Okay, let's break this down. You're describing a 3-phase, 240V system that appears to be wired in a way that you're attempting to derive a 240V control voltage for the coil using two different phases. The problem is you have 120V to ground on each leg, but 0V between B and C phase. This is highly unusual and points to a wiring problem, specifically related to how the 3-phase is derived.

Here's a breakdown of the issues and troubleshooting steps



1. The Problem: 0V Between B and C Phase (and likely between any two phases). This is the key problem. In a proper 3-phase 240V system, you should have 240V phase-to-phase. The fact that you have 120V to ground on all three phases, but zero volts between any two phases, points to a very fundamental issue with your power source. Likely Scenarios



Missing Phase

One phase of the 3-phase supply is completely missing/disconnected at the main panel or before it gets to your starter location. This is the most common explanation.

Mis-wired Transformer/Derivation

If you're using a transformer to derive this 3-phase power, the transformer is incorrectly wired, resulting in parallel instead of phase separation. It's effectively acting as a single-phase source.

Incorrect Grounding (Very Unlikely, but Possible)

A very unusual grounding configuration (like a ground fault on one of the phases close to the source) could cause something similar, but this is far less likely.

Meter Error

While less likely, verify your meter is functioning correctly and is set to measure AC voltage. Try measuring voltage at the main panel.

2. Why Your Current Wiring Doesn't Work

You're trying to get 240V by tapping 120V from C-phase to ground, and then using the switch on B-phase. However, if B and C are effectively at the same voltage potential (0V between them), then you're only supplying 120V to the coil, if even that. The circuit is incomplete because B and C are not "out of phase" as they should be in a 3-phase system.

3. How 3-Phase 240V Should Work

In a standard 3-phase 240V system, you have three phases (A, B, and C) that are 120 degrees apart from each other electrically. The voltage between any two phases is 240V. This is not 120V to ground + 120V to ground = 240V. It's based on the phase angle difference. The proper way to wire a 240V coil on a 3-phase system is to connect it directly between two of the phases (e.g., A and B, or B and C, or A and C).

4. Troubleshooting Steps (in order)


VERY IMPORTANT: SAFETY FIRST! Work with electricity is dangerous. If you're not completely comfortable and knowledgeable, call a qualified electrician. Turn off the power at the main breaker before starting.2. Check the Main Power Source



Go back to the main panel where the 3-phase power is supplied. Measure the voltage between each phase (A-B, B-C, C-A) at the breaker. You must have approximately 240V between each of those pairs. If not, the problem is upstream, before your starter. If you're using a transformer

Verify the transformer's input and output voltages, and the wiring connections on both the primary and secondary sides. Compare this to the transformer's wiring diagram. Ensure it's wired to provide 3-phase 240V output.

Check for a Missing Phase

Physically trace the wires from the main panel to your starter location. Look for any breaks, loose connections, or blown fuses/tripped breakers on any of the phases.3.

Verify Grounding

Ensure you have a good, solid ground connection at your main panel. Check the ground wire for continuity.4.

Once You Have Proper 3-Phase 240V



Disconnect the wiring from your coil. Wire the coil directly between two of the phases (e.g., A and B). Use appropriately sized wiring for the coil's current draw. Insert your switch in one of the phase wires going to the coil. This switch will now control the 240V supply to the coil.

Example Wiring (After Fixing the Power Issue)

1. Take Phase A (one of the 240V legs) from your 3-phase supply.
2. Connect that wire to one side of your switch.3. Connect the other side of your switch to one terminal of your coil.
4. Take Phase B (another of the 240V legs) from your 3-phase supply.5. Connect that wire directly to the other terminal of your coil.

Important Notes



Fuse/Breaker Protection

Ensure that you have appropriate overcurrent protection (fuses or breakers) in the circuit feeding the coil. The size of the fuse/breaker should be based on the coil's current draw (which you can calculate from the voltage and resistance: I = V/R = 240V / 70 ohms ≈ 3.4 Amps). A 5-amp breaker would likely be appropriate, but consult the motor starter's documentation.

Motor Starter Compatibility

Double-check that the motor starter itself is rated for the voltage and horsepower of your fire siren motor.

In summary, the root of your problem is almost certainly an issue with the 3-phase power source itself. You must resolve the missing phase/wiring problem at the main panel or transformer before you can properly wire the coil. Good luck, and please prioritize safety! Flag for review