Partial Discharge vs Corona vs Arcing: What’s the Difference?

The Short Answer

All three are forms of electrical discharge, but they sit at very different points on the energy spectrum:

• Corona is a luminous, low-energy discharge in air around a sharp electrode or conductor, driven by a strong electric field. It happens at high voltage, often continuously, and is usually benign — until it is not.
• Partial discharge (PD) is a localized breakdown of insulation that does not completely bridge the gap between conductors. It occurs inside solid or gas insulation and slowly degrades it over months or years. This is the one that kills switchgear and cables silently.
• Arcing is a complete, sustained conductive path through air or gas — a full short circuit. It releases enormous energy in milliseconds and causes arc flash, equipment destruction, and injury.

In the field, corona and PD are detected with the same handheld instruments (acoustic, TEV, and RF sensors), while arcing is usually an event you hear, see, and report — or prevent. Knowing which one you are looking at decides whether you write a maintenance note or call the operations manager immediately.

Field rule: Corona is a warning, PD is a countdown, arcing is the event. A utility inspection program that catches PD in the “countdown” phase never has to experience the third.

What Is Corona Discharge?

Corona occurs when the electric field around a conductor or fitting exceeds the dielectric strength of the surrounding air, ionizing it and producing a visible glow (often bluish-purple at night), a faint hissing sound, and ozone. It typically appears on:

• Sharp edges, burrs, and damaged fittings on overhead lines and bushings
• Pollution layers on insulators that concentrate the field
• High-voltage terminations with poor stress control
• Transmission lines in humid or high-altitude conditions

Corona itself is a surface/air phenomenon. It does not immediately destroy insulation, but it is a leading indicator of field stress problems: it corrodes hardware, produces ozone that attacks rubber and polymers, generates radio interference, and can erode the surface of insulator materials over time. In substations, sustained corona on a bushing or cable termination is often the first visible sign of a developing defect that will later become PD or a flashover.

For detection, corona radiates strongly in the ultrasonic band and produces RF/EMI. A handheld acoustic camera or an ultrasonic sensor hears the characteristic hiss; a TEV sensor on a switchgear panel picks up the surface discharge component. Because corona is usually benign in the short term, it is the lowest-urgency finding of the three — but never ignore it on critical assets.

What Is Partial Discharge (PD)?

Partial discharge is the localized electrical discharge that only partially bridges the insulation between conductors. It happens inside the insulation system: voids in cast resin, delamination in cable insulation, tracking along bushing surfaces, or gas bubbles in transformer oil. Each discharge pulse is tiny (picoCoulombs to nanoCoulombs) but repeated millions of times per cycle, and each pulse erodes the insulation chemically and physically.

The danger of PD is its invisibility. There is usually no glow, no smell, and no sound audible to the human ear in the early stages. A medium-voltage switchgear panel with active PD can run for months with zero visible symptoms while the insulation degrades toward a catastrophic failure — a phase-to-phase fault or a cable blowout.

This is why PD testing has become standard predictive maintenance for utilities: it finds the defect years before failure, while it is still cheap to repair. Common PD sources in the field:

• Voids and manufacturing defects in cast-resin transformers and busbars
• Deteriorating terminations and joints in medium-voltage cables
• Pollution and moisture tracking on bushings and insulators
• Loose connections that create floating-electrode discharges in switchgear
• SF6 leak-induced pressure drops in GIS, which lower the gas’s dielectric strength

PD is detected with TEV sensors (on metal-clad switchgear), ultrasonic sensors (airborne PD in cables, terminations, and open switchgear), and HFCT sensors (on cable earth connections and transformer bushings). The SC Series combines TEV and ultrasonic (AE) in a single instrument so a technician can screen an entire substation without changing tools.

What Is Arcing and Arc Flash?

Arcing is the complete breakdown of the insulation or air gap: a conductive plasma channel that carries full fault current. Once an arc forms, it sustains itself through ionization and releases energy at megawatt scale. In switchgear, an internal arc fault can vaporize metal, create a pressure wave, and produce an arc flash that reaches temperatures of thousands of degrees Celsius.

