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5-10 BE1-87G - Testing And Setting
Table 5-1. Recommended Pickup Settings
Vce
(Lower Value)
SFR
1 1.5 2 3 4
20 0.2 0.4 0.5 0.8 1.6
50 0.2 0.4 0.4 0.5 0.8
100 0.2 0.2 0.4 0.4 0.5
200 0.1 0.1 0.2 0.4 0.5
>200 0.1 0.1 0.2 0.4 0.5
Note:
Use next higher setting if CTs are a mix of C classification and T classification.
The saturation factor ratio, SFR, represents an index of the relative performance of the two sets of CTs. This
performance is a function of the relative quality of the CTs (Vce), the relative burdens (R
t
), and neglecting the
remanence.
If SFR is < 1, then use 1/SFR for SFR.
Setting Example Number One
Select the pick-up setting for the motor application in Figure 5-8. In this application, the settings need to be
based on the probability of significant dissimilar CT saturation due to the very slowly decaying dc component
of the starting current. Since the motor is not grounded, no ground current can flow during starting of an
unfaulted motor. Therefore, one-way lead burden is used to determine the total CT burden. Each phase CT
carries just the burden for the lead for that phase.
Figure 5-8. Motor Differential Application
(R
t
)
1
= R
l
+ R
w
= 0.22 + 0.14 = 0.36 ; (Vce)
1
= 50 ; R
t
< 0.007(Vce)
1
= 0.35
(R
t
)
2
= 0.09 + 0.10 = 0.19 ; (Vce)
2
= 100 ; R
t
< 0.007(Vce)
2
= 0.7