Test Instruments, Tool Care, and Shipboard Maintenance Practice

Switch the function to ac before the leads touch anything, and set the range switch to the highest voltage range. Then step down until the pointer sits near midscale, which is where the reading is most trustworthy .</details>

De-energize before any resistance reading

The circuit under test must be completely de-energized, and any component that ohmmeter current could damage must be removed before the measurement is made . De-energize and discharge the circuit before connecting a multimeter at all, and never apply power while resistance is being measured . For the megger, the equipment is de-energized and discharged both before and after the test .

Current is the measurement least often taken in routine testing, because the ammeter has to become an actual part of the equipment and the circuit must be opened to insert it. Ohm's law applied to a voltage reading usually gives the current instead .

Calibration status is read off the label

The calibration status of any item of test equipment is determined from the label or tag on the equipment .

• CALIBRATED — black on white; the instrument is within tolerance on all scales.
• CALIBRATED — REFER TO REPORT — red on white; used where the actual measurement values must be known to use the instrument.
• SPECIAL CALIBRATION — black on yellow; some unusual or special condition in the calibration is being drawn to your attention.
• USER CALIBRATION — you calibrate the instrument rather than sending it to a calibration facility.
• INACTIVE — CALIBRATE BEFORE USE — due for recalibration and not going to be used for some time.
• CALIBRATION NOT REQUIRED — listed in the Metrology Requirements List (METRL) as not requiring calibration.
• REJECTED — failed the acceptance criteria during calibration and cannot be repaired.
• CALIBRATION VOID IF SEAL BROKEN — placed over accessible adjustments where tampering would affect the calibration.

Recall and scheduling of test, measurement and diagnostic equipment into calibration facilities, and the documentation of what the facility did, is handled by the MEASURE system. Equipment configuration changes in the fleet are tracked by SCLSIS, whose two elements are validation and inventory updating, and standardized nomenclature comes from the JETDS system. GPETE covers instruments that can test two or more equipments of basically different design without modification; SPETE is designed for a single equipment or system.

Care of tools decides whether the tool is safe to pick up

Power tools are kept ready for use by keeping the housing intake and exhaust holes clear of dust and dirt, examining cords for exposed loose wires and damaged insulation, wiping cords frequently so oil and grease do not deteriorate them, checking plugs for loose prongs or cracked casings, never holding or dragging the tool by its cord, applying a light coat of oil to cutting surfaces to prevent rusting, and storing the tool in its designated container NAVEDTRA 14256 — Care of Electric Powertools.

For miscellaneous measuring tools the routine is to clean them thoroughly after use, apply a light coat of oil to all exposed metal parts against rust, and store them in proper locations so sensitive tools are not damaged NAVEDTRA 14256 — Care of Miscellaneous Measuring Tools.

Never use a damaged tool, and notify your supervisor of anything broken; a battered screwdriver may slip and spoil the screw slot or injure the user, and a gage strained out of shape produces inaccurate measurements NAVEDTRA 14256 — Where They Cannot Fall On the Floor or. Do not lay tools above machinery or electrical apparatus, where serious damage follows if the tool drops in once the equipment is running. With power tools: know the controls and features before operating, inspect portable tools before use, work only in adequate light, put the switch in OFF before connecting the tool to a power source, and give the tool your full and undivided attention. Hearing protection is required around tools capable of dangerously high noise levels NAVEDTRA 14256 — Ear Protection.

The general rules that carry across the shop: inspect tools and equipment for safe conditions before starting work, advise your supervisor promptly of unsafe conditions or practices, wear proper clothing and protective equipment, operate only equipment you are authorized to use, and report any injury immediately NAVEDTRA 14256 — Safety and Safety Equipment.

