Manual

71M6541 Demo Board REV 3.0 User’s Manual
53 Rev 4.0
Figure 2-14: Wh Load Lines at Room Temperature with 71M6201 and 50 µΩ Shunts
2.6 SENSORS AND SENSOR PLACEMENT
Both sensor self-heating and sensor placement has to be considered in order to avoid side effects that can af-
fect measurement accuracy. These considerations apply in general to both ANSI meters and IEC meters.
Both meter variations will be discussed below.
2.6.1 SELF-HEATING
The effect of self-heating will be most pronounced at maximum current and depends on the following parame-
ters:
• Nominal shunt resistance
• Current through the shunt resistor
• Thermal mass
• Heat conduction away from the shunt (thermal resistance towards the environment)
• Temperature coefficient of copper and resistive material.
It is quite obvious that the nominal resistance of the shunt resistor should be kept as low as possible. Table 1-9
shows a few combinations of shunt resistance and 71M6X0X part number. The parts with part numbers corre-
sponding to higher current capacity are designed to work with low shunt resistance. Lowering the shunt re-
sistance below the recommended limits decreases accuracy and repeatability.
Good heat conduction can help to maintain the shunt temperature. Attaching the shunt to solid metallic struc-
tures such as meter terminal blocks helps decreasing the thermal resistance. This, of course, applies to meters
where the terminals and other mechanical parts can be considered heat sinks, i.e., they do not heat up due to
other effects.
The thermal mass will control how long it takes the sensor to reach its maximum temperature. Meters, for which
only short-time maximum currents are applied, can benefit from a large thermal mass, since it will increase the
time constant of the temperature rise.
The temperature coefficient (TC) of the shunt is a very important factor for the self-heating effect. Shunts with a
TC of just a few PPM/°C can maintain good shunt accuracy even in the presence of significant self-heating.
There are several methods that can be applied in the meter code that can minimize the effects of self-heating.
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0.1 1 10 100 1000
Form 2S - Wh Error [%] at 0°, 60° and 300° Phase
Angle, 240 V/60 Hz
60°
300°
0°