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CT extention to +30m using CAT 6 ethernet cable


Stu_D

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Hi all

I have a Deye 8Kw inverter located in my garage which is just over 30m from the main DB board. I want to place the CT at the main DB board so I can set up zero export to CT for non essential loads.

The twisted cable to the CT according to the manual can only be extended up to 15m, however I have heard that some people have had success extending up to 50m by using CAT 6 ethernet cable.

My question is firstly is this true? and secondly, how many pairs of twisted wires do I use. One person said only to use one pair and another person says he connected two pairs together for each pole. Which is the correct way to connect it?

Thanks in advance.

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57 minutes ago, Stu_D said:

Hi all

I have a Deye 8Kw inverter located in my garage which is just over 30m from the main DB board. I want to place the CT at the main DB board so I can set up zero export to CT for non essential loads.

The twisted cable to the CT according to the manual can only be extended up to 15m, however I have heard that some people have had success extending up to 50m by using CAT 6 ethernet cable.

My question is firstly is this true? and secondly, how many pairs of twisted wires do I use. One person said only to use one pair and another person says he connected two pairs together for each pole. Which is the correct way to connect it?

Thanks in advance.

We have  installed  CT for Sunsynk thats well over 40m. But it must be shielded (STP) or foil shielded (FTP), and ground one end if the shield only. 

One pair is enough, remember the sense current is a very small ratio to the mains current. Two pairs doesnt hurt though. The main issue to combat is noise, so the shield is important. 

If memory serves the burden resistor sits at the inverter side, which further helps noise immunity. 

 

 

Edited by BritishRacingGreen
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26 minutes ago, BritishRacingGreen said:

Two pairs doesnt hurt though.

Strictly speaking, at some point the capacitance of the twisted pair will become too much, so using 2 pairs will probably decrease the length by which you can extend. (and potentially confuse people into incorrectly using one pair per pole)

Edited by P1000
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18 minutes ago, Stu_D said:

So I will have to specify CAT 6 STP or FTP when I purchase the cable.

Yes. IIRC, CAT 7 is always STP (and the whole cable also has a shield), so if you struggle to find STP CAT6, try CAT7.

Edited by P1000
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A CT transformer has a current output.  It is critically important to place the termination resistor at the inverter side.  Your long cable will then be a low impedance current loop which will have excellent noise immunity.  Because the circuit is low impedance, the stray capacitance that @P1000 mentioned will have no effect, especially not at the low mains frequency of 50Hz.

If you where to place the termination resistor at the CT coil side, the picture would change completely.  The termination resistor converts the current into a measurable voltage.  Your cable would then carry a sense voltage with practically no current at all.  It would be a high impedance circuit that would be very prone to noise pickup.  Also, due to the high impedance, the stray capacitance could start to have an effect. 

I would stick my neck out and say that if you run as a current loop, your cable could be unshielded and it would be OK.  However, I agree with @BritishRacingGreen to rather use a shielded cable and play it safe.  It is important to connect the shield to ground.  I agree that this should be done on one side only.   You want the screen to only work electro-statically and want to prevent a current flowing through it.  Simplistically, the screen will act as an aerial for radio-frequencies and short these to ground.  The cables within this shield are then protected and not subjected to such radio waves. 

It is best to only use one twisted pair.  The twist in the cable makes sure that the two wires are tightly bound and any noise that might get picked up does so equally in both wires.  We refer to such noise as common mode noise that is easy for your system to ignore.  If the two wires have a distance between each other, they can be seen as a loop, into which noise could couple differentially and add to the signal.  Differential noise is problematic.

PS. I presume that your inverter would have the termination resistor mounted on a PCB within the inverter itself.  That is perfect.  Then you do not need to worry about it.

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I needed a small extension for mine too - albeit only a meter or so - and installer asked if I have some spare speaker cable lying around which I did. He said that he prefers using speaker cable rather than LAN cable (maybe he never used the correct lan cable previously). 

Edit: I also offered him LAN cable (have plenty), but he wanted speaker cable. 

Edited by Kalahari Cruiser
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The solution to your question is a simple one.

 

  1. Use a shielded cable and ensure that the shield is earthed at the same potential as the system.
  2. If I am right, the CT has a ratio of 2000:1, therefore the maximum current in the secondary coil can be calculated and combined with the electrical properties for the UTP cable you use, a simple calculation will show the power loss over the distance using multiple numbers of wires. If calculated correctly, secondary effects such as capacitance in the wire can be ignored.
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4 hours ago, Ignatius said:

The solution to your question is a simple one.

 

  1. Use a shielded cable and ensure that the shield is earthed at the same potential as the system.
  2. If I am right, the CT has a ratio of 2000:1, therefore the maximum current in the secondary coil can be calculated and combined with the electrical properties for the UTP cable you use, a simple calculation will show the power loss over the distance using multiple numbers of wires. If calculated correctly, secondary effects such as capacitance in the wire can be ignored.

UTP cable is unshielded so a bit confusing. Also What is the calculation you use?

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