This not always the case, however, and the manufacturer should indicate clearly, this minimum value also depends the accuracy and, particular, the constant error. For example, for ammeter, the upper limit described having calibre.UNIVERSAL TEST AND MEASUREMENT
Technical reminders
Number counts
This one the fundamental specifications instruments using analogue-digital conversion. For sinusoidal signal, 1. also the value measurement count unit quantification usually called the "unit".
RMS: Root Mean Square value
By definition, the effective RMS value current the current value which would cause the same heating when flowing through resistor.
Rated calibre
The calibre instrument the value the quantity measured which corresponds the upper limit the measurement range. defined minimum measurable value and maximum measurable value.
REsolution
This the smallest difference value that can measured. they are replaced cheaper models or, even worse, metal element (copper wire, aluminium foil, etc. They are based direct measurement principles: the thermal method (mainly used metrology) and analogue digital calculation methods requiring sophisticated electronic components. Any accessory with even the slightest defect must replaced immediately, can longer guarantee your safety. This why "RMS" measurement instruments are increasingly widely used.), they will longer protect you against possible voltage surges the installation.9 Vrms t
- uses are protective devices. For instrument offering sufficiently linear conversion, may equal the resolution. the constant error too high, becomes impossible measure very low values accurately.111.
Thus, for 230 V/50 network: Vrms 230 Vpeak 325 Vaverage 207 V
Page 18
.
Peak value Crest Factor
The crest factor defined follows: Vpeak Vrms This information can used with the RMS value assess signal's distortion qualitative terms.
Minimum measurable value (or threshold)
This the smallest value measurable. general, allows you define the measurement range and the resolution the basis the value chosen for the rated calibre. - t the weakest element which defines your level protection. Give preference CAT instruments: his means that can withstand higher overvoltages than CAT III product. "average value" measurement instrument measures the average value sinusoidal current after rectification and filtering and then displays the effective value after applying coefficient 1/0. Otherwise, the measurement error may reach therefore advisable opt for "RMS" measurement instruments which are capable giving correct measurement results whatever the waveform the current voltage. This often the case… but not always! external installations which may temporary upstream the proF tective systems, CAT instruments are mandatory. you use accesI sories whose category voltage lower than your measurement instrument's, you reduce the overall safety level offered your measurement system.9 1. Vpeak Vrms
Vrms Vavg For sinusoidal voltage Vpeak Vrms Vaverage 0.) that operate correctly and are good condition.414 Recommendation: When speak 230 mains network, "effective" RMS value. The spread non-linear loads (switching power supplies, light dimmers, variable speed drives compact fluorescent lamps) calls this approach into question "pure" sinusoidal mains supplies are increasingly rare. Conventional measurement instruments (calculating the effective value the basis the average value) are definition only accurate with sinusoidal currents. This indirect measurement method simple and accurate but only gives true value for undistorted sinusoidal currents only tolerates distortion level few per cent. For long time, the linear loads (incandescent lamps, heating) connected the network caused only slight distortion.
Safety rules and good practice
- measurement instruments and accessories which are suitable for the appliU cation and the measurement conditions. accessories that are perfect condition.
Vrms =
1 T
∫ v(t)
T o
2
In the specific case sinusoidal quantity, application the above equation gives:
v Vpk cos Vrms Vpk v pk cos(ωt)2.
Measurement range
This represents the limits within which the digital instrument complies with all its specifications, without the readings being subject level error greater than the maximum error tolerated. 1,000 offers protection against electric shocks 12,000 while CAT IV 600 offers protection 8,000 • f lower-CATegory instrument used, you must first check that the instalI lation equipped with protective systems (disconnecting switch, circuit breaker, etc. The RMS value essential for industrial applications because used define current.dt 2
The amplitude sinusoidal voltage (Vpk) current times its RMS value (Vpk Vrms)
