Troubleshooting DSP MKII Ground-Fault Detection Failure

Removing the unintended neutral-to-ground path restores the designed resistor current, phase-voltage shift, and DSP MKII ground-fault indication. On this 600/347 VAC, three-wire system, the decisive symptom is about 5 A in the test ground but only 0.3–0.5 A through the grounding resistor: most fault current is returning by another path.

Reject the quick fixes first

Do not start by replacing the DSP MKII. It measures the ground-fault current presented to it; it cannot alarm correctly when an unintended bond diverts that current around the intended sensing path. Likewise, adjusting an alarm threshold would hide the grounding defect and leave the system vulnerable to two faults on different feeders.

Do not treat the missing voltage shift as a meter problem until measurements at the transformer neutral, grounding resistor, and test connection agree. A single ground on a correctly operating high-resistance-grounded system displaces the neutral. A near-solid phase-to-ground fault drives the two healthy phase-to-ground voltages toward the 600 V line-to-line value. If all three remain near 347 V, another connection is holding the neutral near ground potential.

Proceed only after recording all three phase-to-ground voltages, grounding-resistor current, and test-path current from the same test condition.

Prove the intended grounding path

The transformer has a 600 V wye secondary with its neutral connected through a 69 ohm resistor to the ground grid. No neutral conductor is distributed with the three-phase feeders. The nominal phase voltage is directly derived:

V_phase = 600 V / sqrt(3) = 346.4 V
I_ground = 346.4 V / 69 ohm = 5.02 A

That calculation matches the stated 5 A ground-current limit. Before injecting another test fault, prove that the physical circuit still matches the one-line diagram.

1. De-energize and apply the site lockout procedure.
2. Inspect the transformer neutral connection, both grounding-resistor terminals, grounding electrode conductor, and ground-grid termination.
3. Check the resistor for visible damage, contamination, loose joints, or a conductor connected to the wrong side.
4. Measure the resistor and bonding path using a method suitable for the equipment. Account for parallel paths before interpreting an in-circuit resistance reading.
5. Confirm that the DSP MKII sensing element surrounds or measures the intended conductor. A conductor routed outside the sensor can carry fault current without being detected.

Do not continue with energized injection if the grounding resistor, its conductor, or its sensing arrangement fails inspection. The check passes when the transformer neutral has one intentional connection to ground through the 69 ohm resistor and the DSP MKII monitors that path.

Apply one controlled test fault

Use the existing 69 ohm test resistor only under an approved energized-test procedure. Connect it with the system de-energized, use equipment rated for 600 VAC, establish the restricted boundary, then energize remotely if the approved procedure permits it.

With a 69 ohm test resistor in series with the 69 ohm grounding resistor, the expected current is:

I_test = 346.4 V / (69 ohm + 69 ohm) = 2.51 A

The same current must pass through both resistors when no alternate path exists. This equal-resistor test does not produce the same voltage displacement as a solid phase-to-ground fault, so do not require both healthy phases to read exactly 600 V during this particular test. Use current equality as the primary acceptance criterion and record all phase-to-ground voltages as supporting evidence.

1. Measure current in the test-ground conductor.
2. Measure current in the transformer grounding-resistor conductor at the same time.
3. Record all three phase-to-ground voltages.
4. Verify the DSP MKII indication and alarm output.
5. Remove the test connection after the approved test interval.

If the test conductor carries near 5 A while the grounding resistor carries only 0.3–0.5 A, stop testing the relay. The current balance proves that an alternate return path must be isolated next.

Divide the plant at the feeders

Because the feeders carry no neutral, a neutral-current search is not available at the MCCs. Divide the system by feeder and watch for restoration of normal grounding behavior. Prioritize equipment added or modified before the symptom appeared, including the Schneider Electric AV9000 real-time reactive compensation unit, transformer-fed 480 V machinery, drives, robot and servo equipment, and control transformers.

1. Schedule the work with the plant stopped; previous testing caused shutdowns.
2. Open one feeder or equipment group at a time using the approved switching procedure.
3. After each isolation, repeat the controlled test and compare test-path current with grounding-resistor current.
4. When the two currents return to approximately the calculated 2.51 A and the voltage displacement returns, leave that branch isolated.
5. Subdivide the identified branch at MCC buckets or equipment disconnects until one load or transformer remains.

Main feeder loads of 100–300 A can mask a small injected signal on phase conductors. Individual loads at 30 A or less provide better resolution after the search reaches the suspect MCC. The feeder check passes when opening one branch removes the alternate current path reproducibly.

Find and remove the unintended bond

Inspect the isolated branch for any connection that references the monitored 600 V system to ground without passing through the transformer grounding resistor. Common targets are a bonding screw left installed, a grounded wye point in newly connected equipment, an incorrectly landed transformer primary, wiring crossed between isolated systems, or insulation failure joining a primary conductor to a grounded secondary circuit.

The grounded wye secondaries serving lighting and office loads are intentional. Their transformer primaries are delta-connected to the monitored system, so the secondary neutral bond must remain galvanically isolated from the 600 V primary. Do not remove a secondary bond merely because it is grounded; test isolation across the transformer and inspect terminations to locate an actual crossover.

Do not increase test current to make a pulse easier to see. A pulsed tracing test using regular 0.5–1 second on and 0.5–1 second off intervals can help identify a branch, but only with an engineered switching assembly, the existing current-limiting resistor, rated instruments, and an approved procedure. Never use a person operating an exposed switch as the timing device.

After removing the suspect bond or correcting the wiring, verify isolation while de-energized, then repeat the controlled current comparison. Both resistors must carry the same test current before reconnecting the branch.

Verify the complete protection chain

1. Restore the corrected feeder configuration and confirm that no neutral conductor has been added to the three-wire distribution.
2. Measure normal phase-to-ground voltages; each should be near the system’s stated 347 V value, subject to normal meter and system variation.
3. Apply the 69 ohm test fault through the approved test connection.
4. Confirm approximately 2.51 A in both the test resistor and transformer grounding resistor.
5. Confirm repeatable phase-voltage displacement rather than three voltages fixed near 347 V.
6. Confirm that the DSP MKII indicates the ground fault and operates the intended alarm path.
7. Remove the test fault and confirm that indication, current, and voltages return to normal.

Do not return the system to service solely because the alarm operates once. Repeat the test from the normal operating lineup and document the phase voltages, both measured currents, alarm response, isolated branch, and corrected connection.

FAQ