follows the path of least resistance, which may include a buried pipe, cable sheath, or reinforcing steel.
- It travels along that metallic asset toward the traction substation or return point.
- Where it leaves the metal and re-enters the soil through an electrolyte, the metal acts as an anode and corrodes.
Damage concentrates at those discharge points, and it can be significant. As a general reference based on Faraday’s law, a continuous discharge of one ampere can remove roughly 9 kilograms of steel per year. Actual rates depend on current magnitude, duration, soil conditions, and coatings, but the figure shows why small, persistent currents matter.
It is also why buried utilities and metallic infrastructure near transit systems may need a dedicated stray current and electrolysis assessment rather than a generic electromagnetic field (EMF) study. An EMF study measures fields; it does not trace current paths through soil or evaluate corrosion exposure.
EMI, EMC, and Electrolysis: How the Terms Differ
These terms are often used interchangeably in project documents. They should not be.
- EMI is the unwanted disturbance itself.
- Electromagnetic compatibility (EMC) is the ability of equipment or systems to operate acceptably in their electromagnetic environment without causing unacceptable interference to anything else. EMC is the goal; EMI is what stands in the way.
- Electrolysis, in this context electrolytic corrosion, is a corrosion mechanism. It is not another name for EMI or EMC, and it is managed through different engineering methods.
Two questions come up on almost every project team: what is EMI, and what is the difference between EMI and EMC? Answering both clearly helps teams write better requirements and avoid scoping the wrong study.
How Engineers Assess EMI and Electrolysis Risk
1. Project and Source Review
The assessment starts with the project record: electrical systems, transit infrastructure, buried utilities, sensitive equipment, grounding arrangements, drawings, operating conditions, and the criteria that apply. This review defines which risks are present and what level of study each one needs.
2. Field Measurements
For existing sites, electromagnetic field surveys or other site measurements quantify current conditions. Measurements should reflect representative operating loads. A reading taken when trains are not running, or when a feeder is lightly loaded, can understate the real exposure.
3. Engineering Analysis and Modelling
When future conditions must be predicted, or when several sources interact, engineers use calculations, magnetic field analysis, finite element analysis (FEA), or other project-specific modelling. At C-INTECH, we use FEA software to model expected field levels from proposed equipment and layouts before construction begins.
4. Stray Current and Electrolysis Assessment
For transportation projects, this step evaluates potential current paths, the conductive assets exposed to them, corrosion risk, and the transit or utility requirements that apply. For developments near TTC infrastructure, studies are graded into levels based on risk and complexity under TTC guidelines.
5. Reporting and Recommendations
The report documents findings, assumptions, risk areas, applicable criteria, mitigation options, verification requirements, and any monitoring that should continue after construction. Where levels or risks are within criteria, the report says so.
Mitigation Strategies for EMI
Depending on the mechanism identified, EMI controls may include:
- Source reduction through equipment selection or configuration
- Equipment relocation or increased separation distance
- Cable and feeder routing changes
- Grounding and bonding improvements
- Magnetic shielding for low-frequency fields
- RF shielding for higher-frequency interference
- Filtering on conducted paths
- Active field mitigation using sensors, controllers, and compensation coils
Shielding should follow source and frequency analysis, not precede it. A shield designed for the wrong frequency, or placed on the wrong side of the coupling path, adds cost without solving the problem.
Effective electromagnetic shielding in buildings is targeted to a specific source and frequency range, and magnetic shielding for sensitive facilities is engineered around the field conditions identified in analysis.
Managing Stray Current and Electrolysis Risk
Stray current mitigation is project-specific. Depending on study findings, measures may include:
- Electrical isolation of buried or embedded metallic assets
- Bonding and grounding coordination between the development and the transit system
- Current return design that keeps traction current on its intended path
- Stray current collection or drainage systems
- Protective coatings on exposed assets
- Coordination with existing cathodic protection systems
- Ongoing monitoring and verification
No single measure applies everywhere. A bond that protects one asset can create a new current path for another, so recommendations must follow the study findings, the affected asset type, and the requirements of the relevant authority. That is the core of how stray current studies protect transport and utility systems: they define the current paths first, then match each control to the asset it protects.
Testing and Verification
Risk control is incomplete until the final installation is verified under realistic operating conditions. A design can look right on paper and still underperform once the system is energized.
- For EMI, verification may include post-installation field measurements, equipment performance checks, or EMI and EMC testing, depending on the project scope.
- For electrolysis and stray current, verification may include electrical measurements such as structure-to-soil potential readings, ongoing monitoring, or authority-specific testing requirements.