Telling it apart

Resistance instruments — separated by the voltage the instrument applies

• Ohmmeter (multimeter resistance function) — works from the meter's own source on a de-energized, isolated circuit and reads in ohms DOE-HDBK-1011 Vol.4 §14-1. Most often misfiled as the tool for proving a motor winding's insulation.
• Megohmmeter — applies 500–1000 V DC and reads megohms; the test for insulation and for locating grounds on motors, generators, cables and switchgear, with readings trended over time to catch gradual deterioration. Misfiled by candidates who expect a single number to be a pass or a fail.
• Wheatstone bridge — precise resistance measurement, not a substitute for either of the above NEETS Module 16 §1.7.

Reading current — separated by whether the circuit must be opened

• Ammeter in series — becomes part of the equipment under test, so the circuit is opened to insert it, and insertion resistance must be low.
• Clamp-on ammeter or CT — reads load current with the circuit intact and running, which is why it is the instrument named for large currents .

Preventive against corrective maintenance — separated by whether a fault has been found

• Preventive — checks to determine whether equipment is functioning properly, plus visual inspection of cabling and equipment for damage and to see whether lubrication is needed .
• Corrective — isolating troubles by test techniques and practices, then realigning, readjusting or otherwise bringing the equipment back up to proper performance.

Working a question

A 440 V bilge pump motor has been standing idle in a wet space and the engineer wants its condition before it is put back on the board.

1. Name the quantity. The question is insulation condition, measured as insulation resistance in megohms, so the instrument is the megohmmeter DOE-HDBK-1011 Vol.4 §14-1. Winding continuity would be a different measurement on a different function.
2. Check the instrument before the machine. Read the calibration label. CALIBRATED, black on white, means it is within tolerance on all scales; a yellow SPECIAL CALIBRATION label means read the condition attached to it before you trust the reading NEETS Module 16 §1.7.
3. De-energize, isolate and discharge the motor circuit. This is the same precondition as any resistance measurement, and for the megger it also applies after the test .
4. Select the test voltage in the 500–1000 V DC band, within the rating of the instrument and appropriate to the machine.
5. Connect winding to ground and read megohms. A low reading on one winding is how the ground is located.
6. Discharge the machine again before disconnecting the leads.
7. Compare against the recorded history for that motor. Readings are trended to catch gradual insulation deterioration, so the previous entries decide the interpretation, not the single figure in front of you.
8. Choose the next instrument deliberately. If the fault turns out to be an open winding rather than a ground, that is an ohmmeter measurement on the same de-energized circuit; if the motor runs and you want its load current, the clamp-on ammeter reads it without breaking the feeder.

Where candidates lose the point

• "Connect the ammeter in parallel." The voltmeter goes in parallel and candidates carry that across to the ammeter. The ammeter carries the current being measured, so it goes in series, and the circuit has to be opened to insert it NEETS Module 16 §4.6.
• "Set the range switch to the range closest to the expected value." It sounds like good practice, and it is how a wrong option is written. Start on the highest range, then step down for a midscale reading.
• Using the ohms function to certify motor insulation. The ohmmeter reads resistance from the meter's own source; insulation resistance is a megohm reading taken at 500–1000 V DC by the megger, and the megger is the instrument named for locating grounds DOE-HDBK-1011 Vol.4 §14-1.
• Treating volts times amps as power. The wattmeter reads true power because it combines a current coil and a voltage coil, and therefore accounts for power factor.
• Confusing two calibration labels. An instrument that is due for recalibration and will not be used for a while carries INACTIVE — CALIBRATE BEFORE USE. REJECTED is narrower: it failed acceptance criteria during calibration and cannot be repaired NEETS Module 16 §1.7.
• Oiling the wrong thing. All exposed metal parts get a light coat of oil on miscellaneous measuring tools NAVEDTRA 14256 — Care of Miscellaneous Measuring Tools. On power tools the oil goes on cutting surfaces, and cords are wiped precisely to keep oil and grease off them NAVEDTRA 14256 — Care of Electric Powertools.
• Keeping a strained gage in service because it still closes on the work. It will give inaccurate measurements, and damaged tools are reported to the supervisor and returned, not used NAVEDTRA 14256 — Where They Cannot Fall On the Floor or.

Check yourself