ST AN2167 APPLICATION NOTE

October 2008 Rev 2 1/5

AN2167

Application note

Using the STPM01 with a shunt current sensor

Introduction

This document describes how a shunt current sensor can be used with the STPM01

metering device in single-phase metering applications without tamper detection.

Special considerations must be taken into account when the shunt is designed:

■ The current density must be kept constant

■ The power dissipation must be reduced, as it can influence the ambient temperature

within the meter housing. In the event of large current overloads, this can become an

explosion hazard.

Note: Special safety precautions must be followed while the meter is connected to the power line

and opened. The ground plane of the meter is at a high voltage level because the shunt has

to measure the current flow through a hot conductor without any galvanic isolation. This is a

safety hazard for the operators and can destroy the connected equipment.

For more information on the STPM01, refer to the full datasheet.

www.st.com

Theory of operation AN2167

2/5

1 Theory of operation

A shunt is a resistor intended for relatively high current levels. This means that Ohm's law

may be applied to it as a first-order approximation model and measure its response to the

primary current: (see Equation 1).

Equation 1

The first problem is that R

is not constant, stable, or perfectly linear. It is affected by

changes in temperature and current density, so a special shape and material must be used

to:

1. keep all of the coefficients as low as possible

2. maintain low levels of mechanical expansion

The second problem is self-heating due to power loss in the conductive material: (see

Equation 2)

Equation 2

The second equation represents the prime limiting factor to the usage of a shunt sensor. For

example, if the user wants to use a 500 µΩ shunt sensor to build a class 0.5 direct meter

with I

NOM

MAX

= 10/80A

RMS

, the voltage output level would be 40 mV

RMS

, and the power

dissipated in the shunt at I

MAX

would be 3.2 W.

However, this meter must be able to withstand an overcurrent of 5000 A

RMS

for 60 ms,

which yields 750 J of energy. This would melt the shunt and, consequently, generate an arc

and overheat the air in the sealed meter housing, causing the meter to explode.

To keep the shunt from melting, a higher volume of material must be used and power

dissipation needs to be reduced. Power dissipation can be reduced by decreasing the

resistance of the shunt. For a 210 µΩ shunt, the output voltage would be 16.8 mV

RMS

, the

power dissipated would be 1.344 W, and the energy level would be 315 J.

The third problem is the meter accuracy. For a class 0.5 direct meter, the accuracy of the

power and energy values produced by the shunt must be within 1% or better, at 5% of

nominal current (I

NOM

). For a 210 µΩ shunt, the sensitivity of the meter must be

±1.05 µV

RMS

This makes it more difficult to deal with electromagnetic interference (EMI) and new error

sources (e.g., Kelvin and Peltier effects) which become important, and may include stray

capacitances and mutual inductances, as well as noise generated by the preamplifier. The

shunt connection to the line and the STPM01 must be assembled very carefully in order to

minimize error source contributions.

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