NSTM Chapter 300, Electric Plant - General Flashcards
For repair activity work conducted shipboard not IAW NSTM Chapter 300, a ___ between shipyards/repair activities and Ship’s Force
Memorandum of agreement must be documented
For naval applications, voltages greater than or equal to ___ are considered high voltage
1000 volts
The resistance of a human body is about ___
500 ohms
When the skin is damp the resistance of a human body is about ___
300 ohms
A situation where electrical current passes through air gaps between ungrounded conductors or between conductors and grounded components
Arc flash
Another product of an arc flash is the incident energy, which is ___
Radiant energy
Radiant energy is expressed in ___
Calories per centimeter squared (cal/cm^2)
Threshold for a second degree burn
1.2 cal/cm^2
Threshold for a third degree burn
10.7 cal/cm^2
Two major hazards with an arc flash
Arc temperature
Incident energy
Occurs when the severity of an arc or short circuit current causes destructive heating and explosive vaporization of both the surrounding air and the metal in the current path
Arc blast
The unobstructed distance from exposed energized circuits within which unprotected skin could receive a second degree burn if an arc flash were to occur
FPB
FPB
Flash Protection Boundaries
The FPB distance aboard a ship for 450-volt circuits is ___
4 feet
The FPB distance aboard a ship for 4160-volt circuits is ___
11 feet
The FPB distance aboard a ship for 13.8K volt circuits is ___
13 feet
An FPB of 4 feet is applicable to circuits with rated voltages between ___
30-1000 volts
An FPB of 11 feet is applicable to circuits with rated voltages between ___
1000 to 5000 volts
An FPB of 13 feet is applicable for voltages ___
Greater than 5000
The FPB does not need to extend past ___
Permanent solid structure
Intentionally connects the non-current carrying conductive parts of electrical equipment, such as the enclosure, frame, or chassis, to ground through a low-resistance path
Equipment grounding
The purpose of equipment grounding
If energized electrical conductor contacts the enclosure due to fault or damage, it shunts the current away from possible human contact
Grounding of metal enclosing cases, bases, frames, and structural parts of electrical equipment shall be IAW ___
MIL-STD-1310 and MIL-E-2036
On metallic hull ships, grounds can be provided by the fact that the metal enclosing cases or frames are ___
In contact with one another and the metal structure of the ship
A ground must be provided for equipment mounted on a hinged door if the voltage exceeds ___
30 volts
Resistance to ground should be less than ___
0.1 ohm
___ systems provide a limited amount of current when one phase is faulted to ground, and thus allows critical equipment to continue operation until steps can be taken to remove the fault
Ungrounded
___ system designed so that a phase conductor fault to ground will produce high current and quickly trip the circuit breaker or blow the fuse
Grounded neutral
Electric utility service for wiring ashore uses ___ system
Grounded neutral
___ system is isolated from the ungrounded power system at the upstream supply transformer
Grounded neutral receptacle
Some ships use ___ for improved personnel safety over that which standard circuit breakers provide
Ground fault current interrupt equipment
Both conductors of a shipboard receptacle have a ___
Potential between the conductors and the ground
Ashore receptacles has one ___
Neutral conductor which is connected to ground at the source
The resistances of an ungrounded system when ___, form the insulation resistance of the system
Combined in parallel
Insulation resistance of the system is measured using ___
500-volt DC megger or installed ship’s active ground detector
Contain capacitors connected from the conductors to the ground
Electromagnetic Interference Filters
Used to reduce interference to shipboard electrical and electronic equipment
Electromagnetic interference filters
Ungrounded high voltage power systems have a high failure rate due to ___
Transient overvoltage surges
The surges in an ungrounded high voltage power system is the result from ___
Oscillations that occur because of the system’s inherent inductance and capacitance
Ships with electrical systems rated ___ are designed with high resistance grounding subsystems
1000 volts and above
High resistance grounding subsystems are normally contained within an enclosure with ___
A three-phase transformer connected in a wye/delta configuration with an open delta secondary. A resistor is connected across the open delta secondary
Absorbs and clamps voltage spikes on the main AC bus
Transient voltage surge suppression systems
Arc Fault Detection systems are installed on ___ where they ___
Switchboards and load centers
Continuously monitor the interior of these enclosures for arcing faults
Arc fault detection systems can sense ___
Pressure, gassing particles, and light
AFD systems consist of ___
Pressure sensors, thermal ionization detectors, photo sensors, and control units that monitor the sensors
In an AFD system the thermal ionization detector is associated with ___
The continuous thermal monitor system
In an AFD system the control circuitry requires ___ to indicate the presence of an arc and trip the switchboard isolation
Both a photo sensor and its corresponding pressure sensor
