Amp rating of current meters

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mbrooke

mbrooke

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Do amp meters have a current limitation?

Basically I want to monitor leakage current on a 3 wire system. A, B, C meters 0-2000 amps, Ground meter 0-30 amps, possible 0-5 amps.

Question is, what if I get a persistent ground fault over 30 amps? Will my meter burn up? Is what I have below even practical?

https://imgur.com/GkxsDXU
 
Staring at the diagram and making a WAG-
Since you're using CTs (2000:5?) the max actual current would be just when the CT goes into saturation. Where it matters is the meter sensing element- mechanical analog meter or burden resistor/volt-meter, and their own time/overload capacity. The Ground meter probably want's it's own CT (50:5?) which then can saturate and "protect" the meter. (I wonder if some of the relay manufacturers have papers on this.)

Past that, what's the source (grounded at all?) and do you only have those three leads available? If it's ungrounded, I can't see how the Ground meter would ever register a current since there's no path outside of those leads. OTOH if the source -is- grounded (assume wye for the moment), then the Ground meter could show an unbalance current (flowing back through the wye center). And, of course, if there is a SBJ it'l be a lot easier to put the CT on that.

It's also possible that I'm missing something obvious.
 
Assuming your CTs are 2000:5 & your 4 ammeters are 0-5A or read full scale at 5A input.

If you have 30A of imbalenced current, the 4th meter will see .075A. A digital meter might read this, but the analog type with a needle is barely going to move.
 
zbang said:
Staring at the diagram and making a WAG-
Since you're using CTs (2000:5?) the max actual current would be just when the CT goes into saturation. Where it matters is the meter sensing element- mechanical analog meter or burden resistor/volt-meter, and their own time/overload capacity. The Ground meter probably want's it's own CT (50:5?) which then can saturate and "protect" the meter. (I wonder if some of the relay manufacturers have papers on this.)

Past that, what's the source (grounded at all?) and do you only have those three leads available? If it's ungrounded, I can't see how the Ground meter would ever register a current since there's no path outside of those leads. OTOH if the source -is- grounded (assume wye for the moment), then the Ground meter could show an unbalance current (flowing back through the wye center). And, of course, if there is a SBJ it'l be a lot easier to put the CT on that.

It's also possible that I'm missing something obvious.
Click to expand...

Source is solidly grounded wye.

The noodle is grounded at the pad, so CT after it is out the question.
 
SG-1 said:
Assuming your CTs are 2000:5 & your 4 ammeters are 0-5A or read full scale at 5A input.

If you have 30A of imbalenced current, the 4th meter will see .075A. A digital meter might read this, but the analog type with a needle is barely going to move.
Click to expand...

Right. But 3 of the amp meters are 0-2000, the other one is 0-30 if not 0-5. Its possible for the 4th meter to over-shoot indefinitely with more than 30 amps on the primary of those CTs.
 
The important thing is the meters ES or end scale. Since you are using CTs all four of the meters would need a 5A End Scale. The scaling 0-2000 or 0-30 only aids the operator to see the current in the circuit. If the 4th meter needs more sensitivity an aux CT will be needed.

Can you give me better meter information. The fourth meter should peg out with 5A input, Just like the other three.
 
Basically this scheme, but with analog meters on the main switchgear:


There will not be an selector switches in my application just 4 amp meters, 3 volt meters and 3 indicator lights.
 
Typically these meters show full scale deflection at 5A (since that is the CT full output, under normal conditions), no matter what is written on the face. If your CTs are 2000:5 then the face must read 0-2000. When the CT is putting 2.5A out the meter reads 1000A. Can I see the sticker on the side. The ES or end scale is written there. You can order these meters with different ES.

For the meter to read properly the scale written on the face must match the CT & the meters ES must match the full CT secondary output.
 
SG-1 said:
Typically these meters show full scale deflection at 5A (since that is the CT full output, under normal conditions), no matter what is written on the face. If your CTs are 2000:5 then the face must read 0-2000. When the CT is putting 2.5A out the meter reads 1000A. Can I see the sticker on the side. The ES or end scale is written there. You can order these meters with different ES.

For the meter to read properly the scale written on the face must match the CT & the meters ES must match the full CT secondary output.
Click to expand...

Make sense now.

So... how easy would it be to ask for such a custom meter?
 
The meters should be easy to get, your problem is the sensitivity of the 4th meter. In the picture (post 7) above the first small line above zero is 50A.

You should consider inserting a small CT in the metering circuit for the 4th meter for greater sensitivity That would make the 4th meter different from the other 3.
 
An auxiliary CT would work. A window type could be used also. The ratios have to be worked out.

Most of the current must shunted around the meter coil which is made up of very small gauge wire & may only require 10-50mA for full scale deflection, like the coil in analog VOM (volt-ohm-meter). A shunt could be called a precision low ohm resistor.

This meter uses an external shunt. Panel meters have the shunts built in. https://www.amazon.com/DROK-4-5-100...qid=1597283078&sprefix=ammetr,aps,195&sr=8-15
 
What is a window type? You mean window type amp meter?

Thanks. Why the shunt? Is that only with the round ones? Do they make meters without shunts? I imagined the whole 5 amp things was to avoid shunts.
 
You'll have to do that research :D, but start with imagining that the windings are probably smaller than 30g copper and work from there. I know the usual D'Arsonval DC movements are measuring microamps (way back when- 100 µ-amp FS was a standard movement). It's about how many windings you can fit on what size core to get the desired flux in the physical space; at least with iron-vane movements, there aren't flexible connections like on moving-coil designs.

I suspect that the old Audel/Hawkins books have some cut-away drawings of interest, can't go look for them right now.
 
zbang said:
You'll have to do that research :D, but start with imagining that the windings are probably smaller than 30g copper and work from there. I know the usual D'Arsonval DC movements are measuring microamps (way back when- 100 µ-amp FS was a standard movement). It's about how many windings you can fit on what size core to get the desired flux in the physical space; at least with iron-vane movements, there aren't flexible connections like on moving-coil designs.

I suspect that the old Audel/Hawkins books have some cut-away drawings of interest, can't go look for them right now.
Click to expand...

Alright. What about rectifier type meters?
 
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