The Difference Between Class A and Class S

Power Quality: What does class A mean?

Power quality monitoring doesn’t often become an issue until incidents such as system malfunctions, equipment failures, process interruptions, data loss, IT disruptions or even power failure have occurred. As a rule, incidents of this sort are very time-consuming because they cause can’t always pinpointed right away. Beyond this, failures are always associated with costs which could have been avoided.

Power quality is defined in EN 50160, which describes the characteristics of the voltage in electrical power supply networks. However, edition 3 of IEC 61000-4-30 specifies the degree of accuracy required for the measurement of the quality of electrical networks. The standard differentiates amongst different device classes. The measured values obtained from different devices manufactured by various suppliers are rendered comparable in the case of a class A (A = advanced), and class A devices are always used when accurate measurements are required. By means of this standard, reliable, reproducible and comparable results are obtained which can be used for billing purposes.

The following measurements are standardized by edition 3 of IEC 61000-4-30, and are mandatory for class A devices:

• Power frequency
• Magnitude of the supply voltage
• Voltage unbalance
• Voltage dips/swells/interruptions
• Rapid voltage change
• Harmonics/interharmonics, THD
• Flicker
• Mains signaling on the supply voltage

Transients can also be measured as an option. The RMS value of the supply voltage in a 50 Hz network is measured over a 10-period time interval. The measured time interval must be contiguous and may not overlap with adjacent intervals. Measuring error may not exceed 0.1% of the agreed upon supply voltage value under the conditions described in the standard.

The Difference Between Class A and Class S

Whereas measuring accuracy is very high in the case of class A devices, measuring accuracy requirements specified for class S devices (S = survey) are much lower – data and events are only logged qualitatively and fewer demands are placed upon measuring accuracy. Furthermore, class S devices don’t have to measure as many quantities. The following measurements are mandatory:

• Power frequency
• Magnitude of the supply voltage
• Voltage unbalance
• Voltage dips/swells/interruptions
• Rapid voltage changes

Overview Table – Class A versus Class S

Normative Power Quality Monitoring: Voltage Events

Figure 1: Possible Voltage Events