Here are frequently asked questions every new power quality (PQ) engineer or electrician should be able to answer and, more importantly, understand in context. These cover foundational knowledge, practical troubleshooting, compliance, and communication.

Given the quality of supply is there a concern about the uptime or susceptibility of equipment or systems? Or, put another way, is the power supply compatible with the connected loads? What is the economic exposure of poor power quality?

These are some of the most common events a PQ meter will detect. Understanding how they behave and what causes each (internally vs. externally) is key to accurate diagnosis. They are all types of voltage disturbances, but they behave differently and result from different root causes.

A transient is a very short-duration, high-frequency sub cycle change in the voltage or current (think microseconds to a few milliseconds).

  • Cause: Can be from lightning strikes, capacitor switching, arcing, loose connections and others.
  • Impact: May not trip breakers but can damage equipment and sensitive electronics, corrupt data, or degrade equipment over time.
  • Detection: Often missed by standard meters. You need a PQ analyzer with sub-cycle transient resolution.

A sag (a.k.a. dip) is a temporary drop in voltage, typically lasting half a cycle to a few seconds, but can be longer.

  • Cause: Commonly caused by motor starts, short circuits, large inrush currents, or utility reclosures.
  • Impact: Can cause dimming lights, IT reboots, drive shutdowns, or contactors dropping out.
  • Tip: Look at the depth (how low it drops) and duration. A deep sag <70% can trip equipment.

A swell is the opposite: a temporary increase in voltage.

  • Cause: Often from load shedding, poor voltage regulation, or sudden loss of large loads.
  • Impact: Can stress equipment, stress insulation, trip breakers, or trigger nuisance alarms.
  • Context: Less common than sags, but still worth watching in systems with lots of switching activity.

 

Power Quality FAQs

ThIs is called directivity, and it typically applies to voltage sags/dips and swells. It determines if the problem originated upstream or downstream of the point being monitored. As an example, if you are monitoring at the PCC or  utility entrance, upstream is the responsibility of the utility, and downstream is the responsibility of the facility. 

Monitoring both voltage and current is essential in determining directivity. By comparing the direction of the magnitude of voltage to the current at the time of the event you can determine if the problem originated from the source or load. Here are two rules of thumb:

  • If the magnitude of the voltage and current are inversely related, then the direction is downstream, or towards the load. As an example, when a large load or motor turns on, the current inrush may simultaneously cause a sag/dip in the voltage – current abruptly goes up and the voltage goes down. The opposite may occur when large loads are turned off or disconnected causing a voltage swell – voltage abruptly goes down and the voltage goes up.
  • If the magnitude of the voltage and current go down simultaneously the issue is most likely upstream from the monitoring point. These can be voltage sags/dips or interruptions. Think breakers tripping, utility interruptions, etc. No voltage – no current.

Good PQ monitoring is helpful in determining directivity.

They are two of the most referenced power quality standards, but they serve very different audiences.

IEC 61000-4-30: How PQ instruments work and measure PQ

  • This is an international standard that defines how power quality measurements should be made. Included are methods to accurately measure voltage, current, harmonics, flicker and other parameters. 
  • Think of it as the “measurement method rulebook”.
  • Compliance in North and South America is usually not required, but is strongly suggested – very ‘nice to have’. For Europe and other parts of the world it is ‘need to have’ – compliance is required to question the power quality from a utility or other power authority. 
  • IEC 61000-4-30 is in its 3rd edition (Edition 3). Being Class A compliant means the PQ instrument fully complies with the IEC 61000-4-30 edition 3 requirements. It sets specifications for accuracy, measurement methods, timing, and consistency across devices.
  • IEC 62586 is a certification standard to test PQ instruments for compliance with IEC 61000-4-30. A certificate to this standard means the PQ instrument has been fully tested for compliance with a certificate to prove it.
  • Why it matters: It means the instrument comes from a reputable manufacturer with repeatability of measurement, especially if it is Class A compliant. The theory is that instruments from different manufacturers measure the same thing at the same time will show the same results.  

IEEE 519: Harmonics at the Point of Common Coupling (PCC)

  • IEEE 519 defines the recommended practice for the measurement of voltage and current harmonics at the PCC with the utility.
  • The intent is for harmonic measurement at the PCC, but IEEE 519 is often used, but somewhat inappropriately, at points downstream within a facility. 
  • This standard focuses on how to measure harmonics and how much harmonic distortion is acceptable.
  • As of the 2014 edition, IEEE 519 uses the same internationally accepted measurement techniques of IEC 61000-4-7 which is also included as part of IEC 61000-4-30. 
  • The 2014 edition also adds two additional harmonic parameters:
    • Very Short Time harmonics: Aggregation of V & I harmonics over 2 seconds
    • Short Time harmonics: Aggregation of Very Short time harmonics over 10 minutes. 
  • Voltage and current harmonic compliance limits are included at various voltage and current levels. 
  • Why it matters: Many utilities require compliance with IEEE 519 at the PCC. Today, with the proliferation of alternative energy power sources, baselining the harmonics before such power sources go online is important to know the source of harmonic problems should they occur.

Locating PQ problems can be a process of elimination and there’s a right and wrong place to measure. Context matters. If an individual load or process is experiencing problems, then it makes sense to monitor the power feeding that load. If problems are being experienced facility-wide, then a good starting point is to monitor at the utility service entrance.

Every PQ tech needs to know this. Harmonics distort the normal sine wave and can overheat equipment, overload neutrals and cause other problems. Generators produce near pure sine waves, but the distribution system and connected loads can cause distortion – that 60Hz or 50Hz sine wave now contains additional frequency components. You don’t need to explain Fourier analysis, but you do need to explain the existence of harmonics, effects and mitigation strategies. For a more in-depth explanation of harmonics, visit the Harmonics FAQs page. 

Prove it by monitoring the quality of the supply to rule power problems in or out. Soft skills matter. You need a structured, professional way to explain findings and show data without escalating. It’s not about winning an argument, it’s about being trusted.

Many electricians are familiar with multimeters or standard energy meters, but they are too general purpose. PQ monitors tuned to the task gives you context—event timing, waveform shapes, direction, and correlation across phases. That’s how you diagnose and prevent repeat issues.

Your job is as much about communicating your findings as it is conducting an analysis. You’ll need to explain sags, transients, harmonics, and root cause analysis to folks in operations, finance, or safety roles who don’t necessarily have the same technical knowledge as you. Keep it simple. Use visuals. Tell them what they can do with the info. Use these FAQs as a helpful guide.