General Catalogue 2007 - Ibemo Kazakhstan Ltd

Time discrimination
Time discrimination is achieved by delaying the opening of the upstream
circuit breaker until the downstream circuit breaker haopened
and cleared the fault. The total clearing time of the
downstream circuit breaker must be less than the time setting of the
upstream circuit breaker and the upstream circuit breaker must be
able to withstand the fault current for the time setting period.
Therefore the upstream circuit breaker must be a category ‘B’
breaker which has been designed and tested for this purpose.
To determine time discrimination it is only necessary to
compare the time/current characteristic curves of the two devices to
ensure that no overlap occurs.
Fig 24
Short circuit discrimination
A more accurate way of checking the discrimination between two
circuit protective devices at short circuit levels is to compare the
energy let-through of the downstream device with the no-tripping or
pre-arcing energy levels of the upstream device.
In order to check current discrimination at short circuit levels
between:
Fuse upstream - fuse downstream
It is only necessary to compare the I 2 t values of each fuse. This
information is usually available in very simple tabular form (see Table
33). If the total let-through energy (I 2 t) of the downstream fuse is less
than the pre-arcing energy (I 2 t) of the upstream fuse, then total
discrimination is achieved at short-circuit levels.
Hager Catalogue 2007 • Technical
Selectivity & Discrimination
ibemo Kazakhstan - 090301 Republic of Kazakhstan, West Kazakhstan Oblast, Aksai, Pramzone, BKKS office complex
Phone: +7 71133 93077 ; Fax: +7 71133 93074 ; E-Mail: info@ibemo-kz.com
3.49
Protection
Devices