2: Measuring Total Wire Length; Four-Wire Slc Style 6 & 7 (Class A); 1: Measuring Loop Resistance - Johnson Controls IFC-200 Wiring Manual

Four-Wire SLC Style 6 & 7 (Class A)
In Figure 2.1, Branches A, B, and C all begin at the SLC terminal, even though Branch B is
T-tapped.
2.2.2 Measuring Total Wire Length
The total wire length of all combined branches of one SLC cannot exceed the limits set forth in
each system's instruction manual. Determine the total length in each SLC by summing all wire
segments. In Figure 2.1 above, the picture on the right shows an SLC with 3 branches. Figure 2.2
below shows the same SLC divided into segments. The total length of the SLC is determined by
adding the lengths of Segment 1 + Segment 2 + Segment 3 + Segment 4 + Segment 5. No segment
should be summed twice.
Branch A
Segment
One
2.3 Four-Wire SLC Style 6 & 7 (Class A)
2.3.1 Measuring Loop Resistance
The total DC resistance of the SLC pair exceed 50 ohms.
• 50 ohms for IFC-320, IFC2-640, IFC-640, LCM-320, LEM-320, JLIB-200A and JLIB-400.
• 40 ohms for IFC-200, IFC-300/400, and JLIB-200
Measure DC resistance as detailed and shown below:
1. Disconnect the SLC channel B (Out) and SLC channel A (Return) at the control panel.
2. Short the SLC at the last device and measure the resistance at SLC Out. Record resistance and
remove the short.
3. Short the SLC at the first device and measure the resistance at SLC return. Record resistance
and remove the short.
The maximum DC resistance of the SLC is the higher of 2 and 3.
Johnson Controls, Inc. SLC Wiring Manual — P/N 51870:G 04/23/2009
SLC Out
Branch
Short Point
Figure 2.1 Measuring DC Resistance of a Two-Wire SLC
Branch B
Segment
Five
Segment
Two
Figure 2.2 Measuring the Total Wire Length of a Two-wire SLC
Branch A Branch B
SLC Terminal
Block
B+ B–
Segment
Three
SLC Terminal Block
Wiring Requirements
Branch C
Branch C
Segment
Four
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