Digital Multimeters/Clamp Meters/Voltage and Continuity Testers Glossary - Multimeters


RMS - Root Mean Square When an AC supply is placed onto a cir- cuit, it produces heat. The RMS value is the equivalent DC supply that would pro- duce the same amount of thermal heat as the actual AC supply.


True RMS


A specific method of measuring the RMS value


of a signal. With induc-


tive and capacitive systems distorting the sinusoidal wave of the mains sup- ply, this method provides the most ac- curate RMS value regardless of the shape of the waveform. Other meth- ods of measuring RMS values exist, such as the rectifier or mean absolute deviation method; however, these methods are accurate only for sine wave signals.


Crest factor


The crest factor describes the ratio of the peak value to the RMS value of an electrical variable (AC voltage and AC current).


High crest factors cause distortion of the reactive power and harmonics in the supply network, and so are undesirable.


With high crest factors electronic instru- ments often display inexact values. This must be considered when selecting an instrument. For example, if an instru- ment measures 20 A AC current, the peak value is around 28 A with a sinusoi- dal waveform. So the instrument must be able to handle much higher peak cur- rents than the RMS value.


Number of Counts


The number of divisions into which a given measuring range is divided. This can be used to evaluate the resolution of an instrument.


Accuracy


A value to show how accurately an in- strument can read a specific value. This is usually written as a percentage (e.g. 5 V ± 5 %).


CAT I CAT II CAT III CAT IV Resolution


The smallest possible change in a signal that would produce a change in the val- ue on the screen of the test instrument.


Overvoltage category


The overvoltage category specifies the highest mains voltage (or lightning strike, short circuit due to incorrect use, etc.) that the instrument can withstand without danger for the tester or for the object being measured. The standard specifies four overvoltage categories. The overvoltage category affects com- ponent sizing via the air gap. The higher the category, the bigger is the distance to the power source.


CAT I - electronic devices, signal level.


CAT II - domestic appliances, portable appliances, single-phase loads, sockets, (>10 m from CAT III; >20 m from CAT IV).


CAT III - three-phase distribution sys- tems, lighting systems in large buildings, distribution panels.


CAT IV - three-phase systems on power stations, electricity meters, outdoor in- stallations and supply cable incoming feed.


True RMS


DC current range (A) Basic accuracy (%) Maximum resolution (µA) AC current range (A) Basic accuracy (%) Maximum resolution (µA) DC voltage range (V) Basic accuracy (%) Maximum resolution (µV) AC voltage range (V) Basic accuracy (%) Maximum resolution (µV) Resistance measurement (MΩ) Basic accuracy (%) Maximum resolution (mΩ) Acoustic continuity test Diode test Capacitance


Frequency measurement Frequency of digital signals


Temperature measurement (Type K sensor) Autocheck®


V / Ω


Conductance (nS) Count


Backlight


Analogue bar-graph IR, RS232 interface


Automatic and manual range selection Automatic switch off


Non-contact eletrical field detection (EF) MAX hold Peak hold Data hold


Recording (MAX / MIN / AVG) Relative value


Compensation for test leads Overvoltage category


Dimensions with holster (mm) Weight with holster (g) CE mark


Part No.:


7. 2


Accessories: page 7.21


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