These Digital Ground Fault Relays employ either the built-in 500:1 or an external 500:1, 1000:1, 2000:1 or 10.000:1 ratio Current Sensor (CS) to measure residual current flowing to ground in electrical power distribution systems.
...The measured value is compared against two setpoints:
The Ground Fault (G/F) setpoint, settable with dipswitches on the unit.
The Trip Inhibit (T/I) setpoint, depending on the use of an external CS and/or the G/F setting.
When the current exceeds the G/F setpoint for the set delay, but does not reach the T/I setpoint, a ground fault is diagnosed and the unit trips by operating its internal relay. Without Trip Inhibit selected, should a fault be approaching a dead short to ground, causing the residual current to exceed the T/I setpoint, then without any intentional delay the unit operates its relay, indicating a Short Circuit. In this case one assumes that the contactor or breaker can handle the high fault currents.
Contactors are not designed to interrupt high fault currents. The Trip Inhibit function is useful when the Digital Ground Fault Relay is used to trip a contactor or motor starter on a solidly grounded system. In this application, there is no defined limit to the magnitude of the ground fault current should a bolted ground fault occur. Under these conditions the upstream short circuit protective devices, fuses or circuit breaker, should interrupt the fault. If the ground fault current exceeds the inhibit level, the output circuit will be inhibited from tripping until the ground fault current drops below the inhibit level. However, should the current fail to drop within 1,2 s, then, as a last resort, the unit will trip, showing the cause of the trip on its red LED.
In relation to IEC standard 60947-4-1 it is now possible to construct a 'type-2 coordinated' installation:
Only minor welding of the contactor or starter contacts is permissible and the contacts must be easily separated.
Following type-2 coordination tests, the switchgear and controlgear functions must be fully operational.
It is advisable to inspect the contactor after a Trip Inhibit event since it was used to break a current possibly outside its rating. Even if the current was inside the rating, minor welding of the contacts can occur. The cause of the trip and the location of the trip is indicated, so after a quick inspection of the affected contactor, the power on the remaining part of the installation can be restored. The units are Class A devices as defined in the IEC 60755 standard; they are therefore fully characterised for operation with sinusoidal AC and pulsating DC currents.
These Digital Ground Fault Relays employ either the built-in 500:1 or an external 500:1 or 10.000:1 ratio Current Sensor to measure residual current flowing to ground in electrical power distribution systems. The measured value is compared against two setpoints:
The Ground Fault setpoint, settable with dipswitches on the unit.
DGF7-3 |
30mA up to 3,0A |
DGF100 |
30mA up to 9,0A |
The Trip Inhibit setpoint, depending on the use of an external Current Sensor and/or the Ground Fault setting.
When the current exceeds the Ground Fault setpoint for the set delay, but does not reach the Trip Inhibit setpoint, a ground fault is diagnosed and the unit trips by operating its internal relay. Without Trip Inhibit selected, should a fault be approaching a dead short to ground, causing the residual current to exceed the Trip Inhibit setpoint, then without any intentional delay the unit operates its relay, indicating a Short Circuit. In this case one assumes that the contactor or breaker can handle the high fault currents.
Contactors are not designed to interrupt high fault currents. The Trip Inhibit function is useful when the Digital Ground Fault Relay is used to trip a contactor or motor starter on a solidly grounded system. In this application, there is no defined limit to the magnitude of the ground fault current should a bolted ground fault occur. Under these conditions the upstream short circuit protective devices, fuses or circuit breaker, should interrupt the fault. If the ground fault current exceeds the inhibit level, the output circuit will be inhibited from tripping until the ground fault current drops below the inhibit level. However, should the current fail to drop within 1,2 s, then, as a last resort, the unit will trip, showing the cause of the trip on its red LED.
In relation to IEC standard 60947-4-1 it is now possible to construct a 'type-2 coordinated' installation:
Only minor welding of the contactor or starter contacts is permissible and the contacts must be easily separated.
Following type-2 coordination tests, the switchgear and controlgear functions must be fully operational.
It is advisable to inspect the contactor after a Trip Inhibit event since it was used to break a current possibly outside its rating. Even if the current was inside the rating, minor welding of the contacts can occur. The cause of the trip and the location of the trip is indicated, so after a quick inspection of the affected contactor, the power on the remaining part of the installation can be restored. The units are Class A devices as defined in the IEC 60755 standard; they are therefore fully characterised for operation with sinusoidal AC and pulsating DC currents.
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