Do not join extension cords longer than ___ together
25 ft
Never join more than ___ extension cords together
2
Extension cords up to ___ are permissible
100 ft
___ use silicon-controlled rectifiers vice mechanical contacts to provide dual power supplies to individual loads
Static automatic bus transfer switches
Electrical systems require a minimum of ___ isolation points in each conductor isolation path
One
All electrical work should be approached with a focus on ___
What could go wrong
CO permission is not required when performing ___
IVV or safe-to-work voltage checks
The safety observer for work on energized circuits must ___
Be trained in first aid, know what breaker isolates the system and be ready to operate it immediately, and not engage in any distracting behavior
For work on energized circuits greater than ___ a safety line shall be attached to the worker
300 volts
IVV
Initial Voltage Checks
When shorting a capacitor to drain the charge it should be done ___
Three times about ten second apart
When energized wiring and non energized wiring are located in close proximity to each other
Capacitive coupling
Detecting stray voltage using Simpson 260 the reading should be ___
Less than fill scale deflection on the 2.5V scale
Detecting stray voltage using DVM set to low impedance mode or Fluke stray voltage eliminator installed the reading should be ___
Less than 3 volts
Capacitive storage is verified as indicated by ___
Minimum of 5 seconds with steadily decreasing voltage indication
To dissipate capacitor stored energy the ground must have a reading of ___
Less than 0.1 ohm
Do not approach or take a conductive object without an approved insulating handle closer than ___ to potentially energized exposed equipment without proper PPE
2 ft
Establish an electrical safety and arc flash area while checking that ___ equipment is deenergized
Greater than 1000 volts
Proximity voltage tester must be ___ from equipment being tested
8 inches or closer
STW
Safe-to-work
If all circuits are ___ STW checks are not required
30 volts or less
Safety devices installed in power and lighting circuits and in control circuits to protect the equipment and circuits from damage due to excessive current
Fuses
Silver plated fuses develop a ___ over time that does not affect performance
black oxide coating
SIlver plated fuses are designated by ___
The letter S following the current rating of the fuse
If the replacement fuse blows, then ___
A fault probably exists
___ before replacing a fuse
Deenergize the circuit
Do not use ___ to grab a fuse that remains in the fuse barrel when the fuseholder is removed
A bare hand
For circuits ___ or less, fuses may be removed or replaced without deenergizing the load
10 amps
Some indicator types of panel mounted fuse receptacles contain riveted connections that carry current. These connections are required to be ___
Soldered to achieve a low-resistance connection
A draw-out type circuit breaker uses a ___ to remove
Racking mechanism
The imaginary plane formed by the opening of an electrical enclosure
Electrical safety plane
If electrical components are mounted on the door, then the plane includes ___
The arc formed by the doors edge as it swings open
No precautions are required for visual inspections of equipment or circuits rated ___ or less
30 volts
For visual inspections, erect a barrier at least ___ from the electrical plane of the equipment
2 ft
Minimum PPE to wipe water off of equipment
Rubber gloves
Have an attached cord, are hand-held or frequently handles while operated, and are plugged into an electric power receptacle
Portable electrical tools and devices
A unit that is not hard-wired, can be moved, but normally is stationary while operating
Mobile electrical equipment
Equipment brought onboard by the owner for personal use
Personal electrical equipment
Use rubber gloves when using metal-cased tools that are not marked as ___
Double insulated
When performing work that could tear the rubber gloves ___
Wear leather gloves over them
If damage or defects are found in portable cord and plug-connected hand-held equipment and cords, then ___
Remove from service, and turn it into tool issue or the electrical safety officer
Isolated receptacle units are isolated from ___ by ___
The main power distribution system
Transformers
Isolated power receptacles circuits are limited in length to reduce the ___ to an acceptable level
Line-to-ground capacitance
Isolated power receptacles limit ground leakage to less than ___
10 mA
Refers to the existence of two separate insulating systems within a tool or device
Double insulated
The term “double insulated” stamped on a device is a ___ designation
Underwriters Laboratories (UL)
Computer power strips of the marine type must have ___
A metal case, double-pole switch/circuit breaker, and dual thermal fuses. Must provide both common and normal mode protection and be listed by a nationally recognized testing laboratory
Marine power strips have the commercial item description of ___
A-A-50622
If the power cord must be more than 25 ft long, the equipment must be ___
Permanently mounted and hard-wired
Commercially available mobile electrical equipment must not be used aboard ship unless ___
They have been approved for shipboard use
Three-conductor flexible cables should be ___
