Voltage sag
BSRIA Power quality guide (AG 2/2000) was written by C C Pearson and V Uthayanan and published by BSRIA in July 2000. It states:
A sag is a decrease to between 10% and 90% of nominal rms voltage or current at the fundamental frequency for durations from 0.5 cycles to one minute. Sags of longer than about 10 to 20 milliseconds may cause computer equipment to malfunction.
The power quality community has used the term sag for many years to describe a short-duration voltage decrease. Although the term has not been formally defined, it has been increasingly accepted and used by utilities, manufacturers, and end users. The IEC definition of this phenomenon is dip. The two terms are considered interchangeable.
Terminology used to describe the magnitude of a voltage sag is often confusing. A "20% sag" can refer to a sag which results in a voltage of 80% or 20%. The preferred terminology would be one that leaves no doubt as to the resulting voltage level: "a sag to 80%" or "a sag whose magnitude was 20%." When not specified otherwise, a 20% sag will be considered an event during which the rms voltage decreased by 20% to 80%. The nominal, or base, voltage level should also be specified.
Voltage sags are usually associated with system faults but can also be caused by energization of heavy loads or starting of large motors. They can be caused by an air conditioning (or other) motor starting or utility problems hundreds of miles away and may not be noticed by the naked eye. Some sensitive machines in a facility are affected (shut down) by voltage sags or the more severe interruptions. Voltage sags are usually far less costly to solve than interruptions, so it is important to monitor power circuits with equipment capable of capturing waveform information.
--BSRIA
[edit] Related articles on Designing Buildings
Featured articles and news
Guide to ISO 19650 for Architecture Firms (2026)
A user gives their low down.
A UK training and membership provider for mould remediation professionals.
Building Safety recap April, 2026
A short and longer run-through of the month, with links to further information and sources.
CIAT May 2026 briefing.
Independent NSI and BAFE study exploring how organisations are changing the way they buy fire safety services.
From medieval scribes to modern word art.
ECA welcomes crackdown on late payment and push for clean energy, whilst CIOB seek fixed cladding removal timeframes.
Cyber Security in the Built Environment
Protecting projects, data, and digital assets: A CIOB Academy TIS.
Managing competence in the built environment
ITFG publishes new industry guide on how to meet the ICC principles.
The UK's campaign to reduce noise pollution: Mythbusting, articles and topic guides.
Setting Expectations on Competence Management
Industry Competence Committee.
New Scottish and Welsh governments
CIOB stresses importance of construction after new parliament elections.
The sad story of Derby Hippodrome
An historic building left to decay.
ECA, JIB and JTL back Fabian Society call to invest in skills for a stronger built environment workforce.
Women's Contributions to the Built Environment.
Calls for the delayed Circular Economy Strategy
Over 50 leading businesses, trade associations and professional bodies, including CIAT, and UKGBC sign open letter.
The future workforce: culture change and skill
Under the spotlight at UK Construction Week London.
A landmark moment for postmodern heritage.