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Entry type: FAQ Entry ID: 109762375, Entry date: 11/26/2018
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# Using current transformers with other secondary currents than 1A or 5A

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Question:

Is it possible to connect CT's with other secondary currents than 1A or 5A?

Problem definition and solution suggestions:

PAC measuring devices with current transformer inputs are defined current signals designed:

SEM3 PAC1600 PAC3100 PAC2200 PAC3200  PAC3200T PAC4200PAC5200
0.1 A x
1 A  *1  x x x x x
5 A   x x x x x x x

*1) Can be parameterized in device. Measuring range resolution is still 5A.

Corresponding transformers are connected 1:1 and parametrized with their respective nominal values in the device. In general, converters with other output currents can be connected as long as they are within the permissible input limits of the device.

1. Measuring range settings

1.1 Example PAC3100 with 250/1A transformer

The meter calculates the measured current depending on the parameters:

IWire = (IMeter / IIn) * (CTPrim / CTSec)

IWire             : The current to be determined in the wire
IMeter           : The current measured by the PAC
IIn                  : Measuring range current input PAC
CTPrim          : Primary rated current of the transformer
CTSec           : Secondary rated current of the transformer

For regular use, CTSec = IIn and only the converter ratio must be set during meter configuration.

In this application, IIn is fix 5A and by a factor of 5 larger than CTsec.Without further adjustments, all values would be underestimated by this factor. However, this can be compensated for by a factor of CTPrim, increased by the same factor.

In this case, the right settings to PAC3100 are:

I-Primary            = CTPrim * 5 = 1250 A
I-Secondary       = IIn = 5 A

Note:  The maximum measurable current remains the primary rated current of the converter (CTPrim).

1.2 Example PAC3200 with 250/0,1A transformer

The current transformer with 0.1A secondary current intended for SEM3 measuring system can also be used on other measuring instruments, analogous to the first example. Due to even smaller secondary current, however, a measuring device that offers an input with a 1A measuring range is recommended.

In this application IIn is fix 5A and by a factor of 10 larger than CTSec. Without further adjustments, all values would be underestimated by this factor. However, this can be compensated for by increasing CTPrim by the same factor.

In this case, the right settings to PAC3200 are:

I-Primary           = CTPrim * 10 = 2500 A
I-Secondary       = IIn = 1 A

Note: The maximum measurable current remains the primary rated current of the converter (CTPrim).

2. Measuring range resolution

For the measuring range of the current inputs, information on the minimum and maximum measurable currents can be found in the technical data sheet of the devices.

Example PAC3200:

The percentage specification always refers to the set measuring range of the current input on the device (IIn) and differs accordingly in the absolute values:

IIn IInMin: 1%IInMax: 120%
1A0.01A1.2A
5A0.05A6.0A

If a connected current transformer does not use the full measuring range, the percentage limit of the minimum measurable current also changes.

2.1 Exampe PAC3100 with 250/1A transformer

The device works with a fixed 5A measuring range (IIn), of which the connected converter uses only 1A (CTSec). The minimum measurable values remains unchanged at 0.05A (IInMin).

This results in a minimally measurable transformer current of:
IInMin / CTSec = 0.05A / 1A = 0.05 means 5%

Based on the measured primary current:
250A * 0.05 = 12.5A

-> Only from 5% of the nominal current or 12.5A can be measured within the specified tolerances.

2.2 Example PAC3200 with 250/0,1A transformer

The device works with a fixed 1A measuring range (IIn), of which the connected converter uses only 0.1A (CTSec). The minimum measurable values remains unchanged at 0.01A (IInMin).

This results in a minimally measurable transformer current of:
IInMin / CTSec = 0.01A / 0.1A = 0.1 means 10%

Based on the measured primary current:
250A * 0.1 = 25A

-> Only from 10% of the nominal current or 25A can be measured within the specified tolerances.

-> The minimal measurable current shifts depending on the used measuring ranges.
In addition, the acutal accurary of the converter used is to be observed, which remains unchanged.

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