Understanding Shunt Selection for Accurate Current Measurement in DC Systems

[Junior01]

I need to measure current in DC system. I have circuit breaker (125A), so 150A shunt is selected. On that shunt ammeter is connected. But that ammeter has 60mV, scale 0-150A. What does this 60mV means on ammeter? Is it correct?

 

[Baarken]

See this: http://www.exalto.com/en-us/vdo-vl-ammeter-shunt-150a-60mv-single-a2c59514047
From the link:

A shunt is a resistor that causes a small voltage drop. This voltage drop is mVolt is proportional with the current in ampère that goes through the resistor

So if you measure 0 V that means 0 A and if you measure 60 mV that means you have 150 A.

 

[Junior01]

Thank you, but what is measuring range then. The instruments (Deif) has measuring range 1mA...40A, but on my instrument scale is 0-150A? What is different between scale and mesuring range?

 

[Baarken]

Take a look at this video and see if it is of any help.

 

[Junior01]

Thank you, but I really have no idea how to select proper ammeter.
Here is a datasheet od Deif instruments, on page 4. I have DC instruments and there is measuring range from 1mA-50A, there is no 150A. So, how my instrument will show on scale 150A, if measuring range is up to 50A.

https://www.deif.com/products/eq#documentation

 

[cnh1995]

So, how my instrument will show on scale 150A, if measuring range is up to 50A.

When you select a shunt, there is a multiplying factor for that shunt which gives you the actual value of the current. If your ammeter's full-scale deflection is 50A and you have extended its range to 150A (using shunt), multiplying factor would be 3. This means when your meter is showing 50A (full scale deflection), the actual current is 150A.

 

[Junior01]

OK, here is a datasheet of deif's shunt 150A/60mV. Which ammeter or voltmeter (I'm now confused, select ammeter or voltmeter? ) can I select from previous datasheet (or picture) to measure proper current 150A?

 

[Baarken]

You need an ammeter.

What I think is confusing for you is that the ammeter reads a voltage drop across the shunt, which is proportional to the current through the shunt, which is seen from Ohm's law.

Look at the circuit below, the resistance is your shunt device.

If you have a ammeter with a scale of 0-50 A, but your shunt can measure 0-150 A then do as described in post #6, but make sure the mV rating is the same for your ammeter and shunt (60 mV).

 

[Junior01]

Thank you.
But, my problem is that I don't know what measuring range means (attached datasheet or picture in post #7). There is 1mA-50A, it's probably not scale, because scale is changable?

 

[Baarken]

Aha, I think that the measuring ranges from datasheet in post #7 is the range the display can measure without having to use an external shunt. If you wish to go above, you need an external shunt.

See this link:
http://www.wpa.ie/Products/ProcessIndicators/AnalogMetersIndicators.aspx

They write:

Direct measuring ranges:
I = 40µA...25A,
U = 60mV...1000V,
Measuring ranges with external shunts: I = 1A...15kA

Maybe @jim hardy hardy can help us to put an end to this.

 

[jim hardy]

c'mon guys, this isn't an electrical problem it's a catalog reading problem.

You know how an ammeter with a shunt works.
So you need a shunt that measures 150 amps. Looks like those come in two flavors - 60 mv and 150 mv. SO you need a meter that shows full scale at whichever millivolts you pick for your shunt.

60 mv looks like a good choice

Delf's ordering looks painless enough

.............................So pick a meter the size you want. And note you can choose 90 degree or 270 degree needle travel...

if you want the smallest size meter i'd guess it's the second one

bigger sizes are further down the table.

figure out how to specify 60mv/1ma sensitivity and scale "0 - 150 Amps"

1 ma input(measuring range) and specify scale --- 150 DC amps ?
Observe note 4 on the catalog page - you'll want to mount it within one meter of the shunt or else order special longer leads...
if that creates a problem it's worth a call to their sales rep.

Focus guys, focus.