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 prospective
ground-fault current to the operating value of the ground-
fault protection. An adequate tripping ratio ensures that
sufficient ground-fault current is available for detection
when a ground fault occurs."
"If the operating value of the ground-fault relays is
greater than the charging current of the largest feeder, and
if a tripping ratio of 5 is selected to ensure adequate
tripping levels and machine-winding ground-fault
protection, the let-through current of the NGR must be
greater than 5 times the charging current of the largest
feeder."
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-fault
protection.
o The trip level should be above the charging
current of the largest feeder
Select an appropriate tripping ratio, usually between
5 and 10.
o The higher the tripping ratio the better the
ground-fault relays will be at detecting high-
resistance ground faults. If there is more
than 10 A of ground-fault current a tripping
system should be used.
There's no "tripping ratio" dial on the relay! Just a trip level!
I'm very confused!
Thanks
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 prospective
ground-fault current to the operating value of the ground-
fault protection. An adequate tripping ratio ensures that
sufficient ground-fault current is available for detection
when a ground fault occurs."
"If the operating value of the ground-fault relays is
greater than the charging current of the largest feeder, and
if a tripping ratio of 5 is selected to ensure adequate
tripping levels and machine-winding ground-fault
protection, the let-through current of the NGR must be
greater than 5 times the charging current of the largest
feeder."
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-fault
protection.
o The trip level should be above the charging
current of the largest feeder
Select an appropriate tripping ratio, usually between
5 and 10.
o The higher the tripping ratio the better the
ground-fault relays will be at detecting high-
resistance ground faults. If there is more
than 10 A of ground-fault current a tripping
system should be used.
There's no "tripping ratio" dial on the relay! Just a trip level!
I'm very confused!
Thanks