Micrologic Relay Setting Calculations in ACB
( Air Circuit Breaker)
Using the Micrologic relay protection settings effectively in an ACB is only possible when the calculations are done correctly. These settings depend on the electrical load, system design, and fault level.
Below is the basic method to calculate each protection setting. This will help you apply correct settings on-site:
1. Long Time Protection (Ir, tr)
Purpose: To protect against overload.
Ir (Overload Current Setting):
This is set close to the main load current with a safety margin.
Ir1.05x Full Load Current
If full load current is 400 A, then
Ir = 1.05 * 400 = 420rho
Formula:
Example:
tr (Time Delay):
This allows time for motor starting or temporary overloads.
Typical values range from 10 to 20 seconds, following an inverse time-current curve.
2. Short Time Protection (Isd, tsd)
Purpose: To protect against short circuits or high inrush current.
Isd (Short Time Pickup):
This is usually set as a multiple of Ir. The multiplier (k) typically ranges from 1.5 to 10.
Ir = 420A and k = 5 then Isd I = 5 * 420 = 2100A
Example:
tsd (Short Time Delay):
This is the time delay to allow selectivity and coordination with downstream devices.
Usually set between 0.1 and 0.5 seconds.
3.Instantaneous Protection (li)
Purpose: To instantly trip the breaker on very high fault current.
li (Instantaneous Pickup):
This is typically set between 6 to 15 times the rated current (In).
It can also be turned OFF if coordination with downstream breakers is required.
Example:
If In = 630A then
Ii = 10 * 630 = 6300A
4. Earth Fault Protection (Ig, tg)
Purpose: To detect ground fault or earth leakage.
Ig (Earth Fault Current Setting):
This is set as a percentage of In, usually from 0.2 to 1.0 times In.
If In 630 A and Ig is set to 0.4 x In, then Ig = 0.4 * 630 = 252A
Example:
tg (Time Delay for Earth Fault):
This delay allows permissible leakage current without tripping.
Typically set between 0.1 and 0.5 seconds.
5. Protection Coordination Logic
Long Time (Ir): Has the longest delay, used for overload protection.
Short Time (Isd): Medium delay for moderate fault current.
Instantaneous (li): No delay for severe, direct faults.
Earth Fault (Ig): Adjustable, used for leakage and insulation faults.
Important Notes
The ACB's rated current (In) is printed on the breaker label.
The Ir setting should never exceed In.
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