Designing Medium-Voltage Systems

Three Lenses Every Engineer Should Consider

When designing a medium-voltage electrical system, it is tempting to start with the voltage rating, select a suitable piece of equipment, and move on to the next item. But a reliable design involves much more than matching a number on a specification sheet.

A medium-voltage system has to work as a complete system. The equipment must be built to the right standards, installed according to the applicable codes, and designed with safe operation and maintenance in mind.

I find it helpful to look at the design through three different lenses: equipment standards, installation codes, and safe-work practices.

1. Equipment Standards: What Are We Actually Specifying?

The first question is not simply, “What voltage do we need?” It is, “What type of equipment is appropriate for this application?”

The ANSI/IEEE C37 series provides the construction and testing requirements for different types of switchgear. These equipment families are not interchangeable, even when they have similar voltage ratings.

For example, metal-clad switchgear, covered by IEEE C37.20.2, is generally selected when a project requires a higher level of compartmentalization, protection, and maintainability. A typical lineup may include drawout circuit breakers, segregated compartments, and grounded metal barriers.

The testing requirements also give us a sense of the electrical stresses the equipment is expected to withstand. A typical 15 kV metal-clad lineup, for example, may have a 36 kV rms power-frequency withstand rating and a 95 kV peak lightning impulse withstand rating.

That does not mean every 15 kV application requires metal-clad switchgear. It means the equipment has been designed and tested for a particular level of performance.

Now compare that with metal-enclosed interrupter switchgear, covered by IEEE C37.20.3. This type of equipment generally uses fused or unfused interrupter switches and can be a practical, cost-effective choice for certain distribution applications. However, its protection, isolation, and maintenance characteristics are different from those of metal-clad switchgear.

Then there is gas-insulated switchgear (GIS), covered by IEEE C37.20.9. GIS places the conductors and switching components inside sealed metal enclosures filled with dielectric gas. Its compact construction can be especially valuable where space is limited.

The important point is that these are not simply different versions of the same product. Each equipment family offers a different combination of protection, maintainability, footprint, and cost.

The engineer’s job is to select the equipment based on the application not just the voltage.

2. Installation Codes: How Does the Equipment Become Part of a System?

Once the equipment has been selected, the next question is how it will be installed.

This is where the National Electrical Code (NEC) becomes important. The equipment may be perfectly suitable on its own, but the installation still has to meet the applicable code requirements.

The 2026 NEC introduced a significant reorganization of requirements for systems operating above 1,000 volts AC. The changes recognize the increasing use of medium-voltage distribution systems outside traditional utility-owned installations.

The new structure includes dedicated articles addressing different parts of the installation, including branch circuits, feeders, services, and grounding and bonding.

For engineers, the practical lesson is that equipment compliance and installation compliance are two separate questions.

A switchgear lineup can meet its product standard and still be installed incorrectly. Conversely, an installation can follow the code while using equipment that is not appropriate for the application.

Both sides have to work together.

3. Safe-Work Practices: What Happens When Someone Has to Maintain the System?

The third lens is perhaps the one that deserves the most attention: how people will safely operate and maintain the equipment.

A system may perform exactly as designed during normal operation, but that does not automatically make it safe to work on.

NFPA guidance emphasizes the hierarchy of risk controls. The preferred approach is to eliminate or reduce the hazard through design and engineering controls before relying on administrative controls or personal protective equipment.

PPE remains important, but it should not be the first and only answer to an arc-flash hazard.

This brings us to arc-flash mitigation.

Passive vs. Active Arc-Flash Mitigation

There is an important distinction between containing an arc flash and reducing the energy released by the fault.

Arc-resistant switchgear, tested under IEEE C37.20.7, is designed to contain and redirect the effects of an internal arc fault away from personnel in the intended operating area. This can provide valuable protection for nearby operators.

However, arc-resistant construction does not necessarily reduce the duration of the fault. The equipment may still experience significant damage, and the incident energy may remain high.

That is why active mitigation methods are also important.

Bus Differential Protection

A bus differential scheme compares the currents entering and leaving a protected bus zone. When the currents do not balance as expected, the scheme can identify an internal fault and initiate rapid tripping.

Because it does not have to wait for the same time delays used in conventional overcurrent coordination, it can clear a bus fault much faster.

Energy Reduction Maintenance Settings

Energy Reduction Maintenance Settings, or ERMS, provide another way to reduce incident energy during maintenance.

When enabled, the protection system uses a faster tripping characteristic to reduce the time the fault remains energized. The actual reduction depends on the system and the protection settings, but the principle is straightforward: less fault-clearing time generally means less incident energy.

Zone Selective Interlocking

Zone Selective Interlocking, or ZSI, allows breakers to communicate with one another so that a fault can be cleared quickly while maintaining selectivity where possible.

The goal is to avoid unnecessary delays for faults that require rapid clearing, without simply making every breaker trip instantaneously for every fault.

Light-Based Arc-Flash Detection

Optical sensors can detect the intense light produced by an arc flash. When combined with appropriate overcurrent supervision, they can initiate very fast tripping.

These systems are particularly useful where reducing the duration of an arc fault is a priority.

Bringing the Three Lenses Together

The best medium-voltage designs do not treat equipment selection, code compliance, and safety as separate tasks.

They are connected.

The equipment standard tells us what the switchgear is designed and tested to withstand. The installation code tells us how it must be integrated into the electrical system. Safe-work practices tell us how to reduce the risk to the people who will operate and maintain it.

When these three considerations are addressed together, the design becomes more than a collection of compliant components. It becomes a system that is intended to perform reliably, protect its equipment, and reduce the consequences of failure.

That is the real goal of medium-voltage engineering: not simply to manage a failure, but to design the system so that failures are less likely to become serious events.