Open delta in depth

In closed delta, three transformers form the triangle. If one fails or is removed, the other two can still deliver three line voltages. That is open delta (V-connection). The voltages remain; the power available falls.

Framing A — two units alone: open-delta bank ≈ 86.6% of (kVA₁ + kVA₂) for balanced three-phase load.

Framing B — lost one of three equals: remaining ≈ 57.7% of the original closed-delta bank capacity (not 66.7%).

Example: 3 × 50 kVA closed = 150 kVA. Open with two → ≈ 86.6 kVA three-phase useful (≈57.7% of 150), not 100 kVA.

Each remaining transformer operates in a different duty than in a balanced closed delta. It is easy to overload one unit while “line voltage looks fine.” Oil/winding temperature and smell beat the customer’s voltmeter.

Open (or closed) delta often has a center tap on one leg for 120/240 V single-phase. Lighting and receptacles hang there. Pile single-phase load on that tap and keep large three-phase motors, and the “lighting” transformer fails first.

Before leaving a plant on open delta: (1) real three-phase kVA demand, (2) open-delta capacity available, (3) expected current per transformer, (4) single-phase share, (5) hours vs forever. If numbers do not close, bring a third unit or shed load.

Closed delta capacity is not “three independent pipes.” Magnetic and current relationships in the triangle mean removing one unit hurts more than removing one-third of the nameplate sum. The 57.7% figure (√3/3) is the standard teaching result for equal transformers—memorize it for emergencies.

List every 120 V and 240 V load on the lighting transformer. Convert to kVA. Compare to that one unit’s rating—not to the bank’s open-delta three-phase rating. Many “mystery” failures are simply single-phase overload on the tapped can.

Say: “You still have three-phase, at roughly half the original bank capacity, and the lighting transformer is carrying extra risk.” Clear language prevents the “temporary” patch from lasting two years.

Translate the theory into a two-minute job briefing: what you will measure first, what reading would change your mind, and what you will leave documented for the next shift. If you cannot brief it, you do not own it yet.

Before you speak, write down:

1. What topology or machine you have in front of you.
2. Voltages and currents with the measurement point.
3. Frequency or rpm if they apply.
4. What the nameplate or diagram says.
5. What changes if you isolate one part of the circuit.

A diagnosis without those data is conversation, not the trade.

1. Explain the central block out loud to an imaginary helper.
2. Rewrite the field case with numbers from a real piece of equipment.
3. Complete the checklist without looking.
4. Mark which rows in the mistakes table have already happened to you.

If you cannot say the core idea in one minute, return to the first third.

Energized measurement needs PPE, a meter of the right category, and a plan if the reading does not make sense. Capacitors, inductive fields, rotating shafts, and power neutrals do not forgive haste. If the procedure says de-energize and verify absence of voltage, that rules.

Situation. Maintenance removed a failed 75 kVA unit from a 3 × 75 kVA delta bank and left open delta. Someone says “we still have 150 kVA.”

How to think. Two × 75 = 150 nameplate sum, but three-phase capability ≈ 0.866 × 150 ≈ 130 kVA—and compared with the original 225 kVA bank, you are near 57.7% (≈130 kVA). If plant demand was 180 kVA, you are already in trouble.

Conclusion: never confuse nameplate sum with open-delta capability.