Power quality: the fault that stops a line and leaves no trace

The symptom is an intermittent stoppage with nothing to inspect

Electronic equipment reacts to brief excursions in voltage that older electromechanical plant simply rode through. A short depression lasting a fraction of a cycle can drop out contactors, trip drives and reset controllers across a whole area at once, which is the distinguishing clue: several unrelated machines stopping at the same moment. Because nothing fails, the fault leaves no physical evidence, and the maintenance record fills with entries blaming individual machines. Correlating stoppage times across the plant is the simplest first diagnostic and often the one that reveals the true pattern.

Disturbances that arrive and disturbances you generate

Some events originate outside: faults elsewhere on the network, weather, switching operations. You cannot prevent those, only make the plant less sensitive to them. Others are created inside the fence by your own equipment — large motors starting, welding plant, furnaces, and the harmonic currents that electronic drives and power supplies draw. Internally generated problems tend to correlate with your own production pattern, which makes them identifiable. The distinction matters because it determines who can fix it and where the money goes, and it can only be established by measuring at the boundary as well as internally.

Measure before buying anything

The market offers filters, conditioners, isolation and ride-through equipment, and each addresses a different phenomenon. Fitting the wrong one solves nothing and is expensive, so recording what is actually happening comes first. That means monitoring at the incoming supply and at affected equipment for long enough to capture events, ideally including the periods when stoppages occur. Interpretation belongs to an engineer familiar with power quality phenomena, since raw event logs are easy to misread. A supplier proposing a remedy without site measurement is proposing a product, not a diagnosis. Keep the measurement records afterwards, since they become the evidence for any conversation with the network operator about events originating outside your boundary.

Sensitive equipment is now the normal case

Variable speed drives, programmable controllers, servo systems, machine vision, measurement equipment and the computing that supports them all tolerate less disturbance than the equipment they replaced, and modern plants contain far more of them. Meanwhile some of that same equipment is a source of harmonic current. So a factory can become both more sensitive and more polluting as it modernises, which is why plants that ran for years without trouble start experiencing stoppages after an automation programme. Anticipating this during equipment projects is far cheaper than diagnosing it afterwards across an installed base.

Where responsibility divides and who assesses it

The supply side has obligations for the quality delivered to your connection point, expressed through national requirements and international standards work coordinated through bodies such as the International Electrotechnical Commission. Inside the boundary the installation is yours, including limits on what you inject back. Establishing which side an event came from requires proper measurement at the interface and an engineer competent to interpret it against the applicable requirements. Approach the network operator with evidence rather than complaint, and expect any remedial work on your installation to be designed and certified under the local electrical regime.

Frequently asked questions

How do we tell whether a stoppage was caused by the electrical supply?

Look for simultaneity. If several machines on different circuits and with different functions stop within the same moment, and nothing has physically failed, a supply disturbance is a strong candidate. Drive fault codes often record an undervoltage or supply event, and those logs are worth extracting rather than cleared. Correlating stoppage times with weather, with a neighbouring industrial user's operations, or with your own heavy plant starting frequently points at the source before any monitoring equipment is installed.

Are harmonics actually a problem or just a specification issue?

They can be a real operational problem. Harmonic currents cause additional heating in cables, transformers and neutral conductors, can lead to nuisance tripping, and may disturb sensitive equipment sharing the same supply. Whether your installation has a problem depends on how much non-linear load you have relative to the supply capacity, which is measurable. It is worth assessing before adding a large number of drives, because mitigation designed into a project is considerably cheaper than mitigation retrofitted across an existing installation.

Will a standby generator protect us from these events?

Generally no, because the disturbances that stop production are usually brief and are over long before any standby set could start and take load. Standby generation addresses sustained loss of supply. Protecting against short disturbances is a different problem, addressed by equipment that provides ride-through for the specific loads that matter, or by making those loads more tolerant. Deciding which loads justify that protection requires knowing what a stoppage costs and how often the events occur.