Considerations for Instrument Grounding

Many people have heard of the term "grounding", but few fully understand its meaning and importance. Sometimes, even experienced electricians do not treat grounding as a serious issue. The impact of an incorrect or absent grounding ranges from noise interference. resonance or humming during the use of electrical equipment to the worst case where electricity leakage through the chassis causes personal injury or damage to instrument components. Grounding, therefore, is a very practical issue that should be dealt with properly. For those who operate electrical equipment frequently, a complete understanding of grounding theories and applications is necessary in order to become a best-in-class technician.

In the eighteenth century, Benjamin Franklin performed the famous kite experiment to observe how lightning in the sky was conducted to the earth. This experiment led to the invention of lightning rods to avoid lightning strikes. From then on, people began to realize that the vast ground under our feet is a huge electrical conductor. It may not be the best conductor, but it is certainly a good one. It is so enormous in size that it can sustain a tremendous amount of current. That is why the voltage level of the ground is set to be zero. Safety regulations require that all metal parts which do not carry electricity should be kept at zero or the earth voltage level.

There are several reasons for grounding. Some are for safety purposes, and some are for maintaining circuit stability. The following are some examples:

• Power system grounding: As you can see in Figure 1. this design is to prevent the secondary side from being damaged by the high voltage on the primary side, as the current will be conducted to the ground through the Grounding Wire to protect human lives.
• Instrument grounding: By connecting the equipment or chassis to the ground, operators can be protected from electric shocks if there is electricity leakage.
• Signal grounding: A zero voltage reference or a loop-back path is provided for all integrated signals to ensure proper operation or accurate measurements.
• Shielded grounding: This is used to prevent static electricity from being accumulated. Ground isolation or conduction can help to reduce noises and electro-magnetic interference. Examples include shielding rooms, cables, wirelines, guarded terminals of instruments, transformers and filters.

Types of Instrument Grounding

Figure 2 shows a commonly used instrument grounding on inputs and AC power. In this case. the input signal ground is connected to the power ground and when you are making a measurement, it is important to make sure that the input signal ground is not short-circuited directly to any point where there is a voltage difference to the earth ground. This is very common when measuring commercially available low-cost circuits. To reduce costs, these circuits usually do not use power isolated transformers. Instead, the AC power is directly connected to the circuit. As a result. a loop is formed between the circuit itself and the earth ground, and a voltage difference occurs. If the AC power happens to be plugged in the reverse way, or a considerable voltage difference exists between the neutral line and the earth ground, the combined factors could lead to very unpredictable results. Therefore, caution must be exercised before the input is connected for measurement.

To avoid the problem described above, some instruments provide floating inputs as shown in Figure 3. Each of the inputs is floating from the earth ground. Ideally, as long as the voltage difference between these two inputs is within an acceptable range, the inputs can be connected to any voltage point.

Summary

When installing the equipment in a building, make sure to have an electrician check on the impedance to the ground and the grounding device to see if they comply with electrical regulations. 8 AWG wireline should be used as a minimum wire type for instrument grounding.

Use the three-pole AC power socket for the instrument. Make sure the polarity of the hot line and the neutral line is correct (see Figure 6). The voltage difference between the neutral and the ground lines should be less than 1 V. At the socket end. the impedance between the neutral and the ground lines should be lower than 1 Ω.

Find out the appropriate way to do measurements. i.e. whether the instrument's input/output terminals should be grounded or be floating.

Check the stability of the AC power (ex. +5% to –10% within 120 V) and whether there are unpredictable impulses, which may cause the measurement to fail or even damage the instrument. Generally, the transient voltage fluctuation should not exceed ± 15% within 120 V and the voltage should be restored to 120 V within 0.5 second. The total harmonic component should be less than 5%.

Verify the grounding of the equipment or device under test (OUT). If voltage differences exist among equipment, connecting them together may cause conflicting situations. The sudden pulses generated when the equipment is powered on may also damage vulnerable modules. If this happens, the links between the equipment and devices under test should be disconnected before the AC power is turned on. Each equipment and OUT should be reconnected only after all equipment and devices have stabilized. In so doing, the possibilities of damage can be minimized. However, this is not the way to eradicate the problem completely. The best solution is to identify the root causes and fix them.

Reduce and remove unwanted static, interference and noise through proper grounding.

Keysight Test and Measurement Service Centers have been providing comprehensive and precise repair and calibration services to customers for many years. Our pursuit of quality and technical innovation enables us to offer better services to our customers. By sharing this article with you, we hope to help you improve the accuracy of measurements and the life span of your instruments.