Electrical Fault Isolation and Troubleshooting Procedure

What the rule requires

Maintenance divides into two categories, and one question separates them: has the equipment already failed? Preventive (routine) maintenance is the mechanical, electrical and electronic checks made to determine whether equipment is operating properly, along with visual inspection of cabling and equipment for damage and to determine whether lubrication is needed. Corrective maintenance isolates the failure by means of test techniques and practices, replaces defective parts, and realigns or readjusts the equipment to bring it back to proper performance. NEETS Module 16 §1.5

Troubleshooting is not simply the repair. You must also evaluate equipment performance, both before and after the repair, by comparing how the equipment should operate against how it is actually performing. Performance standards books for each piece of equipment carry that comparison data — what a waveform should look like at a given test point, what amplitude a voltage should be — measured against the baseline, meaning the initial operating conditions on installation or after overhaul when the equipment was operating according to design.

A logical and systematic approach is of the utmost importance. Many hours have been lost to hit-or-miss methods, which the trade calls easter-egging.

The order of work

Any repair of electric or electronic equipment follows a fixed sequence. The first four steps of the six-step procedure are where the exam lives, because they are the steps that get skipped: NEETS Module 19 Ch. 1 — General Use ¶1

1. Symptom recognition — recognising a disorder or malfunction in the equipment.
2. Symptom elaboration — obtaining a more detailed description of the trouble symptom.
3. Listing probable faulty functions — from what you have gathered, where could the trouble logically be? This step applies only to equipment containing more than one functional area or unit.
4. Localizing the faulty function — determining which of the functional units is actually at fault.

Step 3 carries a trigger. On a single-unit item there is nothing to list, and a question that offers "list probable faulty functions" for a one-unit device is offering you a step the procedure itself makes conditional.

Isolate before you open anything

Electrical and electronic circuits often have more than one source of power. Study the schematics or wiring diagrams of the entire system to ensure that all sources have been disconnected. NEETS Module 16 §1.5.5

• All circuit breakers and switches from which power could possibly be supplied are secured — locked if possible — in the OPEN or OFF position and danger tagged.
• The tag is removed only by the same person who signed it when the work began.
• Fuses are removed and replaced only after the circuit has been de-energized, and the replacement is the same type with the same current and voltage ratings. Use a fuse puller for cartridge fuses.
• Safety devices — interlocks, overload relays, fuses — are never altered or disconnected except for replacement, and never modified without specific authorization.
• In wet or damp locations, put a rubber mat or other nonconductive material on top of a dry wooden platform or stool, and use insulated tools and molded insulated flashlights on exposed parts.

Repairs on energized circuits must not be made with primary power applied except in an emergency, and then only after specific approval. When approval has been given, keep one hand free at all times, behind you or in your pocket. Never use a finger to test a hot line; use an approved voltmeter or other voltage-indicating device. NEETS Module 4 §3.5.3

Grounding is the precaution most often skipped aboard ship. Unless a chassis is physically grounded to the hull, it can float several hundred volts above ship's ground, and a man in contact with both the chassis and the hull becomes a low-resistance path back to the ship's generators. Navy electronic equipment is therefore always grounded to the hull, and approved rubber mats are required in every space where electronic equipment is present. NEETS Module 6 — Capacitor to Be Tested With the Power Off Before ¶10

Two checks: visual, then signal tracing

The visual check comes first and it splits at the moment you apply power. Before energizing, look for shorts — any terminal or connection close to the chassis or to another terminal, stray drops of solder, bits of wire, nuts or screws; shake the chassis and listen for rattles. A discolored or leaking transformer is a sure sign of a short somewhere, and if the fuse did not blow, find out why: too large a size may have been installed, or there may be a short across the fuse holder. Loose, broken or corroded connections get fixed. A resistor that is discolored or charred has been subjected to an overload; an electrolytic capacitor shows a whitish deposit at the seal around the terminals. NEETS Module 7 — Test Equipment You Are Using Is Properly Grounded and That ¶1

After energizing, only two things are on the list. Smoking parts or boiling or sputtering sounds mean power comes off immediately and you go back to the ohmmeter starting near the smoking part. Tap or shake the chassis: sparking means a loose connection or a short.

