# Indirect current measurement

#### minhtam

Joined Aug 28, 2023
7
Hi guys,

I have a question and I hope you guys can help me answer it.
I have a circuit below

I don't know how current is measured and how current transformers wiring like this is possible? (A1 is the device that measures current and power and activates alarms if a value exceeds the limit. )

Minh Tam

#### ronsimpson

Joined Oct 7, 2019
2,894
In your picture it matters little where the current transformer goes. It now is above the switches but could be below.
A CT is isolated (voltage) from the line. In your picture one of the secondary leads connect back to the line voltage and it does not have to. I think the picture is wrong. I will redraw it and post it soon.

1) I moved B3 and B1 so the CT for line 3 is on the same phase. Voltage and current are on the same input on the box "A1".
2) Inside A1 the red circles are voltage meters that measure from line to Neutral.
3) The blue circles are current meters. The current meter inputs could go from A1 directly to the CT.

Here I redrew B3 (the third CT) to show a direct connection from CT to the current inputs on A1. In this case A1 needs 3 inputs. Two for the CT and one for the voltage. In your example there are only two inputs. Works either way.

Last edited:

#### minhtam

Joined Aug 28, 2023
7
In your picture it matters little where the current transformer goes. It now is above the switches but could be below.
A CT is isolated (voltage) from the line. In your picture one of the secondary leads connect back to the line voltage and it does not have to. I think the picture is wrong. I will redraw it and post it soon.

1) I moved B3 and B1 so the CT for line 3 is on the same phase. Voltage and current are on the same input on the box "A1".
2) Inside A1 the red circles are voltage meters that measure from line to Neutral.
3) The blue circles are current meters. The current meter inputs could go from A1 directly to the CT.
View attachment 301525
Here I redrew B3 (the third CT) to show a direct connection from CT to the current inputs on A1. In this case A1 needs 3 inputs. Two for the CT and one for the voltage. In your example there are only two inputs. Works either way.

View attachment 301526
In your picture it matters little where the current transformer goes. It now is above the switches but could be below.
A CT is isolated (voltage) from the line. In your picture one of the secondary leads connect back to the line voltage and it does not have to. I think the picture is wrong. I will redraw it and post it soon.

1) I moved B3 and B1 so the CT for line 3 is on the same phase. Voltage and current are on the same input on the box "A1".
2) Inside A1 the red circles are voltage meters that measure from line to Neutral.
3) The blue circles are current meters. The current meter inputs could go from A1 directly to the CT.
View attachment 301525
Here I redrew B3 (the third CT) to show a direct connection from CT to the current inputs on A1. In this case A1 needs 3 inputs. Two for the CT and one for the voltage. In your example there are only two inputs. Works either way.

View attachment 301526
Hi ronsimpson. I really appreciate your reply!. The thing that I dont understand is that how they can connect the line directly to the secondary side of the CT? I thought the secondary side induces current and no contact to the primary side. But in this case, the second side touches the line. Could you please explain what is the principle behind it?

#### Ya’akov

Joined Jan 27, 2019
8,973
Welcome to AAC.

This appears to be coursework. Is that the case?

#### ronsimpson

Joined Oct 7, 2019
2,894
Following the red wires, the volt meter looks at Line_1 and Neutral.
The small wires from the CT need to go to the Amp meter. They could use the blue wires to connect directly to the meter. To save wires, one wire (red) connects the CT to the A meter. The other wire connects to L1 and then jumps to the A meter at the meter.

Sorry for the bad picture. The A meter could be wired in red or in blue and the results are the same.

#### schmitt trigger

Joined Jul 12, 2010
843
I am not going to mince words: whoever drew that “schematic”, should be kept away from anything powered from anything larger than an AAA battery.
Least he harms himself and destroys equipment.

#### minhtam

Joined Aug 28, 2023
7
Following the red wires, the volt meter looks at Line_1 and Neutral.
The small wires from the CT need to go to the Amp meter. They could use the blue wires to connect directly to the meter. To save wires, one wire (red) connects the CT to the A meter. The other wire connects to L1 and then jumps to the A meter at the meter.

Sorry for the bad picture. The A meter could be wired in red or in blue and the results are the same.

View attachment 301600

#### minhtam

Joined Aug 28, 2023
7
Welcome to AAC.

This appears to be coursework. Is that the case?
No, it's from a real drawing.

#### Ya’akov

Joined Jan 27, 2019
8,973
No, it's from a real drawing.
So you are not working on a coursework-related project or assignment by answering this question?

#### minhtam

Joined Aug 28, 2023
7
So you are not working on a coursework-related project or assignment by answering this question?
I'm learning how to read electrical drawing from a real drawing and the way they wired CTs confused me. That's why I asked you guys about it.

#### Ya’akov

Joined Jan 27, 2019
8,973
I'm learning how to read electrical drawing from a real drawing and the way they wired CTs confused me. That's why I asked you guys about it.
OK. The reason I asked is the rules here on AAC require all coursework related questions to be posted in the Homework Help forum so they can be treated properly. Members are not allowed to provide direct answers to coursework questions, only guidance based on the student's own work.

There's nothing wrong with learning on your own, and for that sort of question where there is no direct credit as a result it's just like anyone else—we are all students, at least if we are doing things right. It's just that some of us get graded.

Welcome to AAC, good to have you with us.