Short- and long-term outages

Short- and long-term outages: Causes, impact and solutions for your installation

In the ideal world, electrical voltage is a constant sine wave available 24/7. However, the reality in industry, hospitals and data centres is unruly. A short- or long-term interruption - often colloquially called a power failure or blackout - means that the voltage on one or more phases is almost completely lost. Even an interruption of a few milliseconds in modern facilities can lead to the shutdown of critical processes, data loss and significant financial losses.

At HyTEPS, we see that many installation managers focus on preventing long-term outages, while it is precisely the short interruptions(transients and short voltage dips that change to zero voltage) that are more common and more difficult to diagnose. In this article, we explain the technical background, the distinction between short and long according to standards (such as EN 50160), and - crucially - how to make your installation resilient against these unavoidable events.

What exactly is a voltage outage?

To take the right measures, we first need to sharpen the definitions. According to European standard EN 50160 and international guidelines, we speak of an interruption when the supply voltage at the transfer points drops to less than 1% of the agreed rated voltage. In practice, the 5% or 10% limit is often used, depending on the sensitivity of the connected equipment.

We distinguish between two types, based on duration:

1. Short-term interruption (KU): this refers to a situation where the voltage is lost for a period of up to 3 minutes. In many cases, however, this lasts much shorter, from a few milliseconds to a few seconds. These are often caused by the operation of automatic switches in the medium-voltage grid (e.g. 'Automatic Reconnection' or AWI) that try to isolate a temporary fault, such as a branch on the line.
2. Prolonged interruption (LU): In this case, the voltage is absent for more than 3 minutes. This indicates a permanent fault in the grid operator's network or in your own installation, such as excavation damage to a cable, a defective transformer or a fire in a distribution system. Recovery requires human intervention or complex switching.

Nuance - Dip vs interruption: it is essential not to confuse a voltage dip (sag) with an interruption. In a dip, the voltage drops temporarily (e.g. to 50%), but energy is still present. In an interruption, the energy supply is effectively zero. A PC can sometimes continue to work during a short dip, but will irrevocably fail during an interruption without a UPS (Uninterruptible Power Supply).

How do outages occur and how do you recognise the source?

A common mistake is to point directly at the grid operator when a breakdown occurs. Although external factors play a role, many problems arise within one's own installation or at the interface (Point of Connection).

Common causes:

1. External causes (The public grid):
   • Weather effects: Lightning strikes or storm damage to overhead lines.
   • Excavation: Physical damage to cables.
   • Grid manoeuvres: Switching of connections by the grid operator.
   • Short circuit elsewhere: A short circuit at a neighbouring company can cause a deep dip or short interruption at your site until the protection there intervenes.
2. Internal causes (Your installation):
   • Selectivity issues: If a protection device (fuse or circuit breaker) trips 'too early' or unintentionally during a switch-on peak, a localised interruption occurs for that group.
   • Human error: incorrect switching operations during maintenance.
   • Obsolescence: failing components such as cable sockets or transformers.
   • Harmonic pollution: Although harmonics usually lead to overheating, in extreme cases they can cause unjustified trips of protective devices, resulting in outages.

The role of measurement and analysis: Because a short interruption is sometimes over so quickly that the light does not even flash but a machine stops, permanent monitoring is necessary. With advanced Power Quality meters, we can use Continuous Waveform Recording to see exactly what happened in the milliseconds before, during and after the outage.

What can you do against voltage outages?

Completely preventing dips in the public grid is impossible; after all, the grid operator cannot influence weather or excavation damage. You can, however, make your installation resilient against it. We distinguish three levels of solutions:

You cannot control the public grid, but you can make your installation immune (resilient). HyTEPS' approach is based on the 'Structural Model': first analyse, then optimise.

1. Quick wins (Operational)

• Check settings: Are your protection devices (circuit breakers, earth leakage circuit breakers) set correctly? Are they selective with respect to each other?
• Restart protocols: Ensure that machines do not restart automatically all at once after an interruption. This prevents another peak load that immediately leads to another outage.

2. Hardware and Engineering (Structural)

• Uninterruptible Power Supply (UPS) systems: For critical IT and control systems, a UPS is indispensable. It bridges the time between failure and recovery (or start NSA).
• Emergency power (NSA): Long-term outages require a generator. Note that this does not solve short interruptions due to start-up time.
• Ride-through functionality: Many modern AC drives have settings to bridge a short 'dc bus undervoltage' (Kinetic Buffering). Our engineers can help determine whether this is applicable.
• Active Harmonic Filter (AHF): although an AHF primarily reduces harmonics, it can reduce the load on transformers and cables, creating more 'space' in the installation and reducing the risk of internal failure due to overloading.

Note on UPS selection: A UPS is itself a non-linear load that can cause harmonic contamination. An incorrectly selected UPS can worsen the Power Quality in the rest of your installation. Get advice on the interaction between UPS and installation.