Signal tracing follows if the visual check turns up nothing, and it is the most rapid method of locating a trouble you cannot see. Observe the waveform at the input and output of each part of the circuit. The point where the signal stops or becomes distorted is the place to look for the trouble. No dc output voltage points to an open or a short; low dc voltage points to a defective part.

Taking the measurement

Voltage measurements can be made as part of either preventive or corrective maintenance, and you can take them without interrupting circuit operation. Point-to-point voltage charts in the equipment technical manual give the normal operating voltages, measured between the indicated point and ground unless stated otherwise. Set the voltmeter on the highest range first so that excessive voltage does not overload the meter, then step down to the range appropriate for comparison with the chart. Make sure the safety observer knows where to secure power, connect the ground lead first, and use one hand where possible. NEETS Module 16 §1.6.1

At 300 volts or more the procedure changes completely and the order is examinable:

1. Shut off circuit power.
2. Discharge all filter capacitors with a shorting probe.
3. Temporarily ground the point to be measured.
4. Clip the proper test lead onto the high-voltage point.
5. Move away from the voltmeter.
6. Turn on circuit power and read the voltmeter.
7. Turn off circuit power.
8. Discharge all capacitors before disconnecting the meter.

Current measurements are rarely taken in preventive maintenance or testing, because the ammeter must become an actual part of the equipment: the circuit has to be opened, usually desoldered, to connect it in series. Take a voltage measurement instead and calculate current by Ohm's law.

You are asked to record 450 volts dc at a rectifier output. Your first action?

Shut off circuit power. Anything at 300 volts or more goes through the de-energized sequence — power off, filter capacitors discharged with a shorting probe, point temporarily grounded, lead clipped on, then step away before energizing to read. Placing a probe on the point with power applied is the procedure for less than 300 volts only.

Telling it apart — which instrument, and whether the circuit may be live

The criterion is what the meter does to the circuit it is connected to.

• Voltmeter — measures voltage, connected across the circuit, and reads with power applied. Its own internal resistance loads the circuit, which is why loading effect is minimized by using a voltmeter whose internal resistance is many times higher than that of the circuit being measured. NEETS Module 3 — Appendix a
• Ammeter — measures current, and must be inserted in series, so the circuit must be broken to fit it. Sensitivity for an ammeter means the current that causes full-scale deflection.
• Ohmmeter — measures resistance and checks continuity, and cannot be used with power applied. Disconnect one end of a resistor before measuring it whenever a parallel path may be connected across it. NEETS Module 6 — Equipment You Are Using Is Properly Grounded and That the ¶2
• Megohmmeter (megger) — measures very large resistance values and is the instrument for insulation breakdown in wires, because it applies a high voltage as it reads resistance. That is what finds a transformer short which only appears when operating voltages are present.
• Multimeter — one instrument combining ammeter, voltmeter and ohmmeter functions.

Working a question

A reefer box is running warm. The space temperature is above the thermostat setting and the cooling coil magnetic valve is not opening. Work it in order rather than pulling the solenoid off the valve body first.

1. Symptom elaboration. Read the temperature carefully at the sensing element and check it against a reliable thermometer. If the thermostat is simply badly located, the remedy is to relocate it to a place more representative of average space temperature; if it is out of adjustment or sticking, calibrate against a good thermometer and clean, adjust or replace it. NAVEDTRA 14075 Ch. 6 — Test Remedytrouble Possible
2. Localize to the unit. With the thermostat proved good and its contacts closed, the fault lies in the solenoid valve — either a sticking valve or a failed coil.
3. Visual inspection first. Check the connections for poor soldering, loose connections or broken wires. Check the plunger for cleanliness, binding, mechanical failure and improper alignment, and check the mechanism the solenoid actuates for proper operation. NEETS Module 3 — Button ¶1
4. Check the energizing voltage with a voltmeter. Low voltage means less coil current and a weak magnetic field, which shows up as slow or poor operation, chatter, or no operation at all. Voltage too high can overheat the coil or cause arcing. Either way, reset the voltage to its proper value.
5. Check the coil with an ohmmeter for opens, shorts and proper resistance. An open coil passes no current and the field is lost. A short means fewer turns and higher current, and the net result is still a weak field. A high-resistance coil also reduces coil current and weakens the field. Any of the three — open, shorted, or changed in resistance — and the coil is replaced.
6. Check the coil for a short to ground. If one is found, remove it to restore proper operation.
7. Remedy. For a failed coil, replace the solenoid coil; for a sticking valve, clean the valve or adjust the pilots.

