Specifications

T4.35
Transformers
T4
Series MT
Measurement Instruments
Function and range
The range of AC-measurement-instruments consists
of 2 main families: analogue and digital.
The analogue family includes:
- voltmeters
- Ammeters
- frequency meters
- hour counters
The digital range consists of:
- voltmeters
- Ammeters
- frequency meters
- kWh meters
- energy meters
- net analysers
On top of this, several accessories complete
the range:
- a complete range of current transformers,
- a complete range of corresponding scale-plates,
- selector switches for switching a single phase
measurement instrument between the different
phases of a 3-phase energy distribution system,
- a very user friendly Windows-95 (and up) software
for use with the net-analyser
- an RS232-RS485/422 signal converter for
interfacing between a PC and the net-analyser.
Terminology
Class
The accuracy or class of a measurement instrument
is the maximum error between the displayed value
and the real value.
For an analogue measurement instrument, the class
is equal to a percentage of the full scale. On a
voltmeter with 300V full scale, a class of 1.5 means
a maximum error on the reading of 4.5V, no matter
what the actual reading is. This means that if a
voltage of 228V is measured, the real value can be
anything in between 232.5 and 223.5V, whereas if
the reading would be 10V, the actual value would be
between 5.5 and 14.5V.
On a digital measurement instrument, on top of the
measurement-error, there is also a rounding error
since the display does not have an unlimited
number of digits. In this case, if the full scale is
300V and the display has 3 digits, a device with a
class of 0.5% ± 1digit can have an error in the
reading of maximum ± 2V, again as above,
independent of the actual reading.
True-RMS versus Average AC-metering
Independently of the electrical signal waveform, a
true-RMS meter (true root-mean-square meter)
measures the correct electrical value (except for the
class-error of course; see above). This means that
a true-RMS-Ammeter would measure exactly the
same current as would be measured by a DC-
Ammeter, metering a current flowing through the
same resistance, provoked by a DC-voltage equal to
the RMS-value of the voltage waveform. Figure 1
shows different waveforms with their respective
RMS-values.
An average-metering instrument on the other hand,
measures the magnitude of the electrical signal and
multiplies it with a factor. As this multiplier is only
correct for one specific waveform (see figure 1), the
measurement is incorrect, when measuring with this
device an electrical signal with a waveform other
than the one for which it was meant to be.
All Series MT analogue measurement instruments
are true-RMS, all simple digital measurement
instruments (V, A and W) are average-metering
instruments and all high-end digital measurement
instruments (kWh and net analysers) again are
true-RMS measurement devices.
fig.1