P74x/EN OP/N
(OP) 5-
1.3.3.2.9 CB Fail alarm
The CB Fail alarm is raised on any timer reached (tBF2 or tBF4)
1.3.3.2.10 Separate external 50BF protection to the busbar protection
This is the most common solution utilising conventional wiring. The 50BF relay is completely
independent of all others. When a circuit breaker failure condition occurs the external
protection trips all adjacent circuit breakers as defined in the separate scheme (DDB Ext. CB
fail).
In view of the connection between the functions of the busbar protection and the circuit
breaker failure protection some operators prefer one of the more integrated system solutions
previously mentioned.
1.4 Three Phase Overcurrent Protection
1.4.1.1 Inverse Time (IDMT) Characteristic
IDMT characteristics are selectable from a choice of four IEC/UK and five IEEE/US curves
as shown in the table below.
The IEC/UK IDMT curves conform to the following formula:
The IEEE/US IDMT curves conform to the following formula:
Where: t = Operation time
K = Constant
Ι = Measured current
Ι
S
= Current threshold setting
α = Constant
L = ANSI/IEEE constant (zero for IEC/UK curves)
T = Time Multiplier Setting for IEC/UK curves
TD = Time Dial Setting for IEEE/US curves
IDMT Curve description Standard K Constant
α Constant
L Constant
Standard Inverse IEC 0.14 0.02 0
Very Inverse IEC 13.5 1 0
Extremely Inverse IEC 80 2 0
Long Time Inverse UK 120 1 0
Moderately Inverse IEEE 0.0515 0.02 0.114
Very Inverse IEEE 19.61 2 0.491
Extremely Inverse IEEE 28.2 2 0.1217
Inverse US-C08 5.95 2 0.18
Short Time Inverse US-C02 0.02394 0.02 0.01694
1.4.1.2 Reset Characteristics
For all IEC/UK curves, the reset characteristic is definite time only.
For all IEEE/US curves, the reset characteristic can be selected as either inverse curve or
definite time.
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