Product Engineering FAQ

DC Ammeter Shunts

Can I continuously operate a shunt at its full rated current?

Shunts were originally designed to provide a millivolt output to an analog (tin wiggler) meter. Since these meters were most accurate at approximately mid-scale a typical application would specify a meter which would normally be at mid-scale (250 amp circuit, 500 amp scaled meter). Therefore, the shunts were and are designed for a maximum of 2/3 of their rated current. However, shunts can be run in excess of 2/3 their rated current provided the shunt temperature does not exceed 80 degrees C.

What will happen to a shunt if it is subjected to an overload?

A. A shunt by definition is a resister. As current is passed through the shunt heat is created. The amount of heat is dependent on a number of variables over which we have no control, i.e. amount of overload, ambient temperature, duration, cooling capabilities, etc. If you have concerns with overload please contact us with the specifics of your application and we will attempt to provide you with a shunt that will suit your needs.

What is the temperature rise on a shunt?

The actual temperature rise of any given shunt is again influenced by the same variables outlined above. It is recommended that equipment be designed with provisions for adequate cooling. A shunt should maintain an operating temperature of less than 80 degrees C. (measured at the center of the manganin strips). Operating temperatures above 80 degrees C may cause the resistance to drift, resulting in reduced accuracy.

How long does it for the shunt to heat up?

Typically a shunt will reach 80% of its operating temperature within 2 minutes.

Where should temperature measurements be taken?

Measurements should be taken as close to the center of the manganin blades (resistance material) as possible. In the case of multiple blade shunts, the measurement should be taken between the blades as close to the center as possible.

What is the operating temperature range?

The operating temperature is dictated by the temperature coefficient of the manganin. We recommend for optimum accuracy that the shunt be between 20-80 degrees C. If the temperature exceeds 80 degrees C. the resistance may drift. If the temperature reaches 140 degrees C the resistance will be permanently altered.

How does Empro calibrate shunts?

Our specialized equipment measures the actual resistance of the shunt.

What is the resistance of a particular shunt?

The resistance of the shunt is based on the amperage and millivolt ratings of the shunt. The nominal resistance is calculated using Ohms law. (voltage drop divided by amperage = resistance)

What is the accuracy of your shunts?

Empro’s shunts are normally calibrated to an accuracy of +/- 0.25%. For a nominal charge we can calibrate to +/- 0.10 %.

Are your shunts certified traceable to N.I.S.T.?

Yes. All of our shunts are calibrated on equipment with current certifications traceable to N.I.S.T. As a matter of practice, a standard certification is contained on your packing slip. If you require additional information or special certifications we will be happy to provide them at an additional charge.

Does Empro provide recalibration and recertification services?

Yes. Many of our customers return their shunts to us on a scheduled basis for recertification.

How frequently should a shunt be returned for recalibration and recertification?

To maintain optimum accuracy we recommend that shunts be checked and recertified annually. At your request, we can place a sticker on the shunt with the next calibration date noted.

Will you recertify shunts made by another manufacturer?

Yes. We frequently certify other brands of shunts. However, we do not rebuild or repair shunts made by anyone else.

Are shunts lineal with respect to the millivolt output across their operating range?

Yes, provided the shunt is kept within its recommended operating temperature range.

I’m looking at your catalog and find quite a few types with the same electrical characteristics. Why, and what is the difference?

The primary reason for different types or styles is to accommodate various methods of installation and mounting.

Specifically, what is the difference between the Type F and Type WT?

The WT is much larger and heavier than the F. It should handle overloads without over heating and damage to accuracy

What is the best way to cool a shunt?

There are a number of ways to reduce the operating temperature of a shunt. Such as:

  • Forced air such as using a fan to blow air across the shunt
  • Increasing the physical size of the shunt
  • Adding heat sinks to the blocks and/or busbar
  • Using a water cooled shunt which we can design

The best way is usually dictated by the application and location of the shunt.

I’m designing a power supply that will require a shunt. What should I be concerned with?

There are many considerations when designing the physical layout of a piece of equipment. Following is a partial list of things that through 60 years of experience we have found create the most problems

  • Cooling, as discussed above, a shunt generates heat therefore be sure to have adequate airflow around the shunt and that the conductors are substantial enough to conduct the heat away.
  • Whenever possible mount the shunt so that the resistance blades are in a vertical plane to allow the heat to rise out of and away from the shunt. Sometimes, a cooling fan may be necessary.
  • Where possible, all shunts should be mounted on the grounded side of the circuit. Panel mounted shunts with insulated bases must be mounted on the grounded side of circuits above 750 volts.

Your website indicates you design and make custom shunts. What if I do not need a special design but would like a standard shunt with some minor modifications, such as a different hole size or pattern?

We can modify any standard shunt to meet a specific requirement. We will work with you to accommodate your application.