Systematic Troubleshooting of Industrial Power Systems

When power is lost in part of a plant, the first hour decides how the rest of the day goes. People who guess reset breakers and swap parts, and sometimes they create a second fault on top of the first. People who work a system ask what the fault must be for the evidence to make sense. This covers that method on the kinds of faults a senior or lead electrician gets called for: feeders that trip, phases that disappear, ground alarms, and failures that keep coming back.

What You Will Be Able to Do

• Work a fault in a fixed order, from evidence to root cause.
• Use a one-line diagram to narrow a fault by halves.
• Read a trip unit and decide whether a breaker can be reclosed.
• Read a primary phase loss from secondary voltages.
• Find ground faults on solidly grounded, resistance grounded and ungrounded systems.
• Stop when the normal method no longer applies.

Warning: Voltage readings on live gear are energized work. They are for qualified people, with a rated meter, the personal protective equipment (PPE) for that equipment and your plant's procedure. Lock out, prove dead and release stored energy before you put hands on conductors. Do not use a jumper to defeat a protective device.

The Method

1. Make it safe. Decide if anyone is at risk now. If there is smoke, heat or an injured person, that comes first.
2. Gather evidence before you change anything. Reclosing a breaker can erase the clue.
3. Define the fault. What failed, what still works, when did it start, and what changed?
4. Form a theory. Name what would explain everything you see.
5. Predict a reading. Say what the meter should show if the theory is right.
6. Test by halves. Use each reading to remove half of the possibilities.
7. Repair the cause. Replace what failed, and fix why it failed.
8. Prove it and write it up. Return it to service by your procedure, and record what you found.

Step 2 is where senior people differ. Take photos of the trip unit, write down the time, look at alarms, check the drive logs and talk to the operator first.

Using a One-Line Diagram

The one-line diagram is your map. Use it to find the common point.

If one motor is down, the fault is usually at that motor or its branch. If everything on a motor control center (MCC) is down, look at the common feed. If a whole unit substation is down, go higher. The fault is at or above the last point that all the failed equipment shares.

Worked Example: Finding the Common Point

Three machines stop at once. They are on three different panels. The one-line shows those panels fed from one 480 to 208Y/120 transformer. Other 480-volt loads from the same switchboard are still running.

1. The common point of what failed is the transformer or its feeder.
2. The switchboard is fine, because other loads still run.
3. Check the primary device for the transformer first.
4. If the primary device is closed, a qualified person measures the secondary at the first panel.

That is two checks, rather than three machine calls.

Old drawings lie. Walk the path where you can, and mark up what is wrong so the next person gets a better map.

Reading a Trip

Modern breakers often record why they tripped. Learn to read the letters, which are usually L, S, I and G.

An L trip with a high load says find the load. An I trip says there is a real fault, so you do not reclose until it is found.

Before You Reclose

Do not reclose a breaker that tripped on a fault until you know why. If it tripped on a large fault, it may have damaged itself. Closing into a fault again can cause an arc flash.

Ask these questions:

1. What did the trip unit show?
2. Are there signs of damage, smell or discoloration?
3. Has a qualified person inspected the circuit and tested insulation, with the circuit locked out?
4. After a high-fault trip, does the breaker need inspection or testing before it goes back into service?

Reading Phase Voltages

A Lost Phase on the Primary

Line-side phase loss on a delta-wye transformer gives a strange pattern. Each secondary phase depends on one primary winding. With one primary phase gone, one winding still sees full voltage, and the other two share it in series.

At light load, you see something like this on a 208Y/120 secondary:

One normal phase and two near half is the fingerprint of a lost primary phase. It sends you to the primary fuses, not the secondary. Under load the numbers shift, so treat them as a pattern and not exact figures.

Other Patterns

Keep in mind that a motor can back-feed a lost phase. A running three-phase motor can generate voltage on the missing phase, so a meter may show something close to normal when a phase is actually open. Measure with loads off where you can, or check current as well as voltage.

Finding Ground Faults

The method depends on how the system is grounded. Know that before you start.

