Ground Loops in Vibration Testing Systems

February 13, 2020

As discussed in the previous lesson, an electronic ground can be a reference to 0 volts. This 0V reference is used as the base for other voltages generated in a testing system. It is the primary reference point and must be appropriately maintained.

Understanding Ground Loops

A ground loop occurs when multiple components that have an earth ground to a building’s electrical power supply circuit are connected to each other with a cable that carries a 0V ground connection. This connection creates multiple paths to ground through multiple system components.

A 0V reference affected by ground loops is no longer exactly 0V, as an additional voltage is now present. This change can cause errors with the measurements that the testing system uses.

Control Systems

Vibration control systems have a complete ground connection through a set of interconnecting cables going from:

  1. The controller output to the amplifier
  2. The amplifier to the shaker
  3. The shaker to the control sensor
  4. The control sensor to the controller input

In addition to this complete loop of single-point ground references, the controller, amplifier, and shaker have safety ground references to earth ground.

The multiple references to earth ground and the interconnected cabling have an extremely high potential for creating ground loops, especially when the different components require different types of power (120V-Single Phase, 480V-3 Phase, etc.). See Figure 1.3 below.

Ground loops vibration control system

Figure 1.3. Connections and potential ground loops in a vibration control system. Note that the power grounds from all three devices are connected via the electrical power supply circuit.

Resistance and Voltage

Electricity seeks balance; as such, the two wires of a circuit must contain an equal amount of voltage or current. Any imbalance is transmitted to ground.

Different voltages are generated depending on the resistance in the imbalanced wires. According to Ohm’s law, voltage equals current times resistance (V = I * R). As the resistance increases, the overall voltage increases, even when the current remains the same.

High-resistance wires and circuits will result in a higher voltage signal being carried on the ground side of the cables. In some cases, this voltage difference can cause actual motion on the shaker; in others, the difference may only be measured by the vibration controller.

Detecting Ground Loop Noise

There are two indications that your vibration control system is experiencing ground loop noise:

  1. The shaker head is moving when the system is powered on, but no test is running.
  2. The graphs display amplitude bumps at the cycle frequency of the electrical power supply (60Hz in North America, 50Hz in Europe).

If you suspect noise is affecting your vibration control system, the following lessons will take you through troubleshooting steps.