The 480V 3 Phase System may be an Ungrounded Delta, or a Center-Tap Grounded Delta.
From the description of how the Contactor drops out, I am thinking the System is some type of Delta, which does not have a Grounded Conductor.
Warrant the services of a Qualified Electrician, and have that Person perform the following:
To verify if the System is an Ungrounded Delta, or a Corner Grounded Delta, take Voltage readings at either the Service Equipment, Panelboard, or Disconnect Switch.
Note: Disconnect the Contactor's Coil Leads prior to performing the Voltage Measurements.
Disconnect both Coil Leads - the Line Side Lead, and the Lead attached to the Metallic equipment.
If there are other connections of this type (L-G), disconnect them also (Both "Sides" of the Load).
Voltage Meters needed:a: (1) Low Input Impedance Voltmeter: A "Wiggy", or Solenoid type Voltage Meter.
b: (1) High Input Impedance Voltmeter: A typical DVM (Digital Voltage Meter), such as the Standard Fluke Meter will work.
Voltage MeasurementsStep#1: Verify the Enclosure is solidly Grounded / Bonded to the Local Grounding Electrode System (GES). The Service Equipment is the best place to take measurements, as the Metallic Enclosures will (likely) be Bonded to the GES... unless someone has disconnected the Grounding Electrode Conductor (GEC) for some odd reason.
Step#2:With a Grounded Enclosure verified, perform Three separate Line-To-Ground (L-G) voltage test with the Low Impedance Voltmeter (Low Z Voltmeter).
Test between:
- Line (Phase) A and Ground,
- Line (Phase) B and Ground,
- Line (Phase) C and Ground.
If the Low Z Voltmeter shows no Voltage on all three of these tests, the System most likely is an Ungrounded Delta.
If the Low Z Voltmeter shows apx. 480V on two of the three tests, and one test shows no Voltage, the System is a 480V Corner Grounded Delta.
The Line with Zero Volts to Ground is the System's Grounded Conductor - and should be Color Coded "White" or "Gray".
If the Low Z Voltmeter has a "Solid" Voltage reading on all three L-G tests, the System is a Center-Tapped 480V 3 Wire Delta.
One L-G Voltage reading will be considerably higher than the other two L-G Voltage readings.
Step#3: With the High Impedance Voltmeter (Hi Z Voltmeter), perform the same Three L-G Voltage tests, as were performed with the Low Z Voltmeter:
Test between:
- Line (Phase) A and Ground,
- Line (Phase) B and Ground,
- Line (Phase) C and Ground.
If the System is a 480V 3 Phase 3 Wire UNGROUNDED DELTA:a: The Hi Z Voltmeter will display a Voltage of apx. 150 to 400 Volts between L-G on all three test connections (Line A, B and C to Ground),
&
b: The Low Z Voltmeter will display Zero Volts (or close to Zero Volts) between L-G on all three test connections.
This type of System does not have a Grounded Conductor, but Metallic Enclosures WILL be Bonded to a Local GES.
-------------------------------------------------
If the System is a 480V 3 Phase 3 Wire CORNER-GROUNDED DELTA:a: The Hi Z Voltmeter will display a Voltage of apx. 480 Volts between L-G on two test connections, with Zero (or close to Zero) Volts between the third L-G Test Connection;
&
b: The Low Z Voltmeter will display a Voltage of apx. 480 Volts between the same two L-G test connections as the Hi Z Meter displayed, with Zero (or close to Zero) Volts between the third L-G Test Connection.
This type of System DOES have a Grounded Conductor - the Line with Zero Volt reading to Ground is the Grounded Conductor.
This Phase Line, along with Metallic Enclosures WILL be Bonded to a Local GES.
Color Code for the Grounded Conductor is either White or Gray.
The Equipment Grounding Conductors may be Bare, Green, or simply the Raceway (Conduit).
-------------------------------------------------
If the System is a 480V 3 Phase 3 Wire CENTER-TAP GROUNDED DELTA:a: The Hi Z Voltmeter will display a Voltage of apx. 240 Volts between L-G on two test connections, with Apx. 415 Volts between the third L-G Test Connection;
&
b: The Low Z Voltmeter will display a Voltage of apx. 240 Volts between the same two L-G test connections as the Hi Z Meter displayed, and "Slightly Lower Than 480 Volts" between the third L-G Test Connection.
The Pointer will be low in the "480V" Field, or high in the "240V" Field.
This type of System DOES NOT have a Grounded Conductor.
The Transformer has one Winding Grounded, via a tap at the Center of the Winding's length.
Metallic Enclosures WILL be Bonded to a Local GES, as will the Center Tap (most likely Grounded at the Transformer + Service Equipment, but sometimes the Grounding is at the Utility Transformer only).
-----------------------------------------------------
NOTE: All the above tests will display 480V between any two Lines (Phases), using either the High Z Meter or the Low Z Meter...
i.e.:
- Phase A and Phase B: 480V,
- Phase A and Phase C: 480V,
- Phase B and Phase C: 480V.
..................................................
..................................................
The reason I am leaning towards a System without a Grounded Conductor, is the way the Contactor drops out randomly.
The System might be an Ungrounded Delta, or a Center-Tapped Delta.
If the System is Ungrounded, Capacitive Coupling between the System and Grounded Enclosures will exist.
This is what gave the Hi Z Voltmeter a reading between each Line and Ground, while on the Low Z Voltmeter, there was no displayed Voltage reading.
The Higher the Impedance between L-G, the Higher the Voltage is between those two points.
As the Coupling effect changes (Impedance is reduced), so is the Voltage between the two points.
For a Contactor Coil, if it has latched in, it will attempt to hold until the Voltage across it becomes too low to develop Magnetic Flux, so it drops out.
If the Coupling Effect does not allow enough Current to flow, the Coil will not engage the Contactor.
Having more than one L-G Connections on the 480V side, would easily allow for a scenario like this.
------------
If the System is a Corner-Grounded Delta:
A 480V rated Coil will function properly, but with one side directly Ground Bonded at the Machine, Currents will be flowing through the Grounded metallic Enclosures & Raceways between the machine and the Service Equipment.
This needs to be corrected.
If the Coil's rated Voltage is less than 480V (240V, 277V, 380V), eventually Smoke will ooze out from the Coil, and it will cease to work.
----------
If the System is a Center-Tap Grounded Delta:
A Coil rated for 440-480V is "Likely to Work" if connected between Line "B" and Ground, as this will result in +/- 416V across the Coil.
If / when the L-G Voltage drops below 360V, the Coil should drop out.
If the 440-480V rated Coil is connected between Line "A" and Ground, or Line "C" and Ground, the Coil "may" function "Randomly", according to the Voltage across the Coil.
Since the Voltage would be around 250V Nominal, the coil might latch in during a rise in Voltage (Voltage and Current Spike or Surges).
For large size Contactors, this would be highly unlikely, as the required Inrush Volt-Amps (VA) to pull-in and latch the Contactor are at least 10X the required VA for holding in a latched state.
------------
Nevertheless, please have the system checked ASAP, by a qualified person.
-- Scott