South Carolina Change Location

Tech Tip 01

Line Impulses Can Cause Clocks to Speed Up



  • Case 1: Demand Recorder Gaining Time
  • Case 2: Digital Clocks Gaining Time


  • Case 1: Line Notching From Rectifiers
  • Case 2: Faulty Air Conditioner


  • Case 1: Apply Harmonic Filter
  • Case 2: Repair/Replace Air Conditioner

Most digital clocks look for a pure sine wave cycle and will count two zero crossings. After the completion of 60 cycles, the clock will advance one second. Sometimes other nearby electrical equipment may interfere with this process and cause clocks to speed up. Case 1 refers to a clock that counts four 70-volt threshold crossings.


A steel company reported an energy demand recorder was gaining time at their facility. The recorder was gaining from a few minutes to a day each month.

At this location the voltage waveform was distorted from line notching caused by the customer's 6-pulse rectifier loads. The line notching was causing the clock to count extra 70 volt crossings. The waveform is shown in Figure 1.

 Waveform of Input Before Filtering

Figure 1: Input Voltage to Recorder Before Filtering

A filter was designed and installed to clean the voltage waveform at the demand recorder. The output of the filter is shown in Figure 2. Since the installation of the filter, the demand recorder has stopped gaining time.

 Waveform of Input After Filtering

Figure 2: Input Voltage to Recorder After Filtering


A Residential customer was plagued with digital clocks gaining time through the day. During a site visit, a voltage waveform analyzer was plugged into the clock’s electrical outlet. The waveform indicated extra zero crossings caused by line notching (Figure3).

 Waveform of Clock Voltage

Figure 3: Voltage Causing Clock Speed Up

It was also noted that when the clock was unplugged momentarily, the reset 12 o’clock numbers flashed near twice the expected 60 times per minute. Moving the clock and switching individual breakers off, the notching was believed to be generated outside the home. After talking with neighbors, it was determined the three houses served off a common distribution transformer experienced similar problems. Using the clock as a trouble-shooting tool, a defective air cleaner was isolated next door. Replacing the air cleaner solved the problem.

The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.