General Catalogue 2007 - Ibemo Kazakhstan Ltd

Typical RCCB Time/Current Characteristics
Fig 36
Discrimination between Residual Current Devices
Down-stream Residual Current Device
3.64
Hager Catalogue 2007 • Technical
Selectivity / Discrimination
Discrimination between Circuit Breakers with add on RCCBs
Having decided on the type and the limit of discrimination of the
circuit breakers in the system, it is very important to consider the
discrimination between any add on RCCBs. In theory it is possible to
achieve current discrimination between RCCBs but the limit of
discrimination is too low for practical purposes. Time discrimination is
by far the best method and is achieved by delaying the tripping of the
upstream RCCB, See Fig 36, which shows the RCCB characteristics
for both instantaneous and time delayed.
Note that the limit of discrimination is the instantaneous setting of the
associated circuit breaker. In other words if the earth fault current is
greater than the instantaneous trip setting of the associated circuit
breaker, the circuit breaker will trip regardless of the time delay on the
RCCB. Table 49 indicates how time discrimination may be achieved
between RCCBs.
Up-stream residual current device
Up-stream
RCCB
sensitivity
IΔn 0.01A 0.03A 0.1A 0.3A 1.0A 3.0A
Downstream
RCCB Time
Sensitivity Delay
IΔn Secs 0 0 0 0.2 0 0.2 0.3 1.0 3.0 0 0.3 1.0 3.0 0 0.3 1.0 3.0
0.01A 0
0.03A 0
0.1A 0
0.2
0.3A 0
0.2
0.3
1.0
3.0
1.0A 0
0.3
2.0
3.0
3.0A
Table 49
0
0.3
1.0
3.0
■ Discrimination achieved
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