Voltage dips and short power cuts: what they do to machines

A voltage dip or a micro-cut lasts a fraction of a second. That is long enough to reset a PLC, trip a frequency drive or drop a contactor. First find out whether the dips come from the grid or from your own machines, by measuring. Then protect the controls, and where a stop is costly, the whole line. Our guide to the right emergency power supply for your business compares UPS, generator and battery for longer cuts. Business battery systems that feed your critical loads keep them running through a dip, because the loads never see it.

The lights blink for a moment. Nobody lost power, the neighbours did not notice a thing. But the press shows an alarm, the drive on the conveyor says "undervoltage" and the PLC wants a reset. Twenty minutes later the line runs again, next to a crate of scrap.

What makes it expensive is that nobody counts these stops. The grid operator registers no outage. The generator never starts, because the voltage is back before it would. The small UPS under the office PC keeps the PC alive, but not the drive in the hall. IT managers on forums describe UPS units that switch off "when the power flickers" instead of switching to battery. So the stops come back, every storm, every few weeks, and each one costs a restart, a batch or a delivery.

A site in Gingelom had the same kind of problem with its solar panels: they regularly dropped out. Our system of 30 kVA with 46.2 kWh of battery put an end to that. The grid is now only a back-up when energy runs short. At a hunting lodge in the woods at Gedinne, only 180 V was left of the 3x230 V grid. Our system turns that into a stable 3x400 V and uses the weak grid only to charge the battery.

What is a voltage dip, and how is it different from a power cut?

A voltage dip is a sudden drop to between 90% and 1% of the normal voltage, for 10 milliseconds up to one minute. That is the definition of the European standard EN 50160, as quoted by Janitza and HyTEPS. Below 1% it counts as an interruption: the power is simply gone.

Too high a voltage is a different problem, with its own procedure at the grid operator. Our article on why a solar inverter trips on high grid voltage covers it. A voltage that is structurally too low on a small connection is explained in our article on a 3x230 V connection that is too weak.

What causes voltage dips and short power interruptions?

Most voltage dips start with a short circuit somewhere on the grid, and the rest with large loads switching on inside your own site. Janitza puts about 75% of dips in the medium-voltage grid: digging work, ageing cables, storms and animals on overhead lines. HyTEPS adds weather and switching by the grid operator.

Short interruptions often come from the grid protecting itself. ORES explains it plainly: micro-cuts usually result from an incident on the grid, which puts itself in safety and switches back on automatically. For you, that is a gap of a fraction of a second to a few seconds. For a drive or a PLC, it is a full stop.

The inside causes are yours to fix. HyTEPS notes that a motor can draw up to 10 times its rated current when it starts. That current pulls the voltage down on your own installation. Starting a transformer and faults inside your own site have the same effect. Our article on how a battery inverter carries the start-up current of a heavy machine explains the physics.

Why does a short voltage dip stop your machines?

Because modern controls react within milliseconds, while the voltage only has to be gone for a moment to confuse them. HyTEPS and Janitza both name the same weak spots: PLCs, frequency drives, machine controls, servers and PCs.

The dip itself lasts less than a second. The real cost is the restart. Think of an injection moulding press, a packaging line or a batch process. A machine that stops halfway through a cycle often leaves parts or a batch to scrap. Then someone has to find the fault, reset and check before the line runs again.

Those are sector figures from Janitza, not a promise for your site. For a small workshop, a stop is mostly lost time and scrap. What does an hour of standstill cost your company, and what can you claim after a long cut? Our article on compensation and the real cost of downtime does the sum.

Why do the lights flicker when a machine starts?

Flicker is a quick, repeated variation of the voltage, and it is caused by loads that switch on with a high current. Powerquality.nl names large motors started directly on the grid, spot welders and induction furnaces.

Flicker is measured with two figures: Pst over 10 minutes and Plt over 2 hours. You do not need them to spot it. When the lights in the office dip every time the compressor or the welder starts, you have flicker. Powerquality.nl notes that high flicker levels can cause headaches, tired eyes and loss of concentration.

The fix is usually on your side. A soft starter or a frequency drive lets the big motor start gently. A heavier connection or a shorter cable to the cabin also helps, but that is expensive and slow. A battery inverter can deliver the start-up current itself, so the grid never sees the peak. Lower start peaks also help against the Flemish capacity tariff, which bills your highest quarter-hour peak of the month. Our article on how the capacity tariff for businesses is calculated explains that part.

How do you find out whether voltage dips are behind your machine stops?

By measuring the voltage at your main switchboard with a power-quality analyser for one to two weeks. HyTEPS advises at least that period, so that weekends and busy days are both in the data.

An ordinary energy meter is not enough. It stores 10-minute averages, and a dip of 100 milliseconds disappears in such an average. HyTEPS recommends a class A analyser according to IEC 61000-4-30, which records every event with its depth and duration.

Put the measurement next to your list of stops. When the dips come from the grid, report them to your grid operator with the measured dates and times. Is the voltage too high for hours on end? Our article on inverters that trip on high voltage explains the complaint procedure at Fluvius.

Which solutions keep your machines running through a voltage dip?

Start as close to the weak spot as you can: settings first, then the controls, and only then the whole line. The cheapest fix is often a setting, the most complete one is a system that feeds the loads permanently.

The UPS type matters. According to Vertiv, an offline UPS leaves a break of about 6 to 8 milliseconds when it switches to battery. A line-interactive UPS leaves about 4 to 6. An online double-conversion UPS has no break at all, because the power runs through it all the time. Whether a few milliseconds is short enough depends on the equipment: ask your machine builder.

UPS or battery system: what protects a production line best?

A UPS protects a few devices for minutes, while a battery system can carry whole circuits for hours and does useful work on normal days. For IT and controls, a UPS is fine. For motors, drives and a full line, a battery system with the right inverter power is usually the better fit.

A battery system can work in two ways. In the first, the grid feeds your loads and the system takes over when the grid fails. Victron states that a MultiPlus-II takes over in less than 20 milliseconds, fast enough to keep computers running. In the second, the inverter feeds your critical loads all the time and the grid only charges the battery. Then your loads never see a dip, because they are not on the grid.

That second way is what we did in Gedinne. The grid delivered only 180 V, so the inverter makes a stable 3x400 V from the battery. A generator is coupled in for power cuts. It is the principle behind our semi off-grid system, in which the public grid acts as a generator.

Our article on dozens of small UPS units or one central battery weighs the two for servers and production lines. The article on which inverter a business actually needs explains the difference between solar, battery and UPS inverters.

What happens when you ask us to help with voltage dips on your site?

We start with what stops, and what it draws when it starts.

1. Step 1: Analysis. We list the machines that stop, their start currents and the circuits they sit on. We measure your actual and peak consumption over a period. If the stops point to the grid, ask your electrician or a power-quality specialist for a class A measurement as well.
2. Step 2: Technical design. We choose which circuits the battery system feeds, the inverter power for your motors and, where useful, a generator as second line.
3. Step 3: Installation and commissioning.
4. Step 4: Monitoring and support. You follow the consumption per phase and the state of charge in our monitoring software.

Our battery systems for businesses are built for back-up power and business continuity, from small SME installations to modular systems up to 1 MW.