HT instruments HT327 - Manual (Page 5)

HT327
EN - 4
2. GENERAL DESCRIPTION
This meter performs the below listed measurements:
 DC and AC TRMS Voltage
 DC and AC TRMS Current
 Resistance and Continuity test
 Frequency
 Capacitance
 Diode
test
All selectable by means of a 10 position rotary selector (including OFF position). Functions
keys are also available (see § 4.2). An analogical bargraph is also available. The selected
quantity is displayed with indication of measuring unit and active functions.
The instrument disposes of an Auto Power Off function consisting in an automatic
switching off 30 minutes after last selector rotation or function selection.
2.1. MEAN VALUE AND TRMS: DEFINITION
Safety testers for AC quantities are divided in two big families:
 MEAN VALUE instruments, measuring only the value of the wave at the
fundamental frequency (50 or 60 Hz).
 TRUE ROOT MEAN SQUARE (or “TRMS”) instruments, measuring the true root
mean square value of the quantity under test.
In presence of a perfectly sinusoidal wave, both families provide identical results. While in
presence of distorted waves, readings are different. Mean value instruments provide only
the value of the fundamental wave while TRMS instruments provide the value of the entire
wave, including harmonics (within the passband of the instrument). Accordingly, if the
same quantity is measured with both kinds of instruments, the measured values are
identical only if the wave is purely sinusoidal. Should it be distorted, TRMS instruments
provide higher values than MEAN VALUE instruments.
2.2. TRUE ROOT MEAN SQUARE VALUE AND CREST FACTOR: DEFINITION
The effective current value is defined as follows: “In an interval of time equivalent to a
period, an alternate current with effective value having an intensity of 1A, by passing on a
resistor, disperses the same energy which would be dispersed in the same period of time
by a direct current having an intensity of 1A”. From this definition comes the numerical
expression: G=

T
t
t
dt t g
T
0
0
) (
1
2 The effective value is indicated as RMS (root mean square).
The Crest Factor is defined as the ratio between the Peak Value of a signal and its
effective value: CF (G)=
RMS
p
G
G
. This value varies according to the waveform of the signal,
for a purely sinusoidal wave it’s worth 2 =1.41. In presence of distortions the Crest Factor
assumes higher values as long as the wave distortion is higher.