i) Design appropriate CT ratios for CTA, CTB, and CTC. (ii) Estimate time of Operation for all three relays for maximum value of fault current to maintain the selectivity. (iii) Determine TSM for relays at bus A and B for maximum fault current. (Use the standard formula for IDMT relays for these calculations.)  (iv) Also use the standard curves for IDMT relays shown in Figure 2 to select TSM for relays at bus A and B for maximum fault current. Compare results obtained in (ii) and (iv).  Verify the design to ensure that selectivity is maintained at minimum fault current for the fault in the same zone

EBK ELECTRICAL WIRING RESIDENTIAL
19th Edition
ISBN:9781337516549
Author:Simmons
Publisher:Simmons
Chapter28: Overcurrent Protection–fuses And Circuit Breakers
Section: Chapter Questions
Problem 11R: State four possible combinations of service equipment that meet the requirements of 110.9 and 110.10...
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FIrst 3 question answers are give below answer the last two parts
answers:

1) relay at C (CT ratio) = 40 : 1

    relay at B (CT ratio) = 60 : 1

    relay at A (CT ratio) = 40 : 1

2) TMS/A = 0.711

3) top = 1.48 sec ( relay will operate as per the fault current level.)


  1. i) Design appropriate CT ratios for CTA, CTB, and CTC.
  2. (ii) Estimate time of Operation for all three relays for maximum value of fault current to maintain the selectivity.
  3. (iii) Determine TSM for relays at bus A and B for maximum fault current. (Use the standard formula for IDMT relays for these calculations.)
  4.  (iv) Also use the standard curves for IDMT relays shown in Figure 2 to select TSM for relays at bus A and B for maximum fault current. Compare results obtained in (ii) and (iv). 
  5. Verify the design to ensure that selectivity is maintained at minimum fault current for the fault in the same zone
5 678910
20
30 40
50
40
30
20
0.6
0.5
TIME DIAL
SETTING
0.4
0.3
0.2
VININI
0.1
0.08
0.07
0.06
0.05
0.04
0.03
0.02
1/2
0.01
2 3 4 5 6 7 8 910
20 30 40
MULTIPLES OF TẠP VALUE CURRENT
TIME IN SECONDS
Transcribed Image Text:5 678910 20 30 40 50 40 30 20 0.6 0.5 TIME DIAL SETTING 0.4 0.3 0.2 VININI 0.1 0.08 0.07 0.06 0.05 0.04 0.03 0.02 1/2 0.01 2 3 4 5 6 7 8 910 20 30 40 MULTIPLES OF TẠP VALUE CURRENT TIME IN SECONDS
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