Solidly Grounded Systems

A ground fault causes a large current, so a device trips. Find it by testing insulation, with the circuit locked out and electronics disconnected. Split the circuit into sections and test each with an insulation tester (megger), starting with the point closest to where it failed.

High-Resistance Grounded Systems

In a high-resistance grounded (HRG) system, a resistor between the neutral point and ground limits ground fault current to a few amps. A single ground fault raises an alarm, and the plant keeps running. NEC 250.36 allows these systems from 480 to 1,000 volts, with conditions. They include qualified people maintaining the system, ground detection, and no line-to-neutral loads.

Many HRG systems have a pulsing feature. It switches the fault current on and off in a steady beat. A qualified person uses a clamp meter on each feeder, following the pulse to the faulted circuit.

The alarm does not mean you can ignore it. A second ground fault on a different phase is a phase-to-phase fault through the ground path, and that one will trip breakers, often in two places. Find the first fault within the time your procedure sets.

Ungrounded Systems

An ungrounded delta has no intentional ground. A first fault does not trip anything. Ground detection lamps or a meter show it by the voltage to ground. The faulted phase reads near zero to ground, and the other two rise to line voltage. Finding it means shutting off feeders one by one, in a planned way, and seeing when the indication clears.

A warning for the last two: a voltage to ground reading on a resistance grounded or ungrounded system can be misleading. Phases can sit at full line voltage to ground during a fault. Check the rating of your meter and your approach.

Intermittent Faults

Some faults leave no trace by the time you arrive.

• Write down each event, with the time, the temperature, the load and what was running.
• Look for a pattern. Same time each day often means a process, a schedule or a utility capacitor switching. Hot afternoons point to heat.
• Leave a recorder on it. Use a power quality analyzer or a data logger.
• Scan with infrared under load. Compare the three phases to each other, because like against like shows problems best.
• Look for vibration, which can make a loose lug open and close.

Do not close the call because the fault went away by itself. It will come back.

Finding the Root Cause

Replacing a failed part fixes today. A lead asks why it failed. Keep asking until the answer is something you can change.

Worked Example: A Burned Lug

A 400-amp feeder lug burns off.

1. Why did it burn? It was hot.
2. Why was it hot? The connection was loose.
3. Why was it loose? It was never torqued after the last job, and the feeder cycles from heavy loads to none.
4. Why was it not torqued? The work order had no torque step.
5. What will you change? Add a torque step, check the other lugs on that panel, and add the panel to the thermal scan route.

Replacing the lug alone leaves every other lug on that job in the same state.

When the Normal Method Does Not Apply

Stop and get engineering or specialist help in these cases:

• The fault is on medium-voltage equipment you are not qualified for.
• The problem involves protective relays or coordination settings.
• Equipment shows signs of an internal arc.
• The result could change the available fault current or arc flash labels.
• Evidence does not fit any theory after a fair amount of testing.

Saying "this needs someone else" is part of the job.

Common Mistakes

Check Your Understanding

1. A 208Y/120 panel reads 118, 61 and 59 volts to neutral at light load. Where do you look?

Upstream on the primary. One normal phase and two near half points to a lost primary phase on a delta-wye transformer.

2. A feeder breaker tripped on instantaneous, and production wants it closed. What do you do?

Do not reclose yet. An instantaneous trip means a real fault. Lock out, inspect and test the circuit first.

3. A ground alarm appears on an HRG system, and the plant is still running. Is it safe to leave it until the weekend?

Treat it as urgent, and follow the plant's time limit. A second fault on another phase can trip feeders. Use the pulsing feature to find the circuit.

4. A breaker trips every weekday around 7:15 a.m. What do you check first?

What starts at that time. Compare trip records with process schedules and record the feeder current across that period.

5. Three lugs on the same panel show the same heat pattern on a scan. What does that suggest?

A workmanship or procedure problem, not one bad part. Plan an outage, re-torque, and find out how that panel was installed.

Key Takeaways

• Gather evidence before you change anything, including before you reset.
• Use the one-line to find the common point, then test by halves.
• Do not reclose on an unexplained fault.
• One normal phase and two near half points to a lost primary phase on a delta-wye.
• Know how the system is grounded before you hunt a ground fault.
• Keep asking why until you find something you can change.