Arcing is rarely a gradual process on its own — it is usually the end stage of a developing defect. The chain is familiar to every protection engineer:

1. Insulation weakens (often because of unnoticed PD or corona)
2. A tracking path or void grows until the gap can no longer hold voltage
3. A flashover bridges the phase gap; fault current flows; the arc ignites
4. Protection operates, but not before significant energy is released

Arcing has unmistakable signatures: brilliant light, loud noise, strong RF, and a sudden current spike that protection relays detect. It is not something you “test for” with a portable detector — it is something you prevent by catching the corona and PD that precede it. Portable instruments do, however, play a role: ultrasonic sensors can hear the pre-arcing corona and PD activity, and thermal cameras detect the hot spots that accompany loose connections and failing components.

Corona vs PD vs Arcing: Comparison Table

The table makes the maintenance logic obvious: corona warns you about field stress, PD warns you about insulation health, and arcing is the failure you must prevent. A complete inspection program measures all three.

How Each One Is Detected

Modern handheld instruments blur the line between these detection methods because they share physics. Here is how to use them in practice:

• TEV (Transient Earth Voltage): Metal-clad switchgear — a PD discharge inside the panel induces a transient voltage on the earthed metal skin. A TEV sensor on the panel door measures this in dBmV. This is the fastest way to screen a row of panels. The HPD200 measures TEV in dBmV and ultrasonic in dB, giving a two-channel picture in one pass.
• Ultrasonic / Acoustic: Airborne PD, corona, and loose connections all emit ultrasound in the 20–100 kHz band. A parabolic microphone or an acoustic camera localizes the source visually. This is how you find corona on bushings and PD at cable terminations outdoors.
• HFCT (High-Frequency Current Transformer): Clamped around the earth strap of a cable or transformer, HFCT captures the high-frequency PD pulses traveling in the conductor. This is the method of choice for cable networks and transformer bushings.
• RF / UHF: PD and corona radiate electromagnetic energy. UHF sensors inside GIS detect internal PD with high sensitivity. Portable UHF detectors are used for GIS verification.
• Thermal: Not a direct PD detector, but a thermal camera like the EasIR finds the hot connections and overloaded components that generate or accompany discharge activity. Combined thermal + PD surveys catch both the cause and the effect in one visit.

For a typical utility walkdown, the workflow is: screen panels with TEV (fast), confirm and localize with ultrasonic/acoustic imaging (precise), and record patterns with a device that shows PRPD/PRPS plots (diagnostic). The HPD500 adds pattern analysis so an engineer can distinguish corona, surface PD, internal void PD, and floating-electrode discharge from the shape of the discharge pattern alone.

Pro tip: The discharge pattern (PRPD/PRPS) is the fingerprint that tells corona from PD. Corona clusters at the voltage peak, void-type PD clusters near the zero crossings, and floating-electrode discharges produce two symmetric clusters. Pattern analysis removes most of the guesswork.

Severity, Cost, and Urgency

Severity determines the response:

• Corona (low urgency): Log it, monitor it, plan remediation at the next outage. On transmission assets, corona on hardware is a long-term erosion problem, not a tomorrow problem. On cable terminations, treat it sooner because it can transition into PD.
• PD (high urgency): PD activity above the equipment’s baseline demands scheduling. Trending matters — a single reading is a snapshot; rising dBmV or pC over weeks is a defect that is growing. Depending on severity, repair can be planned for the next outage or accelerated to an emergency intervention.
• Arcing (immediate): If you hear or see arcing, de-energize or isolate per safety procedures and involve protection and maintenance immediately. Arc flash is a life-safety hazard, not just an asset problem.

The cost asymmetry is dramatic. Catching PD early costs one technician-day and a portable detector. Missing it until arcing costs a failed transformer, a panel rebuild, downtime, and potentially an arc-flash injury. This is the entire business case for a PD inspection program — and it is why utilities in Latin America are rapidly adding PD testing to their preventive maintenance routines.

The Bottom Line

Corona, partial discharge, and arcing are three stages of the same physics: insulation stress turning into failure. Corona is the early warning at the surface, PD is the countdown inside the insulation, and arcing is the final event. A modern inspection program detects corona and PD with handheld TEV/ultrasonic instruments and pattern analysis, then schedules repairs while the defect is still cheap to fix. If your team carries only a thermal camera today, adding a partial discharge detector is the single highest-ROI upgrade for medium-voltage asset reliability — because by the time a problem gets hot enough to see with infrared, the insulation may already be failing.