HT instruments HT78 - Manual (Page 5)

HT78
EN - 4
2. GENERAL DESCRIPTION
HT78 meter can performs the herewith features:
• AC TRMS current measurement up to 3000A
• AC leakage current measurement with 0.1mA resolution
• DC voltage analogical output for connection with external data loggers
• 150Hz low-pass filter for reduce harmonic components of input signal
Each feature can be selected by using rotation selector on meter’s front panel. The Data
HOLD, FILTER and LOCK functions are still available (see § 4.2). The selected quantity
appears on a LCD display with indication of measurement units and functions. Analogical
bargraph it’s also displayed on meter. The meter disposes of an Auto Power Off function
consisting in an automatic switching off 10 minutes after last operation.
2.1.
TRMS AND MEAN VALUE MEASURING INSTRUMENTS
Safety testers for alternate quantities are divided into two big families:
• MEAN VALUE instruments: instruments which measure only the value of the wave at
the fundamental frequency (50 or 60 Hz).
• TRUE ROOT MEAN SQUARE instruments, also defined as TRMS: instruments which
measure the true root mean square value of the quantity under test.
In presence of a perfectly sinusoidal wave, both families provide identical results. In
presence of distorted waves, instead, the readings are different. Mean value instruments
provide only the value of the fundamental wave while True RMS 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,
True RMS instruments provide higher values than medium 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 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.