What is low Z on multimeter?

What is low Z on multimeter?

Dual impedance digital multimeters have feature to control the input impedance of the multimeter. LoZ stands for Low Impedance (Z). This feature presents a low impedance input to the circuit under test. A typical digital multimeter has typically around 10 megaohms input impedance, which loads the circuit being measured very little, but can pick up noise easily in some applications. For example when working with electrical power AC circuits, the multimeter can easily pick up noise from nearby wiring. LoZ feature in some multimeters is designed to reduce the possibility of false readings due to ghost voltages and improves accuracy when testing to determine absence or presence of voltage.

The “ghost voltage” reading you get from in multimeter screen is a real voltage and although high in amplitude it is potentially very weak due to it being sourced via a high impedance. A high input impedance measurement device (o-scope, multimeter etc.) can measure this voltage with little or no obvious reduction in the amplitude of the voltage but, a low impedance device will try and take current through the inherent high impedance in series with the voltage, and the voltage will collapse in amplitude, sometimes just to a few milli volt.

Ghost voltage is just a voltage that is in-series with a very high impedance. For instance you can capacitively couple to AC voltages in house wiring and this coupling might only be 50 pF. An oscilloscope will display the voltage on your body and tell you that the voltage is a few volts peak to peak yet, 50 pF has an impedance at 50 Hz of 63.7 Mohm. This will form a potential divider with your o-scope input (usually 10 megaohms) and the 120 VAC might drop to around 10 or 20 volts. Prior to connecting the multimeter the voltage would be higher. These voltages are real, not “ghosts”. Very many digital multimeters have also around 10 megaohms impedance on voltage measurements range.

Ghost voltages are voltages that exist where they shouldn’t. Many technicians use the term “ghost voltages” to refer to compromised voltage sources. However, technically, ghost voltages are those that are inductively or capacitively induced by the proximity to other wiring. One example is when two wires run in parallel, which effectively acts like a capacitor. A capacitor consists of two metal plates separated by a distance and can store an electrical charge. When the voltage changes, or it is an AC voltage, it reduces the overall impedance (actually reactance, but let’s simplify) between the two plates, which can cause a voltage to appear in the nearby wire.

AC current is flowing, but it is small. If you substitute voltmeters having lower internal resistance, you’ll measure lower AC voltages. The LoZ feature on your multimeter takes advantage of a “voltage divider” concept. The conclusion here for each resistance in a series circuit is: “The higher the resistance, the higher the voltage. The lower the resistance, the lower the voltage“. Those high voltages that show at with high impedance measurement mode but disappear on LoZ mode are “ghosts”

Traditionally most electricians and plant maintenance professionals have used some form of solenoid tester to determine whether circuits were energized or not. Because of their low impedance circuit, solenoid testers are not fooled by ghost voltage. Adding LoZ to multimeter potentially avoids the need to carry an extra tool with you in addition to a multimeter.

LoZ is very useful for electrician type applications. But it is not suitable for tracing a electronics circuit with intentionally high resistances. Sensitive electronic devices or components on control boards may not tolerate the additional load introduced by the LoZ function. If you use LoZ in such cases, it can disrupt the intended behavior of the circuit and provide inaccurate readings or even damage components.

Overall, understanding the advantages and limitations of the LoZ function can greatly assist in diagnosing electrical issues and ensuring accurate voltage measurements.

