HT instruments HT712 - Manual (Page 5)

HT712
EN - 4
2. GENERAL DESCRIPTION
The instrument performs the following measurements:
● DC and AC TRMS voltage
● AC voltage with 1 test lead
● Frequenza
● Frequency with 1 test lead
● Phase sequence test
● Phase
conformity test
● Resistance
● Continuity
test
selectable by means of a 5-position rotary selector. Three function keys are also available.
The selected measurement is displayed with indication of active functions. In order to save
batteries the instrument automatically switches off 15 minutes after last pressure on keys
or last selector rotation. To resume operation turn the rotary selector on OFF position and
switch it on again.
2.1. TRMS VALUE AND MEAN VALUE - DEFINITIONS
Safety testers for alternate quantities are divided into two categories:
• MEAN VALUE instruments: instruments measuring only the value of the wave at
the fundamental frequency (50 or 60Hz)
• True Root Mean Square (TRMS) instruments: instruments measuring the true root
mean square value of the quantity under test.
In presence of a perfectly sinusoidal wave, both categories provide identical results. But 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 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 - DEFINITIONS
The current effective 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 we get 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 higher the wave
distortion is, the higher the crest factor values get.