Below are a series of preliminary Questions, which should serve to address the issues at hand.

1.: Are the Non-Functioning Lighting Fixtures High Pressure Sodium, or Pulse-Start Metal Halide?

2.: Are the Non-Functioning Lighting Fixtures connected L-N (Line to Neutral - AKA "277V"), or are they connected L-L (Line to Line - AKA "480V")?

3.: What system are the Photocell and Coil(s) of the Lighting Contactor connected to: the 208Y/120V system, or the 480V system?
(i.e.: what are the Voltage Ratings of the Photocell and the Latching Coil(s) of the Lighting Contactor)

4.: The 1 Phase Voltage readings were made L-G (Line to Ground); is there a Grounded Conductor ("Neutral") available? Measuring L-N, instead of L-G.
The Branch Circuits _SHOULD_ have a corresponding Grounded Conductor if they are intended to be 277V Circuits.

5.: Does the Nameplate on the Transformer indicate One of the following specifications:

a1. Primary = 480V Delta, Secondary = 208Y/120V 3 Phase 4 Wire
a2. Primary = 208V Delta, Secondary = 480V 3 Phase 3 Wire
a3. Primary = 208V Delta, Secondary = 480Y/277V 3 Phase 4 Wire
a4. Primary = 480V Delta, Secondary = 208V (or 240V) 3 Phase 3 Wire

b1. Pri. = 480V, Sec. = 208Y/120V 3P 4W
b2. Pri. = 208V, Sec. = 480V 3P 3W
b3. Pri. = 208V, Sec. = 480Y/277V 3P 4W
b4. Pri. = 480V, Sec. = 208V (or 240V) 3P 3W

c1. Hi Volts = 480V, Low Volts = 208/120V
c2. Hi Volts = 480V, Low Volts = 208V
c3. Hi Volts = 480/277V, Low Volts = 208V
c4. Hi Volts = 480V, Low Volts = 208-240V

d. Primary = 208V, Secondary = 480V

Per Question #1:
The Pulse Ignitors used for starting HPS and MH Lamps may have failed, resulting in Non-Functioning Lighting.

Check Lamps for obvious aging indications.
Place a known operating Lamp of equivalent Wattage in the Circuit, in lieu of the existing Lamp, then energize the Circuit.
If Lamp starts, existing Lamp was bad.
If Lamp does not start, replace Ignitor with known operating Lamp, then re-energize Circuit.
If the Lamp still does not start, proceed to "Question #2"


Clients are notorious for not accurately describing the real scenario, when it comes to the failure of something.
(They tend to leave out pertinent information, eluding to extremely odd and unlikely scenarios)
Example: "The Lights Just Stopped Working" means the last functioning Light Fixture has recently stopped working, leaving the entire area dark.

Try not to be swayed by abnormal claims of sudden Doom and Gloom occurring, especially when Lighting is involved.
Simple lack of Lighting Maintenance may be the culprit, wherein the claims of "Everything Worked Until Yesterday" may have been described to you per the Troubleshooting Request, leaving you to believe there has been some type of major system failure.

Check the most apparent failure types first, then dig into least possible failure issues as necessary.


Per Question #2:
Verify if the Ballasts are connected "L-N" (277V), or "L-L" (480V).

Please indicate if the Ballasts are connected as L-N (277V), or L-L (480V).

Verify the Voltage present at the Ballast's Input Section (where the Branch Circuit connects to the Ballast)
Use a LOW INPUT Z METER for these tests, in order to obtain an accurate reading.
If you only have a typical DVM / DMM, then either leave the Ballast + Lamp connected, or use a "Wiggy" in Parallel with the DVM.

This section of questions will determine if there is any mismatching of Ballast input Voltages, what level of Voltage Drop is being experienced, and if the original Branch Circuit Wiring may have been damaged (i.e.: New Sprinklers have been installed, etc...)


Per Question #3:
If the Photocell and Lighting Contactor Coil(s) are rated as 120V or 208V, and connected to the 208V system, that makes sense of why the Controls operate properly.
In addition, if these items have a rating of 480V and are connected L-L to the output side of the Transformer, they will continue to operate properly.

If the Controls were rated for 277V, and connected L-N to the Transformer's output, the extremely Low L-N Voltage measured at Open Contactor State would not allow for the latching of the Contactor, therefore you would not be able to obtain a L-N reading with the Contactor in Latched position (AKA "Closed").


Per Question #4:
Lack of a Grounded Conductor included with the Branch Circuits, typically indicates L-L (480V) Branch Circuits, instead of L-N (277V) Branch Circuits.

In addition, the odd L-G Voltage readings point to an Output from the 75 KVA Transformer being 480V 3 Phase 3 Wire - as an Ungrounded Delta, rather than a Solidly Grounded system.

The changes in L-G Voltage on the Output of the 75 KVA Transformer when the Lighting Loads are connected, is indicative of L-L connections, and an Ungrounded system.

As measured L-G at a relatively close proximity to the Transformer, with the Contactor Open (no Ballasts connected to the Transformer), the L-G Voltage is found to be dramatically less than what would be found on a Solidly Grounded Wye system.

The Phase Voltage on a 480Y/277V system will be 277V, meaning the L-N and L-G Voltage will both equal 277V near the Transformer (within 50 feet of the Transformer; as the length increases, Voltage Drops increase too).

For this system to be a solidly Grounded "Star Point Grounded" system, the L-G Voltage would not be so dramatically effected if the Ballasts were or were not connected.
In both cases, the L-G Voltage would measure 268V - 280V

Using a High Input Impedance Meter, the measured Open Circuit L-G Voltage of 160V to 178V, and the Closed Circuit L-G Voltage of 210V to 340V, definitely points to Capacitive Coupling effects being read by the High Z Meter.

Try testing L-G with a "Wiggy" to see if there is any Voltage indication.
Most likely, the Wiggy will indicate no Voltage measured L-G, but may show a rapid pulse, then "Drains Off" to indicate zero Voltage.
If connected L-L, the Wiggy will indicate 480V, with a nice solid pull from the Solenoid!


Per Question #5:
Please describe what the Transformer's Nameplate indicates.

If you have recorded what Terminals are involved per function, this information would be helpful too.

If Terminal "X0" is connected to the Grounding Electrode System, what are Terminals "X1", "X2" and "X3" connected to?
* A branch Circuit Breaker on the 208V system?
* A branch Circuit Breaker on the 480V system?

Most likely, what you have is an Ungrounded 480V Delta output, connected to 480V Ballasts.
Ungrounded systems will place large stresses between L-G, due to Capacitive Coupling, which is seen as very large L-G Voltages.
The Integrity of Conductor Insulation, as well as any components Insulation, may easily be exceeded, as it will be subjected to excessive Voltage levels.

For Lighting components, Ignitors and Capacitors may exhibit premature failure.
Ballast Windings may exhibit flash-over, or leak to enclosures.
Lamps and Lamp Sockets may show L-G damages, instead of L-L damages.

Good luck with the Troubleshooting!

Hope this post did not confuse you.

--Scott (EE)

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!