111. 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. 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. Conventional measurement instruments (calculating the effective value the basis the average value) are definition only accurate with sinusoidal currents.9 Vrms t
- uses are protective devices.
Minimum measurable value (or threshold)
This the smallest value measurable. - t the weakest element which defines your level protection. 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. "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. you use accesI sories whose category voltage lower than your measurement instrument's, you reduce the overall safety level offered your measurement system.
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. also the value measurement count unit quantification usually called the "unit".
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. For example, for ammeter, the upper limit described having calibre. The RMS value essential for industrial applications because used define current.
Rated calibre
The calibre instrument the value the quantity measured which corresponds the upper limit the measurement range. For long time, the linear loads (incandescent lamps, heating) connected the network caused only slight distortion. Give preference CAT instruments: his means that can withstand higher overvoltages than CAT III product.
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. general, allows you define the measurement range and the resolution the basis the value chosen for the rated calibre.414 Recommendation: When speak 230 mains network, "effective" RMS value.) that operate correctly and are good condition. This often the case… but not always! external installations which may temporary upstream the proF tective systems, CAT instruments are mandatory. For sinusoidal signal, 1.), they will longer protect you against possible voltage surges the installation. This not always the case, however, and the manufacturer should indicate clearly, this minimum value also depends the accuracy and, particular, the constant error.
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. This why "RMS" measurement instruments are increasingly widely used. Any accessory with even the slightest defect must replaced immediately, can longer guarantee your safety. they are replaced cheaper models or, even worse, metal element (copper wire, aluminium foil, etc.9 1.dt 2
The amplitude sinusoidal voltage (Vpk) current times its RMS value (Vpk Vrms).UNIVERSAL TEST AND MEASUREMENT
Technical reminders
Number counts
This one the fundamental specifications instruments using analogue-digital conversion. They are based direct measurement principles: the thermal method (mainly used metrology) and analogue digital calculation methods requiring sophisticated electronic components.
Thus, for 230 V/50 network: Vrms 230 Vpeak 325 Vaverage 207 V
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. the constant error too high, becomes impossible measure very low values accurately. accessories that are perfect condition.
REsolution
This the smallest difference value that can measured. Vpeak Vrms
Vrms Vavg For sinusoidal voltage Vpeak Vrms Vaverage 0. This indirect measurement method simple and accurate but only gives true value for undistorted sinusoidal currents only tolerates distortion level few per cent. defined minimum measurable value and maximum measurable value. For instrument offering sufficiently linear conversion, may equal the resolution.
Safety rules and good practice
- measurement instruments and accessories which are suitable for the appliU cation and the measurement conditions
