GE 469 Instruction Manual page 30

2.2 ELECTRICAL
Note that an error is produced on each RTD equal to the voltage drop across the jumper on the RTD return. This error
increases with each successive RTD added.
V = V
RTD 1 RTD 1
V = V + V
RTD 2 RTD2
V = V + V
RTD3 RTD3
This error is directly dependent on the length and gauge of the wire used for the jumpers and any error introduced by a poor
connection. For RTD types other than 10 Ω Copper, the error introduced by the jumpers is negligible. Although this RTD
2
wiring technique reduces the cost of wiring, the following disadvantages must be noted:
1. There will be an error in temperature readings due to lead and connection resistances. This technique is NOT recom-
mended for 10 Ω Copper RTDs.
2. If the RTD Return lead to the 469 or any of the jumpers break, all RTDs from the point of the break will read open.
3. If the Compensation lead or any of the jumpers break, all RTDs from the point of the break will function without any
lead compensation.
c) TWO-WIRE RTD LEAD COMPENSATION
An example of how to add lead compensation to a two wire RTD may is shown in the figure below.
The compensation lead L2 is added to compensate for Hot (L1) and Return (L3), assuming they are all of equal length and
gauge. To compensate for leads RL1 and RL2, a resistor equal to the resistance of RL1 or RL2 could be added to the com-
pensation lead, though in many cases this is unnecessary.
d) RTD GROUNDING
Grounding of one lead of the RTDs is done at either the 469 or at the motor. Grounding should not be done in both places
as it could cause a circulating current. Only RTD Return leads may be grounded. When grounding at the 469, only one
Return lead need be grounded as they are hard-wired together internally. No error is introduced into the RTD reading by
grounding in this manner.
If the RTD Return leads are tied together and grounded at the motor, only one RTD Return lead can be run back to the 469.
See the figure below for a wiring example. Running more than one RTD Return lead to the 469 causes significant errors as
two or more parallel paths for the return current have been created. Use of this wiring scheme causes errors in readings
equivalent to that in the Reduced RTD Lead Number application described earlier.
2-16
Courtesy of NationalSwitchgear.com
J 3
+ V
, etc.
J 3 J 4
469
Hot
A1
Compensation
A2
RTD Return
A3
Figure 2–20: 2-WIRE RTD LEAD COMPENSATION
Figure 2–21: RTD ALTERNATE GROUNDING
469 Motor Management Relay
Motor Control
Terminal Box
RL1
L1
Rcomp
L2
RL2
L3
2 INSTALLATION
Motor
+
RTD1
–
808719A1.CDR
GE Multilin