Mains voltage fluctuation and frequency over a week: results

The PicoLog graph below shows the measured results.

Screen capture of PicoLog

Figure 3: screen capture of PicoLog

The results appear to show a cycle occurring daily between Monday and Friday. The voltage at night rises to around 240 volts. During office hours, the voltage drops significantly below 240 V, which may indicate a higher demand on the National Grid. This higher demand is probably due to heavy plant machinery and office usage.

During the weekend, the voltage is generally at a higher level and does not show the daily fluctuations described above.

Using PicoLog, the Minimum, Maximum and Average values for the mains voltage and frequency were recorded and calculated as shown below:

Parameter Min Max Avg
Voltage (V) 232.432 243.846 239.384
Frequency (Hz) 47.97 50.24 49.99


  • The average frequency is very close to the specified frequency.
  • The average voltage is approx. 10 V above the specified voltage.
  • The voltage remained within tolerance during the week.
  • The frequency appears to have a couple of low spikes.


Originally, the UK mains voltage supply was specified at 240 V RMS ±6%. Some while ago, the specifications were changed (230 V +10%, –6%) to allow harmonization across Europe. The average value clearly indicates 240 volts.

At first sight, the spikes in the frequency data are outside the specified tolerance, indicating that the generating company is not meeting their published specification. Closer research into the specification small print shows that the frequency is allowed outside the specification provided that the average frequency over one minute is within the spec. The experiment sampled the frequency every thirty seconds, and therefore the spikes are reasonable. A one minute sampling rate would be more suitable for checking compliance.

Credits, comments and further info

This experiment was written by James Carey at Pico Technology.