How to determine whether the ground on the multimeter probe can be directly connected to the ground of the board being tested?

1. Unsafe operation: floating measurement

Some engineers have a habit of disconnecting the protective ground of the power plug when measuring high-voltage signals, and using ordinary passive probes to directly perform high-voltage floating ground measurements. In fact, doing so is harmful.

Examples of common phenomena: Touching the oscilloscope casing and feeling an electric shock

Check: 1. Check if the power ground of the oscilloscope is disconnected manually or has poor contact; 2. Change the socket strip; 3. The ground of the building is not properly connected.

Reason: Y capacitors are capacitors connected across the live and ground wires, and the neutral and ground wires of the power supply, as shown in Figure 1. They mainly play the role of filtering protection and suppress common-mode interference. They are safety capacitors. After the capacitor fails, it will not cause electric shock and will not endanger personal safety. When the protective ground of the power plug is disconnected, the 220V voltage is divided by the Y capacitor, and the middle 110V voltage is directly added to the metal shell of the oscilloscope. When a person touches the live area, an electric shock phenomenon like being pricked by a needle will occur. Although it will not endanger personal safety, it is also a dangerous operation.

Figure 1 Power supply circuit schematic

2. Unsafe operation: Pick up the probe and connect the wires for measurement

Examples of common phenomena: tripping/tested circuit board burned through/oscilloscope or probe burned out

Check: 1. Whether the ground of the measured signal and the ground of the oscilloscope calibration signal are in common; 2. Whether a single passive probe is used directly to measure the AC power.

Before analyzing the cause, we should first understand what is the mains electricity and the composition of the power supply line. The specification of my country's mains electricity (residential electricity) is AC 220V@50Hz. The power supply line, that is, the wires in the three-plug power supply, consists of a live wire, a neutral wire, and a ground wire, as shown in Figure 2.

Live wire (L): Also called phase wire, provided by power station or substation, voltage 220V, human contact may be dangerous;

l Neutral line (N): provides a loop for the live line and is grounded at the power station or substation. Since it is remotely grounded, the potential at the user end of the residential building is not necessarily zero and may carry weak electricity, but it is relatively safe.

Ground wire (E): zero potential reference point, connected to the ground at the user end of a residential building, zero voltage, safe.

Figure 2 The relationship between the three lines

Reason: Power system testing often requires measuring the relative voltage difference between live wires or between live wires and neutral (neutral) wires in three-phase power supply, while all channels of ordinary digital oscilloscopes are grounded and not isolated from each other. All signals applied to and provided by the oscilloscope have a common connection point, which is usually the oscilloscope case. The probe ground wire is connected to a test point by using the third conductor ground wire in the power cord of the AC power supply device. If a single-ended probe is used for measurement at this time, the ground wire of the single-ended probe is directly connected to the power line, resulting in a short circuit.

3. Standardized operation

1. Self-check the ground before wiring measurement

How can we determine whether the ground on the probe can be directly connected to the ground of the board under test before wiring? It is very simple, and there are three steps to achieve self-check:

Prepare tools: board to be tested, multimeter, oscilloscope

(1) Preparation: Turn on the multimeter and select the AC mode;

(2) Wiring: Connect the oscilloscope and the board under test to the power supply respectively, but do not power on for the time being; connect one end of the multimeter probe to the ground of the oscilloscope calibration signal, and the other end to the ground of the board under test. The simple wiring diagram is shown in Figure 3;

(3) Measurement: Power on each device and measure the AC value.

If the measured value is not 0 at this time, it means that there is a voltage difference between the board under test and the ground of the oscilloscope, as shown in Figure 4. In this case, a single passive probe ground clip cannot be used to directly connect to the ground on the board under test.

Figure 3 Wiring diagram for measuring differential pressure

Figure 4: When there is a voltage difference, the two "grounds" cannot be directly connected

1. "AB" pseudo-differential measurement of mains power

When using an ordinary passive probe to measure the AC power using the "AB" method, the negative terminals (ground) of the two-channel probes should be connected to the power ground wire, the probe tip (positive terminal) of one channel should be connected to the neutral wire, and the probe tip (positive terminal) of the other channel should be connected to the live wire (as shown on the left of Figure 5). The measurement difference between the two channels is the AC power waveform.

While this method is subject to measurement errors, it can be used when the signal is low frequency and the signal amplitude is large enough to exceed any noise of concern.

Figure 5 Recommended wiring diagram for measuring mains power

1. Measurement method: using a high voltage differential probe

Using a high-voltage differential probe for measurement is a safe and accurate method. The solution for floating ground measurement and measuring AC power (wiring diagram as shown in the right of Figure 5) is to use a differential probe with a high common-mode rejection ratio, because there is no grounding problem at both input ends, and the differential operation of the two input signals is completed in the front-end amplifier of the probe. The signal transmitted to the oscilloscope channel is a differential voltage, which can achieve safe measurement.

ZP1500D high-voltage differential probe is independently developed by our company. The probe's standard 50× and 500× gears can switch between 5M and 100M bandwidths. The attenuation ratio and bandwidth gears can be flexibly used to help you measure low-frequency signals and high-speed signals.

Figure 6 ZP1500D high voltage differential probe

IV. Summary

Although high-voltage differential probes are more expensive than ordinary passive probes, they can ensure more accurate and safer measurement results when measuring high-voltage signals. They are recommended for mains and floating ground measurements. As a commonly used measuring instrument in life, we should follow the safe use of electricity rules and perform standardized wiring operations to avoid accidents.