Current Shunt. DC Current Shunt. DC Shunt. Direct Current Instrument Shunt. Shunt Resistor. Current Limiter. Ammeter Shunt. Whatever you may call them, current shunts are extremely useful and can be used in many applications. But what is a current shunt?

Ram Meter Inc Current Shunts

In electronics, a shunt is a device which allows electrical current to pass around another point in the circuit by creating a low resistance path. The origin of the term is in the verb "to shunt" meaning to turn away or follow a different path. In the metering industry, current shunts are used to extend the range of ammeters whenever the measured current is too large to be passed through the indicating instrument (the ammeter), typically currents over 50 amperes. The shunt, a manganin resistor of accurately known resistance, is placed in series with the load so that all of the current to be measured will flow through it. Therefore, the shunt is a sort of diverter which is used to "shunt" the current around the indicating instrument. The voltage drop across the shunt is proportional to the current flowing through it and since its resistance is known, a millivolt meter may be connected across the shunt and can then be scaled to directly display the current value.

Current shunts are not only used to extend the range of ammeters, but can also be used where low current must be applied to a metering panel,or even where the metering panel is quite a distance from the circuit carrying the current to be measured and it would not be feasible to run large cables or bus bars to the panel or switchboard.

Ram Meter Inc. current shunts are rated by maximum current and the voltage drop at that current. For example, a 500 amp, 50 mV shunt would have a resistance of 0.10 milliohms, a maximum allowable current of 500 amps and at that current the voltage drop would be 50 millivolts. Ram Meter Inc. shunts are standardized to deliver either 50 mV or 100 mV when operating at their full rated current, and most ammeters consist of a shunt and a voltmeter with full-scale deflections of 50 mV or 100 mV .

However, we also recognize that there are times when other millivolt outputs are desired or other physical sizes or configurations are more suitable for an application. In these cases, please feel free to CONTACT US or scroll to the bottom of this page for our "Special Shunt Requirement Form"

SHUNT DESIGN

Since the shunt is a resistor which operates in a varying temperature environment and generates heats whenever current flows through it, the shunt resistance must be designed with a special metal alloy which has, essentially, a constant resistance value through its operating range. In addition to using a precision resistance alloy with a negligible temperature coefficient of resistance, the alloy must have a low resistivity in order to keep the self-heating to a minimum and it must also have a low thermal E.M.F. against copper. All Ram Meter Inc. shunts are designed using this special shunt manganin alloy, which meets all of the above criteria.

Ram Meter Inc Current Shunts

Another design consideration is the size. The size of the current shunt relates to self-heating as well as its cooling characteristics to dissipate the heat generated by the resistance alloy. Ram Meter Inc. shunts have been optimally designed for size based on these various factors. Generally, the higher current shunts (bus-bar type shunts) are designed with large end blocks (current terminals) for switchboard use so that they can be installed in the bus or connection bar structure of the circuit where the current is to be measured.. The lower current shunts (portable types) are designed with an insulating base and are intended to be mounted to any flat surface. These lower current shunts may also be used for switchboard applications where the current is relatively low and connection bars are not used.

All Ram Meter Inc. commercial shunts are designed in accordance with IEEE STD 316-1971. However, we also offer 3 series of shunts (the 20M, 21M & 22M series) which are designed to meet Military spec# AA55524. Check out our Military to Commercial Part# Conversion Chart for assistance in crossing your Military Stock #s over to our Ram Meter Inc. Part #s.

AC APPLICATIONS

Ram Meter Inc. direct current (DC) shunts may be used in alternating current (AC) applications with the following understanding: shunts have a very small inductance and exhibit a negligible effect (AC impedance) up to 1 kilohertz, and can be used up to 5 kilohertz where a slightly noticeable effect (AC impedance) takes place. Therefore, we recommend investigating the use of a conventional direct current (DC) shunt in AC applications to establish whether or not it is suitable for the application.

SHUNT SELECTION

Selection of current ratings should be based on a normal operating current of 2/3 the rated value of the shunt.

ACCURACIES

Adjustment Accuracy

Shunts are adjusted to within ±0.25% nominal value with an applied current that will not raise the temperature of the terminal blocks more than 10°C with an ambient room temperature of 25°C (77°F) ±2°C.

Rated Accuracy

Standard Shunts up to 1000 Amps have an accuracy of ±0.33%
Standard Shunts 1000 Amps and ABOVE have an accuracy of ±0.5%
Lightweight Shunts (20M, 21M & 22M series) have an accuracy of ±0.6%

These allow for self-heating variations at the higher current values and variance in connections.

For all "Custom Shunt" requests, fill out the following form and return it via Fax or E-mail.

Ram Meter Inc. Special Shunt Requirement Form (PDF)

Ram Meter Inc. Special Shunt Requirement Form (WORD document)