Mike Dolt Enterprises
Mike Dolt Enterprises
User: MD88
Asked on: Friday, April 8, 2011 at 11:51 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:59 am EDT
Question: Hello,Does anyone know what "tripping ratio" is? I'm talking about NGRs, and ground fault relays... here's the paragraph in question:"Tripping ratio is defined as the ratio of prospectiveground-fault current to the operating value of the ground-fault protection. An adequate tripping ratio ensures thatsufficient ground-fault current is available for detectionwhen a ground fault occurs.""If the operating value of the ground-fault relays isgreater than the charging current of the largest feeder, andif a tripping ratio of 5 is selected to ensure adequatetripping levels and machine-winding ground-faultprotection, the let-through current of the NGR must begreater than 5 times the charging current of the largestfeeder."For example, I've calculated the charging current to be a little less than 5A. So, by this, I should choose an NGR at 25A. But what should the trip setting be?Then, in the manual we have this:Determine the desired trip level for ground-faultprotection.o The trip level should be above the chargingcurrent of the largest feederSelect an appropriate tripping ratio, usually between5 and 10.o The higher the tripping ratio the better theground-fault relays will be at detecting high-resistance ground faults. If there is morethan 10 A of ground-fault current a trippingsystem should be used.There's no "tripping ratio" dial on the relay! Just a trip level!I'm very confused!Thanks
Tripping Ratio
Tripping ratio is a safety margin that ensures ground-fault protection devices (such as ground-fault relays or neutral grounding resistors) can reliably detect and clear ground faults. It is calculated as the ratio of the prospective ground-fault current (the current that would flow if a ground fault occurred) to the operating value of the ground-fault protection device.
Purpose of Tripping Ratio
An adequate tripping ratio ensures that: Sufficient ground-fault current is available to detect and clear the fault. Machine windings and other equipment are protected from damage due to ground faults.
Example
In your example: Charging current is 5A. Tripping ratio is 5.This means that the prospective ground-fault current must be at least 5 times the charging current, or 25A. Therefore, you would select an NGR with a let-through current greater than 25A.
Trip Setting
The trip setting of the ground-fault relay is not explicitly mentioned in the paragraph you provided. However, it is typically recommended to set the trip level above the charging current of the largest feeder. This ensures that the relay will not trip on normal charging current but will detect and clear any ground fault that generates a current above the set level.
Tripping System
If there is more than 10A of ground-fault current, a tripping system should be used. This typically involves using a separate current transformer and relay to detect ground faults and initiate tripping.
Conclusion
Tripping ratio is an important consideration when selecting and setting ground-fault protection devices. It ensures the reliable detection and clearing of ground faults while minimizing the risk of false tripping. The trip setting of the ground-fault relay should be set above the charging current of the largest feeder, and a tripping system may be necessary if the ground-fault current is expected to exceed 10A. Flag for review