Sustained interruptions

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:

Variations in supply voltage outside the normally accepted limits of ± 10% are subdivided according to the length of the variation.

Long duration variations encompass rms deviations at power frequencies for longer than one minute. EN 50160 specifies the steady state voltage tolerances expected on a power system. A voltage variation is considered to be of long duration when the limits are exceeded for more than 3 minutes (the international standards use a period of one minute).

Long duration variations can be:

• sustained interruptions
• undervoltages
• overvoltages.

When the supply voltage has been zero for longer than 3 minutes, the long duration voltage variation is considered a ‘sustained interruption’. Voltage interruptions longer than one minute are often permanent and require human intervention to repair the system for restoration. The term sustained interruption refers to specific power system phenomena and, in general, has no relation to the usage of the term outage. Utilities use outage or interruption to describe phenomena of similar nature for reliability-reporting purposes. However, this causes confusion for end users who think of an outage as any interruption of power that shuts down a process. This could be as little as one-half of a cycle. Use of the term interruption in the context of power quality monitoring has no relation to reliability or other continuity-of-service statistics. Thus, this term has been defined to be more specific regarding the absence of voltage for long periods.

Typically caused by a breaker opening or a fuse blowing to clear a short circuit off the line, these are necessary for protection of equipment and safety concerns. They may be caused by short circuits of more permanent nature such as when high winds cause trees to knock down power lines. To enable a personal computer to “ride-through” momentary outages, an uninterruptible power supply (UPS) is recommended. The UPS is only as good as the battery and unfortunately many UPS devices are connected to poor circuits which cause the UPS to exercise the battery too often. This means that the battery may not be there when you need it.

--BSRIA

[edit] Related articles on Designing Buildings

• Alternating current and direct current
• Ampere.
• BSRIA articles on Designing Buildings.
• BSRIA definitions.
• BSRIA.
• Overvoltage.
• Power.
• Undervoltage.
• Voltage.
• Watt.