What are voltage sags, momentary power interruptions, and brief power outages? Causes, effects, and countermeasures

Voltage sags and momentary power interruptions are said to occur about 12 times a year on average. What exactly are voltage sags, momentary power interruptions, and brief power outages? This article explains their causes, effects, and the key points of countermeasures.

*All amnimo IoT devices and video IoT devices with recording functions have protection against momentary power interruptions.

What are voltage sags, momentary power interruptions, and brief power outages?

Momentary power interruptions and brief power outages

A momentary power interruption is a power outage that lasts only a very short moment. How short depends on the case, but power companies generally use the term for outages of less than one minute. Similar terms include brief power outage and short interruption. According to the Japanese Industrial Standards (JIS), a short interruption is also defined as a loss of supply voltage that lasts no more than one minute, and it seems to be treated as having the same meaning.

What is a voltage sag?

A related term is voltage sag. A voltage sag is a momentary drop in voltage. It differs from a momentary power interruption, in which the voltage falls to zero. JIS calls a voltage sag a “voltage dip” and defines it as “a sudden reduction of the voltage at a point in the electricity supply system, followed by voltage recovery after a short period.”

The difference between voltage sags and momentary power interruptions

In Japanese, the terms for a voltage sag and a momentary power interruption are pronounced the same (“shuntei”), but they are completely different things. In a voltage sag, the voltage drops momentarily but does not reach zero. In a momentary power interruption, the voltage falls to zero temporarily. Keep this difference in mind.

Causes and mechanisms of voltage sags and momentary power interruptions

Causes in power supplied by power companies

When power supplied by a power company has voltage sags or momentary power interruptions, the cause is most often lightning. Lightning strikes on transmission lines in particular can cause momentary power interruptions. Lightning accounts for nearly 70% to 80% of cases.
Other causes include power lines cut by flying objects such as sheet metal or signs blown by typhoons and storms, and lines cut by earthquakes or landslides. During long periods of heavy snow, the weight of snow on the lines can also break them. Momentary power interruptions are also caused by many other things, such as bird nests, damage to utility poles from vehicle collisions, and fires near utility poles.

How voltage sags and momentary power interruptions occur

Here is how voltage sags and momentary power interruptions occur. In Japan, electricity from power plants is always sent over two circuits. When an accident or failure such as a lightning strike occurs at one point on one of the two circuits, a fault current flows and the voltage drops temporarily. This voltage drop affects not only the point where the lightning struck but all connected circuits (transmission line B is also affected). It occurs at the moment of the strike and may recover automatically. If it does not, it continues until a circuit breaker isolates the fault point. This is a voltage sag. It lasts from the fault until the breaker operates or the voltage recovers automatically, so it ranges from several tens of milliseconds to about 2 seconds at most.

Meanwhile, the circuit with the fault (transmission line A) is temporarily without power because the breaker has isolated it from the rest of the system. However, power is restored after about one minute, and the outage that lasts until then is a momentary power interruption. If re-energizing fails, the outage continues until the cause is removed. When a line is broken, an outage usually results.

Why voltage sags and momentary power interruptions occur inside facilities even with a stable supply

Even when the power supplied by the power company is stable, voltage sags and momentary power interruptions can occur at factories and other sites that use large amounts of power. For example, when electrical equipment that needs a lot of power starts up, insufficient supply or insufficient capacity in the power wiring can lower the voltage to other machines on the same circuit and cause a momentary power interruption. Poor contact in power cables can also cause momentary power interruptions. Take particular care if you have a high-voltage power receiving contract. Such sites use large amounts of power, and momentary power interruptions can occur even when the power company’s supply is stable.

What are the effects?

Consumer home appliances generally restart automatically or are not damaged when power is cut, so major problems are rare.
Industrial equipment is a different story. Let’s look at the effects by industry and situation.

