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9/99
This impedance equation simplifies to the form in Figure 5 when all IED loads are identical. If the loads are not
identical, the general equation listed above must be used to calculate the load.
Z
total
The calculated load impedance for the architecture presented in Figure 5 is 333.33 ohms. In this example the IRIG B
receiver/converter must be capable of sending a three milli-amp TTL-level signal to a 333.33 ohm load. If the source
is not matched with the load impedance, IRIG B will not operate correctly.
The cable recommended to connect the IRIG B devices shall have the following characteristics:
Capacitance:
less than 40 pF per foot line to shield
Construction:
2-wire twisted pair shielded with PVC jacket
The maximum lead length of the entire relay is to be no more than 1000 feet. Cable types and vendors recommended
and supported by ABB to interconnect the IRIG B devices are:
BELDEN 9841, BELDEN YM29560, or equivalent
An example of the terminal to terminal daisy-chain interconnection of three units is illustrated in Figure 6.
Applications Notes
Load presented
to the IRIG B
=
Generator as per
example in Figure 1
Z
= 1/(N*(1/1000))
total
Z
= 1/(3*(1/1000))
total
Z
= 333.33 ohms.
total
Thus the Source must be capable of driving a 333.33 ohm load.
Figure 5. Load Impedance Calculation
website:www.abb.com/papd
ABB Distribution Protection Unit 1500R/2000R
1000 ohms = 1 unit load.
1000
1000
1000
Ohms
Ohms
Ohms
where N = number of DPU/GPU/TPU 2000/R Units.
13-5
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