Tackling the most common power problems

Ryan Jones, business development manager, Riello UPS, highlights some of the most common issues that can affect electrical equipment.

While the primary role of a UPS system is to provide standby power in the event of mains supply failure, it can also help mitigate many other power problems.


This is a complete mains supply failure leading to a power cut that lasts anywhere from a few milliseconds through to several minutes, hours, or days.

They tend to occur when there’s damage to the electricity grid or essential infrastructure, often as a result of extreme weather or unexpected equipment failure.


Also referred to as a dip, a sag happens when there’s a brief drop in voltage of at least 10% below the usual mains supply.

They tend to take place when a load draws a high inrush current at start-up, such as motors, air conditioning systems or industrial equipment. Did you know that a motor can draw as much as six times its normal running current when it first fires up?

Another reason is when there’s an issue on the electricity distribution network, for example where a fault needs clearing.

Sags can lead to power supply units (PSUs) drawing down more current than they usually would, which generates additional heat and places stress on key components. Drops in voltage can also see voice processing hardware fail or inadvertently reboot.


These are longer versions of a sag – whereas they only tend to last for at most several cycles, a brownout can persist for a few minutes, hours, or even days!

Such undervoltage tends to be the result of severe weather or increased power demand putting the electricity network under extreme pressure.

Brownouts typically result in flickering lights, sketchy internet connections, and electrical appliances turning on and off. Sustained undervoltage poses particular problems for the non-linear loads found in computers.

Power surge

This is the opposite of a sag, namely a short period where there’s at least a 10% increase in voltage above the usual mains supply.

Power surges occur in the immediate aftermath of turning off a large load or on high-impedance neutral connections.

When there’s a power surge, automatic protection mechanisms kick-in, often resulting in a system crash. They can also trip downstream circuit breakers and generate excess heat, which speeds up component deterioration.

Fitting power line conditioners can reduce the risk of damage from a power surge. Likewise, many modern UPS provide built-in protection as standard.


Also known as transients, a spike is a fast-moving burst of high energy that lasts just a few milliseconds. They’re different to a power surge as they are shorter in duration but much larger in magnitude. For example, a spike can reach several thousand volts (i.e. some measure 6 kV and above).

Spikes are common following lightning and during electrical storms and can cause damage to circuit boards and processors, as well as corrupting data.


These are distortions of the waveform generally caused by the load itself. Harmonics cause issues mainly with non-linear loads that draw their power from regular, modulated pulses from the mains, rather than in a continuous line.

Harmonics distort the mains supply voltage, overheating circuits, conductors, and transformers. This often leads to nuisance tripping, while they can also damage equipment.

Electrical noise

Electrical noise is an unwanted high-frequency disturbance either between the phase and neutral (Normal Mode Noise) or between the supply line and earth (Common Mode Noise).

It can be ‘internal’ from within the electrical system, caused by poor design or switchgear faults, or ‘external’ as a result of electrical storms, radio transmitters interfering with grid frequency, or electromagnetic interference.

Electrical noise disrupts circuit operation, which can potentially damage equipment and cause data corruption.

Frequency variation

This tends to be an issue at sites with standby generators. While most IT equipment these days can handle minor shifts in generator frequency, sensitive devices or motors that depend on regular power cycles can be impacted over time.

If a motor runs too fast or slow to match the input frequency, such inefficiency can speed up the rate of wear and tear.

How does a UPS protect against these power problems?

There are three main types of UPS system, with each topology offering differing levels of protection:

  • Offline or standby UPS (VFD – Voltage and Frequency Dependent): these offer the most basic protection. When there’s a mains failure, there’s a tiny (4-8 millisecond) break in supply when the inverter switches on and powers the load from the batteries, rather than the mains.
  • Line Interactive UPS (VI – Voltage Independent): works in a similar way to an Offline UPS but also incorporates automatic voltage stabilisation technology that protects against sags, brownouts, surges, spikes, and electrical noise.
  • Online UPS (VFI – Voltage and Frequency Independent): offers maximum protection against all issues because the load isn’t directly connected to the mains. Instead electricity goes through a double conversion process, first from AC to DC then back from DC to AC, before powering the load. This dual conversion ‘cleans’ any abnormalities from the power supply, and if there’s a mains failure there’s no break in supply whilst the batteries take over.

Last week, Ryan Jones highlighted some of the recent advances in UPS technology. Find out more about Riello UPS here.