Using 51N on a 3 wire system
Using 51N on a 3 wire system
(OP)
We plan on using a 51N relay on a 13.8kV system, 3 phase, 3 wire to provide for ground fault protection. A co-wowrker syas we can't use a 51N device since we don't have a neutral.
Any comment or input would be appreciated.
Thanks.
Any comment or input would be appreciated.
Thanks.






RE: Using 51N on a 3 wire system
If ground faults are low, and you have cable feeders, you may want to get additional sensitivity by using a CT around all three cable cores.
RE: Using 51N on a 3 wire system
RE: Using 51N on a 3 wire system
If you have a three wire, solidly grounded, system the 51N is probably sensitive enough detect your ground faults. The 51G can be more sensitive since it can use a CT with a lower ratio. If you have 400:5 phase CTs, the 51N is based on that ratio, but if you use a 50:5 core balance CT, you have 8 times the sensitivity.
Which one is right? It depends. Either will work and it is up to you to evaluate the pluses and minuses of each to determine the best course. Many relays allow you to do both.
RE: Using 51N on a 3 wire system
RE: Using 51N on a 3 wire system
There is no standard definition of the difference between 51N and 51G. These terms are somewhat interchangeable. In the US, the "N" is conventionally used when dealing with a residual CT connection and "G" is used when the relay is connected to a CT in a grounded neutral conductor. However, Schweitzer relay documentation is just the reverse of this, as I recall.
RE: Using 51N on a 3 wire system
Is there any difference in the the two, When one make the same type of connections. Assuming a 4th coil is available for the ground fault detection?
Both being a summation of the 3 phase's individual CT?
Or is it just a difference in measurement method?
51N being protection from a calculation from 3 OC coils?
RE: Using 51N on a 3 wire system