A transient overvoltage is a very short burst of voltage above the normal waveform, lasting from billionths of a second to a few milliseconds. Transients rarely cause a dramatic failure on the day. More often they wear down insulation, power supplies and surge protectors a little at a time, until a drive, a UPS or a medical system fails “for no reason”. This guide explains the two kinds of transient, what causes them, and what it takes to actually record one.
Two kinds: impulsive and oscillatory
IEEE 1159 separates transients by their shape, because the shape tells you where they came from.
| Type | What it looks like | Typical duration | Typical causes |
|---|---|---|---|
| Impulsive | A single sharp spike in one direction | Nanoseconds to about a millisecond | Lightning, electrostatic discharge, switching of inductive loads |
| Oscillatory, low frequency (under 5 kHz) | A ringing wave riding on the sine wave | Up to tens of milliseconds | Switching a capacitor bank on, transformer energisation |
| Oscillatory, medium and high frequency | Faster ringing, often local | Microseconds | Back-to-back capacitor switching, cable switching, power electronics |
Typical characteristics as classified in IEEE 1159; magnitudes vary widely with the network.
What causes them
- Capacitor switching. When a utility or plant capacitor bank switches on, the voltage dips to near zero and then overshoots, ringing at a few hundred hertz. This is one of the most common transients in industry, often happens at the same time every day, and is a known cause of nuisance trips on variable-speed drives.
- Lightning. A direct or nearby strike induces fast impulsive transients on overhead lines that travel into buildings.
- Load switching inside the site. Opening a circuit that feeds a motor, transformer or welding set releases stored magnetic energy as a spike.
- Power electronics. Drives, inverters and UPS systems switch at high frequency and can create notching and high-frequency ringing on the local bus.
- Loose connections and arcing faults. Intermittent contact produces repeated small transients that can show up in the record long before the connection fails.
What transients do to equipment
- Drives trip on DC-bus overvoltage when a capacitor-switching transient is magnified by the drive’s own input filter.
- Insulation ages faster. Repeated stress on motor windings and cable insulation shortens their life without any visible event.
- Surge protectors wear out. Every surge absorbed uses up some of the device’s capacity; a surge protector that has been absorbing daily transients for years may no longer protect anything.
- Electronics fail early. Switch-mode power supplies, communication ports and control boards are the usual casualties. In hospitals, that includes the power supplies of imaging and laboratory equipment (see the imaging guide).
Recording a transient: sampling rate matters
Whether an instrument can see a transient depends on how fast it samples:
- Oscillatory transients from capacitor switching ring at a few hundred hertz to a few kilohertz. A power quality recorder sampling in the tens of kilohertz captures their shape well.
- Fast impulsive transients from lightning or static discharge rise in microseconds or less. Capturing their true peak needs a dedicated high-speed transient recorder sampling in the megahertz range. A slower instrument will show that something happened, but it will understate the peak.
Before buying an instrument for transient work, ask for its sampling rate and its transient-capture specification, not just “captures transients”. Our guide to choosing a power quality analyzer covers the other questions to ask.
Finding the source
- Look at the timestamps. Transients that repeat at the same time each day usually point to scheduled switching, such as a utility capacitor bank or a large load starting on a timer.
- Compare phases. A transient on one phase only points to a local source; the same transient on all three phases points upstream.
- Measure at two points. If the transient is larger at a distribution board than at the incomer, the source is downstream of the incomer.
- Check the waveform shape. Ringing at a few hundred hertz is almost always capacitor switching. A single sharp spike points to lightning or inductive switching.
- Check the surge protectors. Replace any whose status indicator shows the end of its life, and review whether the protection is coordinated from the incomer down.
How Inspector helps
Inspector records transients together with dips, swells and the other events it classifies, sampling at 32 kSPS per input with a 24-bit converter. Every event is stored with 5 cycles of waveform before and after, so capacitor-switching transients and their effect on the supply are recorded with their timing. That makes the daily pattern visible, which is usually the fastest route to the source. For lightning-speed impulses, pair it with surge counters or a dedicated high-speed recorder. Running 24/7, Inspector shows when transients happen, how often, and whether a change you made reduced them.
Seeing unexplained drive trips or failed power supplies? Talk to an Inspector engineer about what to measure first.
