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The ends of the shunt strips are embedded in heavy copper blocks. The blocks are attached to the meter coil leads and the line terminals. To ensure accurate readings, you should not interchangeably use the meter leads for a particular ammeter with those for a meter of a different range. Slight changes in lead length and size may vary the resistance of the meter circuit. If this happens, current will also change and cause incorrect meter readings. External shunts are generally used where currents greater than 50 amperes must be measured. SHUNT SELECTION. - When using an external-shunt ammeter, you should select a suitable shunt so that the scale deflection can be easily read. For example, if the scale has 150 divisions and the load current you want to measure is known to be between 50 and 100 amperes, a 150-ampere shunt would be the correct choice. Under these conditions, each division of the scale represents 1 ampere. In other words, a full-scale deflection of the pointer would rest on the 150th division mark, indicating that 150 amperes of load current is flowing. At half-scale deflection, the pointer would rest on the 75th division mark, indicating that 75 amperes of load current is flowing. A shunt having exactly the same current rating as the expected normal load current should never be selected. If you were to select such a shunt, higher than normal load currents could possibly drive the pointer off scale and damage the meter movement. A good choice of shunt values will place the indicating needle somewhere near the midscale indication when the load current you are reading is normal. For example, assume that the meter scale is divided into 100 equal divisions and you want to measure a current of 60 amperes. The shunt to use would be a 100-ampere shunt. This would make each division of the scale equal to 1 ampere. The meter indication would fall on the 60th division showing that 60 amperes of load current is flowing. Therefore, an allowance (40 amperes) remains for unexpected surge currents. Q.10 A good choice of shunt resistance will place the indicating pointer near what part
of the meter scale with a normal load? INTERNAL SHUNTS FOR METERS IN THE 0- TO 50-AMPERE RANGE. - When measuring current ranges below 50 amperes, you will most often use internal shunts (Rshunt). In this way, you can easily change the range of the meter by means of a switching arrangement. A switch will select the correct internal shunt with the necessary current rating and resistance. Before you can calculate the required resistance of the shunt for each range, the total resistance of the meter movement must be known. For example, suppose you desire to use a 100-microampere D'Arsonval meter with an internal coil resistance of 100 ohms to measure line currents up to 1 ampere. The meter will deflect to its full-scale position when the current through the deflection coil is 100 microamperes. Since the coil resistance is 100 ohms, you can calculate the coil's voltage (Ecoil) by using Ohm's law, as follows:
When the pointer is deflected to full scale, 100 microamperes of current flows through the coil and 0.01 volt drops across it. Remember, 100 microamperes is the maximum safe current for this meter movement. Exceeding this value will damage the meter. The shunt must carry any additional load current. The meter coil has a 0.01-volt drop across it, and, because the shunt and coil are in parallel, the shunt also has a voltage drop of 0.01 volt. The current that flows through the shunt is the difference between the full-scale meter current and the line current being fed into the shunt. In this case, meter current is 100 microamperes. Full-scale deflection is desired only when the total current is 1 ampere. Therefore, the shunt current must equal 1 ampere minus 100 microamperes, or 0.9999 ampere. Ohm's law is again used to provide the approximate value of required shunt resistance (Rshunt), as follows:
To increase the range of the 100-microampere meter to 1 ampere (full-scale deflection), place a 0.01-ohm shunt in parallel with the meter movement. You can convert the 100-microampere instrument to a 10-ampere meter by using a proper shunt. The voltage drop for a full-scale deflection is still 0.01 volt across the coil and the shunt. The meter current is still 100 microamperes. The shunt current must therefore be 9.9999 amperes under full-scale deflection. Again, this is an approximate figure found by the application of Ohm's law. You can also convert the same instrument to a 50-ampere meter by using the proper shunt resistance, as follows:
INTERNAL SHUNTS FOR METERS IN THE MILLIAMPERE RANGE. - The above method of computing the shunt resistance is satisfactory in most cases; however, it can only be used when the line current is in the ampere range and the meter current is relatively small compared to the load current. In such cases, you can use an approximate value of resistance for the shunt, as was done above. However, when the line current is in the milliampere range and the coil current becomes an appreciable percentage of the line current, a more accurate calculation must be made. For example, suppose you desire to use a meter movement that has a full-scale deflection of 1 milliampere and a coil resistance of 50 ohms to measure currents up to 10 milliamperes. Using Ohm's law, you can figure the voltage (E coil) across the meter coil (and the shunt) at full-scale deflection, as follows:
The current that flows through the shunt (I shunt) is the difference between the line current and the meter current, as figured below:
The shunt resistance (Rshunt) may then be figured, as follows:
Notice that, in this case, the exact value of shunt resistance has been used rather than an approximation. The formula for determining the resistance of the shunt is given by Rs = I m/Is times Rm, where Rs is the shunt resistance in ohms; Im is the meter current at full-scale deflection; Is is the shunt current at full-scale deflection; and R m is the resistance of the meter coil. If the values given in the previous example are used in this equation, it will yield 5.55 ohms, the value previously calculated. SWITCHING SHUNT VALUES. - Various values of shunt resistance can be used, by means of a suitable switching arrangement, to increase the number of current ranges that can be covered by the meter. Two switching arrangements are shown in figure 3-5. View A is the simpler of the two arrangements when a number of shunts are used to calculate the values of the shunt resistors. However, it has two disadvantages: Figure 3-5. - Ways of connecting internal meter shunts.
1. When the switch is moved from one shunt resistor to another, the shunt is momentarily removed from the meter. The line current then flows through the meter coil. Even a momentary surge of current could easily damage the coil. 2. The contact resistance (resistance between the blades of the switch when they are in contact) is in series with the shunt, but not with the meter coil. In shunts that must pass high currents, this contact resistance becomes an appreciable part of the total shunt resistance. Because the contact resistance is of a variable nature, the ammeter indication may not be accurate. The generally preferred method of range switching is shown in (figure 3-5, view B). Although only two ranges are shown, as many ranges as needed can be used. In this type of circuit, the contact resistance of the range-selector switch is external to the shunt and meter in each range position. The contact resistance in this case has no effect on the accuracy of the current measurement. Ammeter Connections When you are using ammeters, a primary rule of safety is that such current-measuring instruments must always be connected in series with a circuit, never in parallel with it. When an ammeter is connected across a constant-potential source of appreciable voltage, the low internal resistance of the meter bypasses the circuit resistance. This results in the application of the source voltage (or a good portion of it) directly to the meter terminals. The resulting excessive current burns up the meter coil and renders the meter useless until repaired. Q.11 In what manner are current-measuring instruments connected to a circuit? |
 
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