it can be quite difficult to take accurate current measurements in modern industrial and office facilities, where personal computers, motor controls and equipment that draw current in short pulses instead of an even sine format are the norm. This type of equipment means that common instruments used for the rectified recording of average values always reveal errors to some extent. If you have ever searched for the reason why a fuse has been triggered, perhaps the multimeter was the cause.
When we talk about measurement values for AC current, we usually mean the effective value, i.e. the heat power, in the RMS (Root Mean Square) data sheets. This value is equal to a DC current that provides the same heat power in a resistor as the measured AC current. The most common method of measuring this heat power with a multimeter is to rectify the AC current, determine the average value of the rectified signal and multiply the result by the factor 1.11. This factor represents a ratio between the average value and the effective value on a perfect sine curve. If the curve is not perfectly sinusoidal, however, this ratio will no longer apply. This is the reason why instruments used for the rectified recording of average values most often give incorrect values when measuring current in modern electrical systems.
Linear loads consist primarily of resistors, coils and capacitors that always draw a sinusoidal current. For this reason, such loads do not cause any problems (see Figure 1). Non-linear loads, such as frequency converters and switched current generators in office electronics, draw a distorted current (see Figure 2). Measuring the effective value with instruments used for the rectified recording of average values can give a measured value that is up to 50 % too low. This can result in you wondering why a 14 A fuse is constantly being triggered, despite the fact that the multimeter shows 10 A.
In order to measure on such distorted curve shapes, you can first check the curve shape with a graphic instrument and only use an instrument for rectified recording of average values if the curve is perfectly sinusoidal. If you do not want to take any chances, you should always use an instrument that measures the true effective value directly. A modern multimeter for measuring the true effective value uses an electronic measuring technique to always display the correct effective value of an AC current, regardless of whether the curve shape is sinusoidal or whether it is distorted.