What is the input impedance of an instrument? What are ghost voltages and how to detect them?
https://www.youtube.com/watch?v=htx6oLx1gBs

How To Determine If Voltage Is Real Or a Ghost Voltage Using your Fluke Digital Multimeter
https://www.youtube.com/watch?v=mRwsxmn2P-s

How to Turn Off Ghost Voltages with Dual Impedance (LoZ) Meters
https://www.youtube.com/watch?v=JRXTFxsA_lA

Some technical details of LoZ on some multimeters:

Low Impedance Voltage Measurement (Lowz) – Kaiweets HT206B User Manual
https://www.manualslib.com/manual/2074243/Kaiweets-Ht206b.html?page=11 says:

Low impedance voltage measurement (LowZ)
LowZ mode allows you to measure AC voltages with a low impedance (approx. 300 kΩ).
In this mode, the multimeter lowers the internal resistance to prevent ‘phantom’ voltage readings.
As a result, the circuit is more heavily loaded than in the standard measuring mode.
Set the rotary switch to “LowZ” to use the function.
The LowZ voltage measurement must not exceed 600 V.
Due to the reduced impedance, this measurement function is not suitable for continuous
measurement. A max. The measurement duration of 1 minute must be observed

Fluke document at says https://www.fluke.com/en-us/learn/blog/digital-multimeters/dual-impedance-digital-multimeters

With dual impedance meters, technicians can safely trouble shoot sensitive electronic or control circuits as well as circuits that may contain ghost voltages, and can more reliably determine whether voltage is present on a circuit.

On the Fluke 114, 116 and 117 DMMs, the meter’s regular Vac and Vdc switch positions are high impedance. Use these switch positions for most troubleshooting scenarios and especially on sensitive electronic loads.

The Fluke low impedance function is called Auto-V/LoZ. Auto-V stands for automatic or volts. This feature automatically, determines whether the measured signal is ac voltage or dc voltage, selects the correct function and range, and displays the correct information. LoZ stands for Low Impedance (Z). This feature presents a low impedance input to the circuit under test. This reduces the possibility of false readings due to ghost voltages and improves accuracy when testing to determine absence or presence of voltage. Use the Auto-V/LoZ switch position on the DMM when readings are suspect (ghost voltages may be present) or when testing for the presence of voltage.

Most digital multimeters available today have an input impedance that’s high enough to show the capacitively coupled voltage, giving a false impression of a live conductor. The meter is actually measuring voltage coupled into the disconnected conductor. However, these voltages, at times, can be 80-85% of what the “hard” voltage should be. If not recognized as a ghost voltage, additional time, effort and money will be lost troubleshooting circuit problems.

The most common places to encounter ghost voltages are blown fuses in distribution panels, unused cable runs or electrical wiring in existing conduit

With a dual impedance meter, select the Auto-V/LoZ function. This function has a low input impedance on the order of three kilohms. When the leads are placed on an open circuit that contains a ghost voltage, the low input impedance will cause the ghost voltage to dissipate and the meter will display a reading near zero volts indicating no voltage present.

When the multimeter has an input impedance of few kilo-ohms, the downside is that if put in high voltage, a lot of power is lost inside multimeter. It seems that some LoZ multimeters use an adaptive input impedance, that is few kilo-ohms at signals that have amplitude of several volts (meaning up to few mA current), and increases the impedance when the input voltage gets higher (to something like 100 kOhms at full mains voltage, meaning around 1-2 mA current through multimeter).

UNI-T says at https://meters.uni-trend.com/faqs/acv-loz-low-impedance/ on LoZ:

ACV LOZ Low impedance

Low impedance input: False voltage generated by large capacitive equipment can be measured.
The input impedance of a low-impedance multimeter is 300kΩ. It can discharge the residual voltage and determine whether the residual voltage exists.
The input impedance of an ordinary multimeter is as high as 10MΩ. Therefore, residual voltage cannot be identified, resulting in misjudgment or security risks.
There will be residual voltage after powering off large capacitive equipment.
Applied to all electrical equipment, such as reactive power compensation cabinets, large commercial fluorescent lamps.

Low impedance voltage testing (LoZ) discussion at https://electronics.stackexchange.com/questions/463151/low-impedance-voltage-testing-loz has following information bits on how LoZ mode is implemented on some multimeters:

On BM829s, Lo-Z gives you 500kOhm at 1000V, 320kOhm at 600V, 125kOhm at 300V, 18kOhm at 100V: an extremely convenient autoranging capacitor discharge device!