DSE 8610 MKII, Руководство оператора, Страница: 161 / 188

Protections
Page 161 of 188 057-254 ISSUE: 2
6.10 DEFAULT CURRENT PROTECTION TRIPPING CHARACTERISTICS
The graph on the following page shows the default settings for the IDMT tripping curves for the Over
Current, Short Circuit and Earth Fault protections.
The default setting for the Over Current alarm allows for an overload of an alternator to the limits of
the Typical Brushless Alternator whereby 110% overload is permitted for 1 hour or 200% overload is
permitted for 36 seconds. In an over current situation the alternator begins to overheat. The aim of the
Over Current IDMT Alarm is to prevent the windings being overload (heated) too much. The amount
of time that the alternator can be safely overloaded is governed by how high the overload condition is.
The default setting for the Short Circuit alarm allows for an alternator to supply a high current caused
by a genuine short circuit or an inrush current of a motor/transformer. Whereby 300% overload is
permitted for 0.17 seconds or 600% overload is permitted for 0.06 seconds. In a short circuit situation
the alternator begins to overheat to the point the insulation breaks down, potentially causing a fire.
The aim of the Short Circuit IDMT Alarm is to prevent the insulation from melting due to excessive
heat. The amount of time that the alternator can be safely in a short circuit condition is governed by
the alternator’s construction.
The default setting for the Earth Fault alarm allows for an alternator to supply a fault current caused
by an imbalanced load, a high impedance short to earth or motor drives. Whereby anything less then
10% is considered normal (caused by imbalanced loads) and permitted, 12% fault current is permitted
for 3.83 second or 20% fault current is permitted for 1 second.