The decision that matters is at step 4. A weak field caused by low supply voltage and a weak field caused by a shorted coil produce the same complaint at the box, and the only thing that separates them is a voltmeter reading at the coil terminals before the ohmmeter comes out.

Where candidates lose the point

Setting the range switch to the value closest to the expected voltage. It reads as the careful, accurate choice, and it is how you finish — but not how you start. Highest range first, then down to the appropriate range for comparison with the chart. NEETS Module 16 §1.6.1

Clearing a component because the voltage across it read normal. When the probes go across an open resistance, the meter's own resistance completes a circuit path, so the reading can look normal or near-normal while the true circuit condition is abnormal. If the internal resistance of the voltmeter is about the same value as the resistance under test, the meter indicates considerably lower than the voltage actually present.

Fitting a larger fuse "until the cause of the overload is found." The replacement is the same type with the same current and voltage ratings, fitted after the circuit is de-energized. Blocking, shorting or upsizing a protective device is the same error as blocking open an interlock switch. NEETS Module 16 §1.5.5

Letting the relieving watch pull the danger tag. Only the person who signed the tag removes it. Not the job inspector, not the crew leader.

Passing a transformer on an ohmmeter check. An open winding or a winding shorted to the casing will show on a continuity check, but if turns of one winding are shorted together you may not be able to detect any difference in winding resistance. Shorts that appear only under operating voltage are found with a megger. NEETS Module 6 — Equipment You Are Using Is Properly Grounded and That the ¶2

Filing "alignment after a repair" as preventive maintenance. Realigning or readjusting equipment to bring it back to proper performance is corrective work by definition; routine lubrication and mechanical inspection are the preventive side. NEETS Module 16 §1.5

Check yourself

You have opened the breaker feeding a control panel and tagged it. Are you clear to work?

No. Electrical and electronic circuits often have more than one source of power. Study the schematics or wiring diagrams of the entire system to ensure that all sources have been disconnected, and secure and danger tag every breaker and switch from which power could possibly be supplied. NEETS Module 16 §1.5.5

A shipmate says 115 volts is not enough to hurt anyone. Correct him with a number.

A fatal shock can occur from 0.1 ampere of current, and voltages as low as 30 volts have been recorded as causing sufficient current to be fatal. All live electrical circuits are treated as potential hazards at all times. NEETS Module 1 Ch. 3 — Summary ¶2

Which diagram do you use to find the test points for voltage and resistance checks?

The wiring diagram, used in conjunction with the schematic. The wiring diagram gives all wiring, connectors, terminal boards and components with wire numbers or color coding; the schematic shows the electrical connections and functions and is what you use to identify possible circuit malfunction locations. A terminal diagram is for connecting wiring to terminal boards, relays and switches. NEETS Module 4 §3.6 ¶1

A capacitor reads 300 kilohms on an ohmmeter. Verdict?

More than likely bad. A capacitor suspected of being leaky is checked with an ohmmeter, and a reading of less than 500 kilohms condemns it. Because capacitor troubles may be intermittent or appear only under operating voltage, the best proof is substitution with a known-good unit. NEETS Module 6 — Equipment You Are Using Is Properly Grounded and That the ¶2

You need the current drawn by a stage and the circuit is fully soldered. What do you do?

Take a voltage measurement and calculate the current with Ohm's law. Current readings are rarely taken during preventive maintenance or testing because the ammeter must become an actual part of the equipment and the circuit has to be opened to put it in series. NEETS Module 16 §1.6.1

A solenoid valve chatters instead of pulling in cleanly. Where do you look before condemning the coil?

Visual inspection, then the energizing voltage. Chatter, slow operation or no operation at all can be caused by low energizing voltage producing a weak magnetic field, so measure at the coil with a voltmeter before going to the ohmmeter for opens, shorts and proper resistance. NEETS Module 3 — Button ¶1

You have found and repaired a loose connection during the visual check. Is the job finished?

No. Note what you found and what you did, then prove the equipment is operating properly and that no other troubles exist. A good technician takes nothing for granted, and you must evaluate equipment performance both before and after the repair. NEETS Module 7 — Test Equipment You Are Using Is Properly Grounded and That ¶1