Shunt Resistors

[RF Lighting]

I've just been looking at some panel mount ammeters.

All the ones I have previously come across work by having the line conductor you wish to measure passing through a CT.

I have just seen some which use a shunt resistor to presumably do the same job as a CT.

Here is an example for those who don't know what they are.

How would you connect this? I presume the line conductor needs to be lugged to each of the big bolts, and you connect the meter across the two smaller screws? Would this be correct?

I think I'll stick with CTs, as they seem much easier to me, but I was just curious to how you would use them, and if anyone knows how they work, that would be smashing

 

[bernardgreen]

I presume the line conductor needs to be lugged to each of the big bolts, and you connect the meter across the two smaller screws? Would this be correct?

Yes that is correct.


Modern shunt amp meters ( with digital displays ) work by measuring the voltage created by the current to be measured when is passing through the known resistance of the shunt.

If the display is a moving coil analogue meter then the current to be measured is divided between the shunt and the meter in the inverse ratio of their resistances.


The advantage of shunts over current transformers is that shunts can measure DC and AC where a CT can only measure AC and generally measurements with a shunt are more accurate.

There are "current transformers" that measure DC but they are not current transformers but a sensor that can measure the magnetic field around the wire.[/quote]

 

[RF Lighting]

Does the shunt have any effect on the supply, or is it a completely passive device?

I'm wondering if I might be better off with shunts. They look a lot smaller than CT's, so would take up less room in the panel.

Do they generate any heat when there is a bit of load on them? The maximum is likely to be around 50A per phase, or does it depend on the size of the shunt?

Thanks.

 

[bernardgreen]

The shunt introduces a slight resistance in the circuit. Without knowing the specification of the actual meter it is impossible to say what value the resistance of the shunt will be.

But as an example for meauring 0 to 50 amps with a meter that takes 0.0 to 0.5 volts ( at 50 amps ) the resistance would be 0.01 ohms R = 0.5 / 50

Heating would be 25 watts ( W = V I = 0.5 x 50 ) at 50 amps

With modern digital displays the voltage at full current could be as low as 0.05 volt R = 0.001 Ohms and heating at 50 amps 2.5 watts

 

[ericmark]

I've just been looking at some panel mount ammeters.

All the ones I have previously come across work by having the line conductor you wish to measure passing through a CT.

This is the normal method used with high current AC

I have just seen some which use a shunt resistor to presumably do the same job as a CT.

This is the method normally used with DC

Here is an example for those who don't know what they are.

It does say DC

How would you connect this? I presume the line conductor needs to be lugged to each of the big bolts, and you connect the meter across the two smaller screws? Would this be correct?

Yes

I think I'll stick with CTs, as they seem much easier to me, but I was just curious to how you would use them, and if anyone knows how they work, that would be smashing

You can't use a CT with DC either the Hall effect or shunt is only way and using the Hall effect it is very expensive.

I will guess you missed the DC label?

 

[Spark123]

Building a distro Rob?

 

[ericmark]

A PS although the shunt resistor in your link looks smaller it would be for low voltage and the types used for mains voltage are as big as a CT coil as surrounded with a protective shield with loads of air space for cooling.

 

[RF Lighting]

Thanks for the info chaps.

It looks like I'm best off sticking with tried and tested CTs then.

It's not quite a distro I'm building. I'm just doing some research for a possible future project