Type SO or ST, black, white, and green
The length of power cords for high-current equipment should be ___
Only as long as required
Testing of electrical equipment other than visual inspections should be done in a workshop equipped with ___
An electrically safe work bench and insulating rubber deck covering
Grounding conductors shall be ___ and have minimum cross-sectional area of ___
Stranded
9000 circular mils (AWG #10)
The conduction of electricity across the surface of a dielectric
Creepage
The shortest distance between two conductors or between a conductor and ground, measured along the surface of the insulating material
Creepage distance
By increasing the creepage distance, the creepage current is ___
Reduced
In order to use air as insulation, a solid insulating material must be used for ___
Support to maintain the required air-spacing distance between uninsulated conductors
Insulation systems are classified by temperature because ___
Temperature has the predominate effect of insulation life
Insulation life will decrease by one half for every ___ above the specification limit
10 - 15 degrees C (50 to 59 degrees F)
Class B or F insulation systems: maximum temperature rise
120 degrees C minus (nameplate max ambient)
Class H insulation systems: maximum temperature rise
145 degrees C minus (nameplate max ambient)
Class N insulation systems: maximum temperature rise
170 degrees C minus (nameplate max ambient)
For top-curved surfaces having a radius greater than 3 inches and for top-flat surfaces, surface creepage distance shall be increased ___ where these surfaces have irregularities which permit the accumulation of dust and moisture
33%
Three methods of taking temperatures in wiring
Thermometer
Resistance
Infrared
The ability to withstand potential difference and is usually expressed in terms of voltage at which the insulation fails due to electrostatic stress
Dielectric strength
Maximum dielectric strength values can be measured only by ___
Testing to destruction
Two reasons for using an ungrounded power system aboard a ship
Improve reliability of power
Prevent galvanic corrosion
Insulation resistance should be verified in reference to the ___ in which the winding is embedded, between ___, and between ___
Metallic structure
Individual windings on the same component
Isolated brush rigging polarities
When testing rotor windings, the test voltage should be applied between the copper conductors and a metallic part of the rotor rather than the ___ in order to ___
Stator
Eliminate the insulating effect of oil in the bearings
For portable tools, appliances, and cable assemblies, measure the insulation resistance of each line terminal to the ___ and to the ___
Equipment frame
Metal shell of the equipment plug
The ratio of the 10 minute insulation resistance value to the 1 minute insulation resistance value
Polarization Index
The polarization index test is useful for ___
Appraising the cleanliness and dryness of a winding
Insulation resistance of a winding will normally ___ with the duration of the test voltage
Increase
The measured insulation resistance of a dry winding in good condition will reach a fairly steady value in ___
10 to 15 minutes
The measured insulation resistance of a wet or dirty winding will reach a fairly steady value in ___
1 or 2 minutes
For most equipment, the minimum acceptable value of polarization index is ___
2.0
Following a DC high-potential test, hold the ground for at least ___
Four times the accumulated test period or 1 hour, whichever is longer
Perform a ___ before a high-potential test
500 volt megger
No more than ___ AC high-potential tests should be made during the repair process
One
An electronic device designed specifically to stress the whole winding’s insulation system by applying a series of pulses between turns of a coil, between coils, between phases of two windings or two legs of a winding, and from the windings to ground to detect short circuits
Surge tester
Two methods of sealing an explosion-proof enclosure
Completely filling the voids with liquid
Hermetic sealing
Being incapable of releasing sufficient electrical energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture
Intrinsically safe equipment and circuits
The ___ of paint from electrical equipment should be avoided
Removal
Repainting of electrical equipment should be done only when ___
Necessary to prevent incipient corrosion due to lack of paint
Paint should never be applied to any ___ in electrical equipment
Insulating surfaces
Water piping near electrical equipment should be ___
One continuous length
Rule of thumb for using electric lamps to keep equipment dry is to provide ___ for each ton of machine weight
100 watts
The temperature in the secured equipment should be kept about ___ above the ambient air temperature
2.8 to 5.6 degrees C (5 to 10 degrees F)
Desiccant should be composed of ___
Silica gel or clay
___ desiccant is required with machines that use carbon brushes
Clay
Silica gel desiccant will cause carbon brushes to ___
Deteriorate rapidly
Bags of desiccant are never to be placed inside a ___
Motor enclosure
Transparent plastic sheet to wrap secured equipment should be at least ___ thick
4 mils
When the relative humidity reaches ___, the desiccant should be replaced
40%
Core testing is mandatory for all motors requiring ___ when repairs are accomplished
Rewind
