Confused with outside lights

[Sparkywise]

I am working on a commercial parking lot. At the entrance I have a sign with LEDs inside and on the back side of the sign is an irrigation system. Both of those come off of a 120/208 electrical panel. Both use the same 120 volt circuit. Never had issues with it.
The light poles are LED and come off of a 277/480 electrical panel. The lights are 480 volt.
Both 120/208 and 277/480 are in the same conduit. This was done many years ago.
The problem is the company use some outside contractor when they lost power to a section of lights. The contractor got the lights working.
Now the sign and irrigation system do not work. I could not understand why because I had 120 volts during the day. Found out after troubleshooting that when the lights come on the sign and irrigation system received about 400 volts across hot and neutral. Neutral to ground is about 280 volts.
If I turn off the 120 volt circuit and remove the 120 volt neutral and turn on the lights a group of the lights will strobe.
I contacted the outside contractor and they will not respond to me.
I am currently trying to map out where the wires are going. They go through several ground boxes and many light poles.
How is the 277/480 circuits working off the 120 volt neutral?

 

[hillbilly1]

The neutrals have a common tie point at the service. The transformer for the 120/208 volt panel if properly bonded, is tied to the same common grounding electrode's. Sounds like the transformer XO terminal was not bonded.

 

[GoldDigger]

Make all your voltage measurements using a low impedance voltmeter. Otherwise capacitively coupled voltages (which cannot deliver useful current) will confuse your readings.
If you are seeing are not just capacitive (phantom) voltages, then your readings indicate much more than a neutral problem. There is a strong possibility that the contractor cross connected the 120/208 and the 277/440 systems in some way.
Be very careful in your troubleshooting that you do not damage any of your 120/208 powered equipment by actually applying 277/440 to it in the course of changing wires around.
Note that by Code and good practice two circuits should not use a common neutral wire unless they are part of a single shared neutral MWBC. And the neutral wires of the two voltage systems must not be connected to each other except via the neutral bar of the source panels and the common neutral to ground bond at the service.

 

[kwired]

Make all your voltage measurements using a low impedance voltmeter. Otherwise capacitively coupled voltages (which cannot deliver useful current) will confuse your readings.
If you are seeing are not just capacitive (phantom) voltages, then your readings indicate much more than a neutral problem. There is a strong possibility that the contractor cross connected the 120/208 and the 277/440 systems in some way.
Be very careful in your troubleshooting that you do not damage any of your 120/208 powered equipment by actually applying 277/440 to it in the course of changing wires around.
Note that by Code and good practice two circuits should not use a common neutral wire unless they are part of a single shared neutral MWBC. And the neutral wires of the two voltage systems must not be connected to each other except via the neutral bar of the source panels and the common neutral to ground bond at the service.

Agree that there should be separate neutral for each system.

Also suggest if there is a shared neutral between systems that it possibly has open circuit back to source(s) to develop 400 volts to another conductor?