DC Ammeter Shunts
General Information
CSI shunts are designed for use with 50 or 100 millivolt
measuring instruments, calibrated in terms of the ampere
ratings of the shunt. The accuracy of our shunts is better
than ±1/4% of the rated value and the temperature
coefficient is ±0.000015.
It is important that the resistance be constant under
different temperature conditions. It is usually assumed that
the maximum temperature will be about 80° C and the
usual range is 40°– 60° C.
For continuous operation, it is recommended that shunts
are not run at more than two-thirds (2/3) the rated current
under normal conditions of use as per IEEE standards.
If the shunt is used in an AC or pulse current environment
make sure that the highest pulse current does not exceed
the recommended two-thirds, the rated current for
continuous operation. In this case the average millivolt
reading using a rms meter will be proportional with the
duty cycle of the AC current.
Installation
By definition, a shunt is a resistor, and will generate heat
with passage of current. Because of this, the resistance
blades of the shunt should be mounted in a vertical
position to promote free convectional flow of air. Where
this mounting position is impractical and in installations
where the shunt is in a confined location forced air cooling
should be provided.
Under no condition should the manganin shunt strip be
allowed to surpass 145°C, as this will cause a
permanent change in resistance.
Shunts should be installed to protect them from damage
by thermal expansion forces in the connecting bus bars or
by short circuit forces. It should be recognized that shunts
are inherently weaker than their current connections and
that special flexibility may be needed at times.
Where possible all shunts should be connected in the
grounded side of the line. Portable type shunts, with an
insulating base, when used on circuits above 750 volts
should be mounted in the grounded side of the circuit.
Where more than one bus bar is to be connected to each
end of a single terminal shunt, these bars should be
distributed as equally as possible on each side of the
shunt terminals.
Leads
In some applications it is not feasible to mount the shunt
close enough to the instrument to permit the use of the
standard lead lengths. If longer lead lengths are necessary,
the additional drop in the leads must be taken into
consideration when ordering analog instruments.
Characteristics of Manganin Shunt Cold Formed – Stress Relieved
% CHANGE OF
RESISTANCE
+ 0.02
0
- 0.02
- 0.04
- 0.06
- 0.08
- 0.10
TEMP. °C
10
20
30
40
50
60
70
Temperature coefficient of resistance: ± 15ppm/°C.
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80
90
100
DC Ammeter Shunts
Shunt Selection Guide
Definition: Instrument Shunt is a particular type of resistor
designed to be connected in parallel with the measuring
device to extend the current range beyond some particular
value for which the instrument is already complete.
Any shunt by definition is a resistor and will generate heat
with the passage of current. Shunt ratings are established
by finding the power required to achieve a specified manganin temperature rise in free air at certain predetermined
conditions.
When operating conditions are significantly different from
the rating conditions, the shunt should be de-rated or
up-rated accordingly to keep the manganin temperature
within reasonable limits in order to prevent premature
failure, reduced reliability, reduced rated accuracy or
causing permanent change in resistance.
CSI’s standard shunts are rated to meet IEEE Standard
Requirements for Direct Current Instrument Shunt. IEEE 6.2
Current Ratings:
Selection of ratings should be based on operation at a
normal current of 2/3 the rated value.
Ratings are based on mountings in accordance with
Reference Test Conditions:
• Reference Temperature: 25°C (±2°C)
• Position: Current terminals and resistance blades,
vertical, allowing free air circulation
• Connections: Tightly bolted to current bus using all
terminal slots and specified depth of blade insertion
– or tightly bolted to lugs with cables or proper
cross-section
Since a shunt is essentially part of a bus-bar assembly, it
depends on the structure to conduct away a major portion
of the heat generated within the shunt and then to radiate
this heat. It is important, therefore, that these connecting
buses be of ample size and properly attached to the shunt.
Design Considerations
When trying to find the correct shunt rating to specify for
a given application, there are four things to take into
account:
A) Duty – continuous, intermittent or surge
B) Ambient – elevated temperature or high altitudes
Although each of these conditions will be considered
separately, any combination of them may exist in a specific
case. When this happens it will be necessary to use
more than one of the following sections to arrive at the
correct rating.
C) Temperature rise – normal or limited
D) Environment – grouped, enclosed, forced air
E) Combinations of A through D
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DC Ammeter Shunts
Duty
For duty which is less than continuous, a shunt need not
be rated as high as a straight calculation based on
resistance and voltage or current would indicate.
