3020-01097-0 Datasheet, PDF (2/4 Pages) C&D Technologies – 50mV and 100mV Base-Mounted DC Shunts

DC SHUNTS
50mV and 100mV PRECISION SHUNTS
4. Continuous-Operation Derating: If continuous operation is required,
a shunt must only be allowed to carry 2/3 of its maximum nameplate
amperage. The 2/3 derating factor provides an adequate safety margin
for convection cooled shunts operating in an ambient temperature of
+25°C. The following two sections contain additional derating that is
required when shunts will be exposed to higher operating temperatures
or pulsed (intermittent) currents.
5. Temperature Derating: If the shunt is to be operated at ambient
temperatures above +25°C, the maximum continuous current must be
further derated from the 2/3 value previously noted.
To find the maximum permissible continuous current (Ie) at an
elevated temperature (Te), one must first calculate the maximum
allowable power dissipation (Pe) at Te, using the following formulas:
Pe = 0.667Pa x [1-(Te-25°C)/100°C], and Ie =
Pe
R
Where
Ie = Maximum permissible continuous current at elevated
temperature Te
Pe = Maximum power dissipation at the elevated temperature Te
R = Shunt’s resistance (see selection guide for values)
Pa = 0.667 x shunt’s rated power at 25°C ambient
Te = Elevated temperature
Example: How much current can a 150 Amp 50mV shunt safely carry
at an ambient temperature of +100°C?
Te = +100°C
Pa = 0.667(150A x 0.050V) = 5.0 Watts
Pe = 5.0W[1-(100-25/100] = 5.0W x (1-0.75) = 1.25W
R = 0.00033 Ohms
Lastly, use the formula Ie =
Pe
R ^{to find the maximum derated}
continuous-current at Te = +100°C:
Ie =
1.25W
0.00033Ω
=
61.5A
6. Intermittent (Pulsed) Operation: Shunts that will not be exposed to
continuous operation at 2/3 rated current can be operated at levels
close to, or even above, their nameplate ratings for portions of short
periods of times (5 minutes maximum) at 25°C ambient.
The following formulas can be used to calculate the maximum pulsed-
current (Ipuls) for a given shunt:
Ipuls = Ppuls/R, and Ppuls = Pa/K1, and K1 = D
Where
Ipuls = Maximum pulsed current
Ppuls = Maximum pulsed power
Pa = 0.667 x shunt’s rated power at 25°C ambient
D = Ratio of ON TIME to ON TIME + OFF TIME
(period not to exceed 5 minutes)
K1 = Pulsed-operation rating factor (the square root of D)
R = Shunt’s resistance (see selection guide for values)
Example: An 800 Amp 50mV shunt will be operated for 15 seconds
each minute. What is the maximum current this shunt can carry
during the 15-second ON time?
D = 15/60 = 0.25 and K1 = 0.25 = 0.5
Pa = 0.667(800A x 0.050V) = 26.7 Watts
Ppuls = Pa/K1= 26.7W/0.5 = 53.4 Watts
Ipulsed =
53.4W
0.0000625Ω
= 924A
0-50mV
OUTPUT
–+
–
+
LOAD
+
–
DC SUPPLY
50mV SHUNT
LOAD CURRENT
POWER
GROUND
(0Vdc)
Figure 2. Typical Low-Side Shunt Connections
0-50mV
OUTPUT
–+
+
–
DC SUPPLY
50mV SHUNT
LOAD CURRENT
+
–
LOAD
POWER
GROUND
(0Vdc)
Figure 3. Typical High-Side Shunt Connections
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