Master Electrician Exam (NEC)General Electrical KnowledgeHard
A master electrician is performing maintenance on a 480V, 3-phase, Delta-connected transformer. The secondary side is 208Y/120V. If one of the primary fuses blows, what will be the resulting voltage configuration on the secondary side?
- AThe secondary will operate in an 'open-Delta' or 'V-phase' configuration.
- BThe secondary will remain 208Y/120V, but with reduced capacity.
- CThe secondary voltage will become a single-phase 240V.
- DThe secondary will lose all voltage due to the open primary.
Show answer & explanationAnswer & explanation
Correct answer: A. The secondary will operate in an 'open-Delta' or 'V-phase' configuration.
In a 3-phase Delta-connected primary, if one fuse blows (opening one phase), the primary effectively becomes an open-Delta supply. This will induce an open-Delta (or V-phase) voltage on the secondary side, meaning the secondary will still provide 3-phase power, but at reduced capacity (typically 57.7% of the original kVA rating) and potentially slightly unbalanced voltages.
Why the other options are wrong
- B. While capacity is reduced, stating it 'remains 208Y/120V' is imprecise as the phase relationship and balance change.
- C. The secondary will not become a single-phase 240V; it will still be a 3-phase system, albeit open-Delta.
- D. Only one primary phase is open, so the transformer will still induce voltage on the secondary, just in an open-Delta configuration.
Open-Delta Transformer Operation
Open-Delta (or V-phase) operation occurs when one phase of a 3-phase Delta-Delta or Delta-Wye transformer bank is removed or fails. It can still supply 3-phase power, but with reduced capacity and potential voltage imbalance.
- Typically occurs due to fuse blowing or one transformer failing in a bank.
- Capacity is reduced to approximately 57.7% of the original 3-phase bank rating.
- Can still provide 3-phase and single-phase loads, but voltage regulation is poorer.
Memory trick: Delta Fuse Blows, V-Phase Shows.