The Biggest Mistakes That Lead to Thermal Hazards within LV Switchgear

What Affects the Operating Temperature

Operating at up to 1,000 volts AC, switchgear enclosures house the circuit breakers, fuses, contactors, busbars, and control instruments responsible for safely distributing power to industrial plants, commercial skyscrapers, and critical data centers. However, this continuous flow of electrical energy comes with an unavoidable byproduct: heat.

Managing the operating temperature within low-voltage switchgear is not merely a matter of operational efficiency; it is a fundamental requirement for safety, reliability, and asset longevity.

Excessive heat accelerates the degradation of insulation, increases the electrical resistance of conductive paths, and can ultimately lead to catastrophic failures such as arc flashes or fires.

This comprehensive technical guide explores the root causes of heat generation within LV switchgear, the environmental and operational factors that influence internal temperatures, the most dangerous mistakes made by operators, and the strict best practices required to keep temperatures within safe, desired limits.

Okay, let’s dive into the details!

1. The Physics of Heat Generation in Switchgear

To control temperature, one must first understand its origins. Switchgear is not a passive system; the components within it actively generate heat during normal operation.

Figure 1 – A standard indoor installation of low-voltage switchgear. Even in climate-controlled rooms, internal heat generation must be strictly managed.

1.1 Joule Heating (I²R Losses)

The primary source of heat in any electrical panel is Joule heating, often referred to as resistive or I²R losses. No conductor is perfect. Even highly conductive materials like electrolytic copper or structural aluminum possess inherent electrical resistance (R). As current (I) flows through the busbars, cables, and breaker contacts, heat is generated proportionally to the square of the current multiplied by the resistance.

Because the heat generated scales with the square of the current, a sudden spike in facility load will result in an exponential, rather than linear, increase in heat generation.

1.2 Component-Specific Heat Generation

Beyond standard conductors, specific components are designed in ways that inherently produce heat:

Fuses: A fuse operates on the principle of thermal melting. During normal operation, the fuse element generates heat. High-amperage fuses can radiate a significant amount of thermal energy into the enclosure.

Circuit Breakers: Molded Case Circuit Breakers (MCCBs) and Air Circuit Breakers (ACBs) contain bimetallic strips for thermal overload tripping, which rely on heat to function. Furthermore, the internal blowout coils and arc chutes add to the thermal load.

Magnetic Coils: Contactors and relays utilize electromagnetic coils to hold contacts closed. These coils constantly draw a holding current, generating localized heat.

When enclosed within an LV switchgear compartment the heat liberated may adequately dissipate by convection and radiation from the “walls” of the enclosure and by heat sink via the conductors. It may however be necessary to assist the liberation to atmosphere by forced ventilation.

Figure 2 – LV switchgear compartments

1.3 Magnetic Losses

While more prominent in medium and high-voltage systems, magnetic losses (specifically eddy currents and hysteresis), can affect LV switchgear. When alternating current passes through heavy busbars, it creates a fluctuating electromagnetic field.

If ferromagnetic materials (like standard steel enclosure panels or mounting brackets) are placed too close to these fields, they act as secondary transformer windings, absorbing energy and generating heat.

Figure 3 – Typical hysteresis loop and magnetic domain morphology og ferromagnetic materials

2. Key Factors Affecting Operating Temperature

The temperature inside a switchgear enclosure is a delicate equilibrium between the heat generated internally and the heat dissipated into the surrounding environment. Several dynamic factors influence this balance.

Let’s talk about these factors.

2.1 Load Characteristics and Harmonics

The most obvious factor is the amount of current being drawn. However, the type of current is equally critical. Modern facilities are loaded with non-linear loads: Variable Frequency Drives (VFDs), LED lighting, uninterruptible power supplies (UPS), and computers.

Furthermore, higher-frequency harmonics exacerbate the skin effect (where current crowds the outer edges of a conductor), increasing the effective AC resistance and generating even more heat.

Figure 4 – The complex arrangement of busbars heavily influences both heat generation (via resistance) and heat dissipation (via surface area)

2.2 Contact Resistance and Joint Integrity

A solid busbar rarely fails from heat unless grossly overloaded. The weak links in any switchgear are the joints. Whenever two pieces of metal are bolted together, whether it is a cable lug to a breaker terminal or two busbars spliced together, there is a transition of electrons.

If a joint is improperly torqued, mechanically stressed, or oxidized, its micro-resistance increases. An increase in resistance of just a few micro-ohms across a joint carrying 2,000 amps will generate hundreds of watts of localized heat, creating a dangerous “hot spot“.

2.3 Enclosure Design, Ingress Protection (IP), and Airflow

The enclosure’s primary job is to protect personnel from live parts and protect the components from environmental contaminants (dust, water). This is rated by the IP (Ingress Protection) or NEMA rating.

There is an inverse relationship between protection and cooling. An entirely sealed IP65 enclosure prevents dust and water ingress but completely traps air, relying entirely on the slow process of radiant and conductive heat transfer through the metal walls. Conversely, an IP20 enclosure with large louvers allows for excellent convective cooling but exposes the internals to environmental hazards.

