Do circuit breakers with ground fault or LSIG trip unit still work on a 3 phase, 3 wire grounded system without a neutral?
Issue:
Application
Product Line:
Circuit Breaker
Resolution:
Yes. On a 3-phase, 3-wire, grounded system with no neutral, the trip unit ground fault function still works. The trip unit still uses the internal phase CTs to vector sum the phase currents, like it does with a 4-wire system with a neutral. The only difference is that you do not have single-phase loads and the trip unit does not have input from the NCT since there is no neutral current.
The trip unit is not looking for an imbalance since a two-phase load would cause that. It is looking for current leaving the system, which only a GF can cause, and therefore causes the current to vector sum to a value greater than zero: the value of the GF current.
On a 4-wire system with a neutral current, you have single-phase loads with a neutral current. This neutral current is measured with the NCT and is also part of the vectorial summing process along with the internal phase CTs. The current sums to zero when no GF exists. If a ground fault exists, then the current sums to the value of the current going to the ground. Again not looking for an imbalance, as a single phase load would cause an imbalance, but would still properly sum to zero.
Application
Product Line:
Circuit Breaker
Resolution:
Yes. On a 3-phase, 3-wire, grounded system with no neutral, the trip unit ground fault function still works. The trip unit still uses the internal phase CTs to vector sum the phase currents, like it does with a 4-wire system with a neutral. The only difference is that you do not have single-phase loads and the trip unit does not have input from the NCT since there is no neutral current.
The trip unit is not looking for an imbalance since a two-phase load would cause that. It is looking for current leaving the system, which only a GF can cause, and therefore causes the current to vector sum to a value greater than zero: the value of the GF current.
On a 4-wire system with a neutral current, you have single-phase loads with a neutral current. This neutral current is measured with the NCT and is also part of the vectorial summing process along with the internal phase CTs. The current sums to zero when no GF exists. If a ground fault exists, then the current sums to the value of the current going to the ground. Again not looking for an imbalance, as a single phase load would cause an imbalance, but would still properly sum to zero.
Released for: Schneider Electric USA
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