WORKING ON DE-ENERGIZED CIRCUITS

When any electronic equipment is to be repaired or overhauled, certain general safety precautions should be observed. They are as follows:

Remember that electrical and electronic circuits often have more than one source of power. Take time to study the schematics or wiring diagrams of the entire system to ensure that all sources of power have been disconnected

• If pertinent, inform the remote station regarding the circuit on which work will be performed.
• Use one hand when turning switches on or off.
• Safety devices, such as interlocks, overload relays, and fuses, should never be altered or disconnected except for replacement. In addition, they should never be changed or modified in any way without specific authorization.
• Fuses should be removed and replaced only after the circuit has been de-energized. When a fuse "blows," the replacement should be of the same type and have the same current and voltage ratings. A fuse puller should be used to remove and replace cartridge fuses.
• All circuit breakers and switches from which power could possibly be supplied should be secured (locked if possible) in the OPEN or OFF (safe) position and danger tagged in accordance with procedures in the Standard Organization and Regulations of the U.S. Navy, OPNAVINST 3120.32.
• After the work has been completed, the tag (or tags) should be removed only by the same person who signed it (them) when the work began.
• Keep clothing, hands, and feet dry if at all possible. When you must work in wet or damp locations, place a rubber mat or other nonconductive material on top of a dry, wooden platform or stool; then use the platform or stool to sit and stand on. Use insulated tools and insulated flashlights of the molded type when you are required to work on exposed parts.

GROUNDING OF POWER TOOLS AND EQUIPMENT

The possibility of electrical shock can be reduced by ensuring that all motor and generator frames, metal bases, and other structural parts of electrical and electronic equipment are at ground potential.

Normally, on steel-hull vessels, such grounds are inherently provided because the metal cases or frames of the equipment are in contact with one another and with the metal structure of the vessel. In some instances where such inherent grounding is not provided by the mounting arrangements, such as equipment supported on shock mounts, suitable ground connections must be provided.

The grounding wire used for this purpose is generally made of flexible material (copper or aluminum) that provides sufficient current-carrying capacity to ensure an effective ground. In this manner, equipment cases and frames that are not intended to be above ground potential are effectively grounded; also, the possibility of electrical shock to personnel coming in contact with metal parts of the equipment is minimized. The secondary purpose of grounding equipment is to improve the operation and continuity of service of all equipments.

Paint, grease, or other foreign matter can interfere with the positive metal-to-metal contact at the ground connection point. Therefore, all bonding surfaces (connection points or metallic junctions) must be securely fastened and free of such matter. In all instances where equipment grounding is provided, certain general precautions and preventive maintenance measures must be taken.

A few of these precautions are listed below:

• Periodically clean all strap-and-clamp connectors to ensure that all direct metal-to-metal contacts are free from foreign matter.
• Check all mounting hardware for mechanical failure or loose connections.
• Replace any faulty, rusted, or otherwise unfit grounding strap, clamp, connection, or component between the equipment and the ground to the ship hull.
• When replacing a part of the ground connection, make certain that the metallic contact surfaces are clean and that electrical continuity is re-established.
• After completing the foregoing steps, recheck to be sure that the connection is securely fastened with the correct mounting hardware. Paint the ground strap and hardware in accordance with current procedures.

Because of the electrical shock hazards that could be encountered aboard ship, plugs and convenience outlets for use with portable equipment and power tools normally are standard three-prong type. Both plugs and outlets are keyed so that the plug must be in the correct position before it can be inserted into the receptacle. To ensure that the safety factors incorporated in these devices are in serviceable condition and are safe for use, you must perform the following precautions and inspections:

Inspect the pins of the plug to see that they are firmly in place and are not bent or damaged.

Check the wiring terminals and connections of the plug. Loose connections and frayed wires on the plug surface must be corrected and any foreign matter removed before the plug is inserted into the receptacle.

Never use a power tool or a piece of portable test equipment unless you are absolutely sure that it is equipped with a properly grounded conductor.

BASIC MEASUREMENTS

Electronic measurements involve the fundamental electrical quantities of voltage and current and the inherent characteristics of resistance, capacitance, and inductance. In circuits being tested, voltage and current are dependent upon resistance, capacitance, and inductance for their distribution; therefore, voltage and current measurements are valuable aids in determining circuit component conditions and in the evaluation of symptoms. Practically any reading obtained from the use of test equipment will depend on these basic measured quantities of resistance, capacitance, and inductance.

VOLTAGE AND CURRENT MEASUREMENTS

Voltage measurements may be made as part of either preventive or corrective maintenance. These measurements are made using a voltmeter. When compared with voltage charts, these measurements are a valuable aid in locating a trouble quickly and easily. However, if the sensitivity of the test voltmeter differs from that of the voltmeter used in preparing the chart, the voltage measurements must be evaluated before the true circuit conditions can be determined. (Sensitivity in voltmeters was discussed in NEETS, Module 3, Introduction to Circuit Protection, Control, and Measurement.)

Since many of the troubles you find in equipments and systems are the result of abnormal voltages, voltage measurements are a valuable aid in locating trouble. You can measure voltage with a voltmeter without interrupting circuit operation.

Point-to-point voltage measurement charts, usually found in equipment technical manuals, contain the normal operating voltages found in the various stages of the equipment. These voltages are usually measured between indicated points and ground unless otherwise stated. When you begin recording voltage measurements, it is a smart and safe practice to set the voltmeter on the highest range before measuring. This ensures that excessive voltages existing in the circuit will not cause overloading of the meter.

Q.13 On what range should you set the voltmeter prior to taking a voltage measurement?

To increase accuracy, you should then set the voltmeter to the appropriate range for the proper comparison with the expected voltage in the voltage charts. When checking voltages, remember that a voltage reading can be obtained across a resistance, even if that resistance is open. The resistance of the meter itself forms a circuit resistance when the meter probes are placed across the open resistance. Therefore, the voltage across the component may appear to be normal or near-normal as you read the meter, but may actually be abnormal when the meter is disconnected from the circuit.

If the internal resistance of the voltmeter is approximately the same value as the resistance being tested, it will indicate a considerably lower voltage than the actual voltage present when the meter is removed from the circuit. The sensitivity (in ohms per volt) of the voltmeter used to prepare the voltage charts is provided on those charts. If a meter of similar sensitivity is available, you should use it to reduce the effects of loading.

The following precautions are general safety measures that apply to the measurement of voltages. Remember that nearly all voltages are dangerous and have often proved fatal to careless technicians. When measuring voltages, be sure to observe the following precautions:

Q.14 When taking a voltage measurement, which lead of the voltmeter should you connect to the circuit first?

Current measurements are not often taken in the course of preventive maintenance or testing. This is because the ammeter (or other current-measuring instrument) must become an actual part of the equipment being tested. The circuit must be opened (desoldered) to connect the ammeter in series with the circuit being tested. Usually, you can take a voltage measurement and use this factor to calculate the circuit current by applying Ohm's law.

Q.15 Is an ammeter connected in series or in parallel with the circuit under test?