The specific volume of the enclosure, the placement of louvers, and the internal compartmentalization (Form 1 vs. Form 4b) drastically alter airflow dynamics and internal ambient temperatures.

Figure 5 – High IP-rated enclosures restrict airflow, necessitating careful thermal derating of internal components

2.4 External Ambient Conditions

Switchgear does not operate in a vacuum. The ambient temperature of the switchroom directly impacts the baseline temperature of the equipment. If a switchgear assembly is rated for a 40°C ambient environment, but the plant room regularly reaches 50°C due to poor ventilation or nearby heavy machinery (like boilers or compressors), the switchgear’s internal temperature will easily exceed safe design limits.

Additionally, altitude plays a surprising role. As altitude increases above 2,000 meters, air density decreases.

Thinner air is a less effective medium for convective heat transfer, meaning switchgear installed in high-altitude locations must be derated (run at lower capacities) to prevent overheating.

Further Study – Current heat losses produced by electrical operational equipment in LV switchgear

Current heat losses produced by electrical operational equipment in LV switchgear

3. The Dangers of Excessive Heat (Hazards)

Allowing temperature to exceed design limits (typically defined by standards like IEC 61439 or UL 891) triggers a cascade of physical and chemical reactions that compromise the switchgear.

3.1 Insulation Degradation and the Arrhenius Equation

Electrical insulation, whether the PVC on cables, the epoxy coatings on busbars, or the molded plastic of circuit breaker casings, has a strict thermal lifespan. This lifespan is governed by the Arrhenius equation, a chemical rule of thumb stating that for every 10°C rise in operating temperature above a material’s rated limit, its expected operational life is cut in half.

If a cable insulation is rated for an ambient of 90°C for 20 years, operating it at 100°C reduces its life to 10 years. At 110°C, it drops to 5 years. As insulation thermally degrades, it becomes brittle, cracks, and loses its dielectric strength, paving the way for short circuits.

3.2 Thermal Runaway

Thermal runaway is a vicious, self-perpetuating cycle that occurs at joints and connections. It begins with a slightly loose connection, which creates a minor increase in resistance. This resistance generates heat. The heat causes the copper or aluminum busbars to expand. When the load drops and the metal cools, it contracts.

Eventually, oxidation accelerates at these high temperatures, acting as an insulator, driving resistance up exponentially until the joint literally melts or causes an arc fault.

Suggested Course – Practical Course for Designing LV Distribution Systems in Houses and Commercial Buildings

Practical Course for Designing LV Distribution Systems in Houses and Commercial Buildings

3.3 Arc Flash and Fire Hazards

The ultimate danger of unmitigated heat is an arc flash. If thermal degradation causes phase-to-phase or phase-to-ground insulation to fail, the electricity will ionize the surrounding air and arc across the gap. An arc flash can reach temperatures upwards of 35,000°F (19,400°C), hotter than the surface of the sun.

It causes an explosive expansion of air and vaporized metal, leading to catastrophic equipment destruction, devastating fires, and severe or fatal injuries to nearby personnel.

3.4 Nuisance Tripping

Even before a catastrophic failure, excessive heat severely impacts facility operations through nuisance tripping. Because thermal-magnetic circuit breakers use heat to detect overloads, an abnormally high internal enclosure temperature can "trick" the breaker into thinking it is drawing more current than it actually is.

This causes the breaker to trip prematurely, resulting in costly, unexpected facility downtime.

Good Reading - Twenty Switchgear Design Flaws That Drive Operators Crazy

Twenty Switchgear Design Flaws That Drive Operators Crazy

4. The Biggest Mistakes That Lead to Thermal Hazards

Despite rigorous engineering standards, human error in the design, installation, and maintenance phases frequently leads to thermal hazards. Here are the biggest mistakes made in the field.

Mistake #1 - Ignoring Harmonic Loads During Design

One of the most frequent errors in modern facility upgrades is failing to account for harmonic distortion. A facility might upgrade entirely to LED lighting and install dozens of VFDs to save energy. The facility manager looks at the total amperage and determines the switchgear is adequately sized.

However, by ignoring the high harmonic content, they fail to realize the neutral bus is slowly burning up due to cumulative triplen harmonics, and the phase conductors are suffering from severe skin effect heating.

Triplen harmonics from the three phases tend to accumulate in the neutral conductor for two reasons:

1. Fundamental currents cancel out: The fundamental frequencies are 2π/3 (120°) out of phase, meaning their sum is naturally zero.
2. Triplen harmonics add together: Unlike the fundamentals, triplen harmonics are completely in phase with one another across all three phases, causing their currents to stack and flow directly into the neutral (see Figure 6).

Figure 6 - Triplen harmonics are in phase and cumulate in the Neutral

Mistake #2 - Poor Joint Maintenance and "Set It and Forget It" Mentalities

Switchgear is not a static monolith; it requires active maintenance. A massive mistake is failing to implement a torque-checking or thermography schedule. Installers torque bolts on day one, and facility managers never check them again.

Due to the thermal cycling mentioned earlier, or the natural "cold flow" of aluminum conductors, bolts loosen. Ignoring periodic maintenance allows thermal runaway to progress unchecked in the dark enclosures.