Appendix A: Protection Curves

Dependent Time Delay Current Protection Types

The following curves show the relationship between current ratio (I/In) and trip time for different protection curve types when the time constant parameter i^t = 1 second.

When a different value for time constant is set, the protection trip time changes proportionally to the time constant. For example, at i^t = 10 seconds, the protection trip time at the same current ratio will increase 10 times.

Protection Curve Types

Type	Description	Standard
SIT	Standard Inverse Time	IEC 60255-3 / BS 142
VIT (LTI)	Very Inverse Time (Long Time Inverse)	IEC 60255-3 / BS 142
EIT	Extremely Inverse Time	IEC 60255-3 / BS 142
UIT	Ultra Inverse Time	IEC 60255-3 / BS 142
RI	RI Type	IEC 60255-3 / BS 142

Protection Curves

Curve Formulas

The operation of the dependent time delay protection conforms to IEC 60255-3 and BS 142 standards.

Standard Inverse Time (SIT): $t = \frac{0.14 \times T}{(I/I_n)^{0.02} - 1}$

Very Inverse Time (VIT/LTI): $t = \frac{13.5 \times T}{(I/I_n) - 1}$

Extremely Inverse Time (EIT): $t = \frac{80 \times T}{(I/I_n)^2 - 1}$

Ultra Inverse Time (UIT): $t = \frac{315.2 \times T}{(I/I_n)^{2.5} - 1}$

Where:

• t = trip time in seconds
• T = time constant (parameter i^t)
• I = measured current
• I_n = rated current (parameter In)

Usage Notes

1. Select the appropriate curve type using parameter i^P:

   • 0 = Independent time delay (constant)
   • 1 = SIT (Standard Inverse Time)
   • 2 = VIT/LTI (Very Inverse Time)
   • 3 = EIT (Extremely Inverse Time)
   • 4 = UIT (Ultra Inverse Time)
   • 5 = RI Type

2. The time constant i^t corresponds to the trip delay time for 10×In

3. To deal with very high amperage currents, the protection has a feature with an independent time delay