Ct Requirements; Phase Fault And Earth Fault Protection - Toshiba GRE110 Instruction Manual

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2.12 CT Requirements

2.12.1 Phase Fault and Earth Fault Protection

Protection class current transformers are normally specified in the form shown below. The CT
transforms primary current within the specified accuracy limit, for primary current up to the
overcurrent factor, when connected to a secondary circuit of the given burden.
5 P 20 : 10VA
Accuracy
Limit (%)
Accuracy limit : Typically 5 or 10%. In applications where current grading is to be applied and
small grading steps are desirable, then a 5% CT can assist in achieving the necessary accuracy. In
less onerous applications, a limit of 10% may be acceptable.
Overcurrent factor : The multiple of the CT rating up to which the accuracy limit is claimed,
typically 10 or 20 times. A value of 20 should be specified where maximum fault current is high and
accurate inverse time grading is required. In applications where fault current is relatively low, or
where inverse time grading is not used, then an overcurrent factor of 10 may be adequate.
Maximum burden : The total burden calculated at rated secondary current of all equipment
connected to the CT secondary, including relay input burden, lead burden, and taking the CT's own
secondary resistance into account. GRE110 has an extremely low AC current burden, typically less
than 0.03VA for a 5A phase input, allowing relatively low burden CTs to be applied. Relay burden
does not vary with settings.
If a burden lower than the maximum specified is connected, then the practical overcurrent factor
may be scaled accordingly. For the example given above, at a rated current of 1A, the maximum
value of CT secondary resistance plus secondary circuit resistance (RCT + R2) should be 10Ω. If
GRE110
GRD110
OC1 / EF1
(inverse time)
OC2 / EF2
Fast Trip
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Feeder Trip
GRE110 GRD110
OC2 / EF2
OC1 HS /
EF1 HS
Figure 2.11.2 Blocked Busbar Protection
Overcurrent Maximum Burden
Factor (at rated current)
 53 
Delayed Back-up Trip
High Speed Block to Incomer for Feeder Fault
Feeder Trip
GRE110
GRD110
OC2 / EF2
OC1 HS /
EF1 HS
6 F 2 T 0 1 7 2
Feeder Trip
GRE110 GRD110
OC2 / EF2
OC1 HS /
EF1 HS