Description
A shunt is a resistor used in parallel with the meter coil to by-pass a major part of the current in a circuit and to enable measurement of higher currents without over-loading the meter movement. Shunts may be internal or external. Meters commonly used with external shunts are generally rated 50 millivolts (sometimes 100 MV). The external shunt is rated to have a drop of 50 millivolts (or 100 MV) across it when rated DC current flows through the shunt. Shunt blades are usually of manganin or other material having a low temperature coefficient.

The shunts shown on the back of this bulletin are light weight military shunts per MIL-S-61 (aircraft shunts formerly listed under AN3200 now fall under MIL-S-61).

Accuracy
Unless otherwise specified, Sigmation shunts are calibrated to a nominal accuracy of 0.5% (using .065 ohms leads). MIL-S-61 requires 0.6%, and MIL-I-1361 requires 0.25% accuracy.

How To Specify
There are several things to consider in determining the proper shunts to use. For example, in very low ranges, such as a 50MA, 50MV shunt, the meter load can appreciably affect accuracy. Therefore, one must think of the amount of current through the meter as well as through the shunt. In this instance, if the meter sensitivity is 5MA, 10% of the current flows through the meter and 90% (or 45MA) flows through the shunt. Therefore, in low current ranges, it is essential to relate the resistance of the meter to that of the shunt. In reality, instead of being a 50MA shunt, this shunt would be calibrated to produce 50MV with 45MA flowing through it (for it would be 1.11 ohms instead of 1 ohm), a difference of 11%. Above 5 amperes, a 5MA, 50MV movement represents less than 0.1% burden on the shunt.

The standard resistance of shunt leads should normally be 0.065 ohm (for total length of both leads). Therefore, the length and size of lead must be taken into consideration, if greatest accuracy is to be maintained. Most 50MV meters are 5MA, 10 ohms, temperature compensated. Therefore, 0.065 ohm represents a 0.65% factor, which must be allowed for in calibrating each shunt. Also, the customer should plan to use the equivalent amount of resistance in his circuit to his meter, by proper selection of wire size; otherwise, he should specify that the shunt must be specially calibrated for the particular resistance in his leads.