General Description; Measuring Average Values And Trms Values; Definition Of True Root Mean Square Value And Crest Factor - HT Instruments HT3010 User Manual

2. GENERAL DESCRIPTION

The instrument carries out the following measurements:
• DC and TRMS AC Voltage up to 600V
• TRMS AC Current up to 400A
• Resistance and continuity test with buzzer
• Frequency with leads and jaws
• Diode test
• Detection of presence of AC voltage without contact with in-built sensor.
Each of these functions may be selected through a rotary switch. The instrument also has
function keys (see § 4.2) and backlight feature. The instrument is also equipped with an
Auto Power OFF function (which cannot be disabled) which automatically switches off the
instrument approx. 15 minutes after the last operation was carried out.

2.1. MEASURING AVERAGE VALUES AND TRMS VALUES

Measuring instruments of alternating quantities are divided into two big families:
• AVERAGE-VALUE meters: instruments measuring the value of the sole wave at
fundamental frequency (50 or 60 Hz).
• TRMS (True Root Mean Square) VALUE meters: instruments measuring the TRMS
value of the quantity being tested.
With a perfectly sinusoidal wave, the two families of instruments provide identical results.
With distorted waves, instead, the readings shall differ. Average-value meters provide the
RMS value of the sole fundamental wave; TRSM meters, instead, provide the RMS value
of the whole wave, including harmonics (within the instruments bandwidth). Therefore, by
measuring the same quantity with instruments from both families, the values obtained are
identical only if the wave is perfectly sinusoidal. In case it is distorted, TRMS meters shall
provide higher values than the values read by average-value meters.

2.2. DEFINITION OF TRUE ROOT MEAN SQUARE VALUE AND CREST FACTOR

The root mean square value of current is defined as follows: "In a time equal to a period,
an alternating current with a root mean square value of 1A intensity, circulating on a
resistor, dissipates the same energy that, during the same time, would be dissipated by a
direct current with an intensity of 1A". This definition results in the numeric expression:
+T
t
0
1

2
G= The root mean square value is indicated with the acronym RMS.
g ) ( t dt
T
t
0
The Crest Factor is defined as the relationship between the Peak Value of a signal and its
RMS value: CF (G)=
sinusoidal wave it is
as wave distortion increases.
G
p
This value changes with the signal waveform, for a purely
G
RMS
=1.41. In case of distortion, the Crest Factor takes higher values
EN – 4
HT3010