Offices

In offices, voltage sags and momentary power interruptions can stop PCs, servers, and storage, corrupting memory data or damaging equipment. Be especially careful with PCs, external HDDs, NAS devices, and similar equipment. A sudden power cut can damage storage areas and lead to the loss of important data. The hardware itself is also expensive, so losses from failures tend to be large.
Failures of network equipment can also cause problems such as lost connections. Now that data is often stored on servers or in the cloud, a broken network path can also lead to data loss.
Air conditioning and lighting may also stop, but this is only temporary and usually not a major problem.

Factories

Many kinds of factory equipment, such as process control PCs, machine tools, inverters, and motors, may fail and stop production lines. In particular, if power is not shut down safely, data may be corrupted and system logs may be lost, so finding the cause and recovering can take a long time. Even without real damage such as equipment failure, companies can suffer losses from the labor cost of recovery work and lost production while the line is stopped.

Hospitals

Measures are especially needed for the power supply to medical equipment on which patients’ lives depend.

Commercial facilities

Temporary stops of air conditioning and lighting and effects on communication equipment are concerns.

Railways

Outdoor power tends to be unstable to begin with, and motors draw large currents, which makes voltage unstable because of magnetic noise. Countermeasures are needed.

How to protect against momentary power interruptions

Countermeasures for voltage sags and momentary power interruptions

Power distribution facilities for transmission, transformation, and distribution are often installed outdoors and are easily affected by lightning, snow, salt damage, and so on. Lightning damage is especially common, and each power company takes various measures. Even so, momentary power interruptions and voltage sags still occur because of natural disasters and other causes. If the risk from momentary power interruptions is high, each company needs to take its own measures.
On the other hand, many businesses have not taken measures because of the cost. The reason is that the frequency of voltage sags and momentary power interruptions and the amount of damage are small compared with the price of protective equipment. Protective equipment also has maintenance costs, so when you introduce it, compare the losses from a momentary power interruption with the cost of the equipment and its maintenance.

Key points of countermeasures

The key point of countermeasures is to stop equipment safely to prevent equipment failure and data corruption. This safe shutdown also needs power, so it is important to secure some power to keep equipment running after a voltage sag or momentary power interruption. For this reason, many businesses use protective devices with energy storage devices called capacitors.
Also, if equipment does not restart automatically when power returns, someone may have to go to the site. Take care when you run equipment in remote locations.

Types of devices that protect against voltage sags and momentary power interruptions

Most devices for protecting against voltage sags and momentary power interruptions use energy storage devices called capacitors. They all use energy storage, but they differ in how they supply power: the series compensation method and the parallel compensation method.

1. Series compensation

This method makes up for the drop in voltage. It compensates for the change in voltage, but it cannot handle brief power outages or momentary power interruptions.

2. Parallel compensation

This method disconnects the grid side and supplies power directly from the capacitors. Uninterruptible power supplies (UPS) are a common example.

Compensation time and capacity differ by product. Some devices handle only voltage sags, and compensation times range from 2 seconds to about 8 hours. You need to choose one that suits your application.

Summary

This article explained what voltage sags, momentary power interruptions, and brief power outages are, and their causes, effects, and countermeasures. Causes and effects vary widely by business and situation. It is important to sort out the specific effects on your operations and consider suitable measures for each.

amnimo IoT devices and recorders with protection against momentary power interruptions

All amnimo IoT devices and video IoT devices with recording functions have protection against momentary power interruptions.

This function works with a high-capacity capacitor built into the device. When the external power supply is cut, the device uses the power stored in the capacitor to keep running for a while.

A demo video of the momentary power interruption protection in amnimo devices is available on YouTube.

This mechanism has three advantages.

1. The equipment keeps running stably even when the power supply is unstable and fluctuating.

2. Operation does not stop at the moment external power is cut, so the device can send an alert about the power cut over the network.

3. In devices that record video, when external power is cut, the device first stops supplying power to the cameras to stop incoming video, processes all data in progress, and then shuts itself down. This reduces how often memory crashes occur (Japanese Patent No. 6954423).

These are the three advantages.

Momentary power interruptions cause problems in many kinds of equipment, and they also seriously affect recorders and IoT devices. If you are interested in recorders and IoT devices with protection against momentary power interruptions, consider amnimo devices.

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