When a Project Needs a Specialized Study
A dedicated study is usually warranted when a project involves:
- Proximity to electrified transit infrastructure, where the TTC or another authority requires EMI, electrolysis, or stray current review
- Sensitive imaging, laboratory, research, data, communication, or control equipment located near strong electromagnetic sources
- New substations, transformers, major feeders, utility relocations, or buried metallic infrastructure where current paths and field conditions may change
- Existing sites with unexplained corrosion, interference, signal instability, image quality problems, or equipment behaviour that changes with nearby electrical operations
Common Mistakes to Avoid
- Treating EMI, EMC, EMF, stray current, and electrolysis as interchangeable. Each term describes something different, and mixing them leads to the wrong study scope.
- Choosing shielding before confirming the source, frequency, and coupling pathway. Shielding selected by default often misses the actual problem.
- Overlooking buried conductive infrastructure near electrified transit. Pipes, rebar, and conduits can carry stray current long before corrosion is visible.
- Relying on a single measurement. A reading that does not reflect realistic operating conditions can understate risk.
- Waiting until commissioning or visible corrosion. By then, mitigation options are fewer and more expensive.
- Skipping post-installation verification or long-term monitoring. Without it, there is no evidence the controls are working.
Separate the Mechanisms, Then Solve Them
EMI is an electromagnetic performance risk: energy that couples into equipment and changes how it behaves. Electrolysis is a potential corrosion risk: current that leaves its intended path and damages conductive infrastructure on the way out. They can share a source, but they rarely share a solution.
Effective risk reduction starts by separating the two mechanisms, then measuring or modelling actual conditions, and finally selecting mitigation based on the source, pathway, affected asset, and project criteria. Done early, that process shapes the design. Done late, it becomes a retrofit.
If your infrastructure, transit, utility, or building project sits near electrified systems or sensitive equipment, request a project assessment and our engineers can help define which studies your project needs and what the results mean for your design.
Frequently Asked Questions
1. What is EMI in infrastructure projects?
EMI is unwanted electromagnetic energy from sources such as transit power systems, substations, transformers, or feeders that disrupts, degrades, or changes the performance of nearby electrical or electronic equipment. It can travel through wiring and grounding paths or through electric, magnetic, and RF fields.
2. What causes electromagnetic interference near transportation systems?
Traction power is the main source. Current flowing through rails, overhead lines, and traction substations produces magnetic fields that vary with train operations, and power conversion equipment can add higher-frequency content. Communication and signalling systems may also contribute. The practical impact depends on distance, field strength, and equipment sensitivity.
3. What are electrolysis risks in transit and utility infrastructure?
The main risk is accelerated corrosion of buried or embedded metal, such as pipes, reinforcing steel, and cable sheaths, caused by stray DC current leaving an asset through soil or another electrolyte. Left unmanaged, it can shorten asset life and lead to leaks or structural repairs.
4. What is the difference between EMI and electrolysis?
EMI affects how equipment performs; electrolysis affects the physical condition of conductive infrastructure. EMI involves electromagnetic energy coupling into systems, while electrolysis involves electrical current flowing through and out of metal. They may share a source but require different assessments and controls.
5. How does stray current cause corrosion?
Stray DC current leaks from its intended return path, travels along a metallic asset, and then discharges back into the surrounding soil or concrete. At the discharge point, the metal acts as an anode and loses material. Corrosion concentrates where the current leaves, which is why damage can be localized and severe.
6. Is EMI the same as EMC?
No. EMI is the disturbance itself. EMC is the ability of equipment or systems to operate acceptably in their electromagnetic environment without causing unacceptable interference. Controlling EMI is one of the ways a project achieves EMC.
7. When is a TTC EMI, electrolysis, and stray current study required?
The TTC typically requires one for developments within specified proximity zones of its tracks, tunnels, or power infrastructure, usually during planning, site plan approval, or permitting. The required level of study depends on proximity and project scope, so requirements should be confirmed with the TTC and the City of Toronto early in design.
8. Can shielding reduce all EMI problems?
No. Shielding is effective when it is designed for the correct frequency range and placed in the right part of the coupling path. Some problems are better solved through separation, routing changes, grounding improvements, filtering, or active field mitigation. Shielding also does nothing to prevent stray current corrosion.
9. How are EMI and electrolysis risks tested and verified?
EMI controls are verified through post-installation field measurements, equipment performance checks, or EMC-related testing. Stray current controls are verified through electrical measurements, such as structure-to-soil potential readings, along with monitoring and any testing the relevant authority requires.