If, for instance the shunt is repeatedly energized for a
short period (not to exceed 5 minutes)… a conservative
rating may be obtained as follows:
K1 = =++
D
Ppuls = Pmax / K1
Pmax = Prated x 0.66
Pmax = maximum continuous power
Prated = catalog rated power
Ppuls = maximum pulsating power
For surge and pulse service a different approach must be
used. In these cases it is necessary to provide enough
thermal mass to absorb the energy to be dissipated and
be sure that the construction of the shunt is adequate
to accommodate the peak power. Therefore, it is recommended that a complete description of the pulse/surge
wave shape or circuit constants and operating conditions
be submitted to the CSI Engineering Department for the
selection of a suitable shunt.
where D is the ratio of ON time to the total period.
Example: A 800 Amp 500mV shunt is energized for 15 seconds of each minute.
++
++
++
++
++
++
++
+ = 0.5 thus,
D = 15 / 60 = 0.25 and K1 = =+++
0.25
Prated = 800 x 0.05 = 40 watts
Pmax = Prated x 0.66 = 40 x 0.66 = 26.4 watts
Ppuls = Pmax / K1 = 26.40 / 0.5 = 52.8 watts
therefore this shunt can be loaded up to 910 Amps.
Ambient
If the ambient temperature of the area in which the shunt
is to operate exceeds the ambient Reference Test
Conditions, a de-rating factor must be applied to prevent
the manganin temperature from going above the safe limit
of 125°C. The following formula may be used to find the
de-rating power.
Pout = Pmax x (1-T/A)
Pmax = Prated x 0.66
where T is the difference between the rated and operating
ambient temperatures, A is the rise allowed above the
rated ambient and Pmax is the Pout at 25°C.
Example: If a 150 Amp 50 mV shunt is to operate in a 100°C ambient, what is the maximum load?
100°C is 75 degrees above the ambient, so T = 75°C, and the rated rise A is 100°C.
Therefore: Pout = Pmax x (1-75 / 100) = Pmax x 0.25 Pmax = 0.66 x Prated = 0.66 x (150 x 0.05) = 4.95 watts
Pout for an ambient of 100°C is Pout = 4.95 x 0.25 = 1.23 watts,
60 Amps will be the maximum load.
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DC Ammeter Shunts
Temperature Rise
In many cases it is desirable to limit the temperature rise
of a shunt for increased accuracy and stability or to
protect other equipment near it. A satisfactory de-rating
factor “K2” may be obtained from:
K2 = 1 - t / 125
where “t” is the difference between the rated and the newly
imposed rise limit.
Example: Suppose it is necessary to limit the temperature rise of a shunt type LC800 Amp 50 mV to 75°C.
What is the maximum safe load?
t = 125 - 75 = 50 K2 = 1 - 50 / 125 = 0.6
Prated x 0.66 = 26.4 watts = Pmax at 25°C
K2 x Pmax = 26.6 x 0.6 = 15.84 watts;
therefore 510 Amps represents the maximum safe load.
Environment
1. Space requirement
Any shunt by definition is a resistor and hence will
generate heat with passage of current. Shunts should
be mounted in a location where there is free circulation
of air to carry away the heat generated. Shunt blades
should be mounted in a vertical position to promote
free airflow. If there is not enough natural convection,
forced air cooling should be applied.
2. Enclosures
Shunts are frequently mounted in enclosures for
protection from the surroundings or to safeguard
operators and personnel. If the unvented enclosure is
large in proportion to the amount of energy to be
dissipated, there is no need for concern. If the power
density is greater than 1/4 watt per square inch of
enclosure surface, provision must be made for cooling
air using vents or fans.
Note: Call CSI Engineering Department for mathematical formulas and examples regarding Venting, Force air cooling,
Grouping, Altitude and Combinations.
Conclusion
The foregoing discussions are intended as a guide to
the design engineer in the selection of the correct shunt
for application and should be adequate under normal
circumstances. Since it is often difficult to determine what
the shunt will actually “see” in service, it is wise to use
a conservative approach in all cases.
If unusual conditions exist or if the engineer is unsure of
the application, CSI Engineering Department is always
ready to help in the selection of shunt for the case at hand.
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Custom Applications
Give us your specs and we’ll give you a solution for your application.
We invite your inquiry for special shunts. See Questionnaire on our Products & Services page.
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