Electrical Test part 5
ELECTRICAL TEST and MEASURING EQUIPMENT 5
(PRACTICAL MARINE ELCTRICAL KNOWLEDGE; Hall, Witherby)
QUESTION 41
Why will a single earth fault blow a fuse in an earthed supply system?
Why will a single earth fault blow a fuse in an earthed supply system?
ANSWER 41
The single E/F completes a low resistance path back to the neutral or centrepoint of the supply with a resulting large fault current to rupture the fuse:
The single E/F completes a low resistance path back to the neutral or centrepoint of the supply with a resulting large fault current to rupture the fuse:
QUESTION 42
If the motor terminal markings are unknown, how could you identify the Start, run and common terminal connections?
If the motor terminal markings are unknown, how could you identify the Start, run and common terminal connections?
ANSWER 42
The start.winding (being short-time rated) has a higher resistance than the run winding so a resistance check should identify the terminals as follows: Using a multimeter on the R x 1 scale, find the two terminals that have the highest resistance between them. These are the start and run terminals. The remaining terminal must be the 'common'. Connect one lead of the meter onto the common terminal and touch the other meter lead onto the other terminals in turn and note the readings. The highest reading indicates the 'start' winding terminal. The other remaining terminal is the 'run' connection. Typically, the run winding is 1.5 - 6♎and the start winding is 6 - 22♎
The start.winding (being short-time rated) has a higher resistance than the run winding so a resistance check should identify the terminals as follows: Using a multimeter on the R x 1 scale, find the two terminals that have the highest resistance between them. These are the start and run terminals. The remaining terminal must be the 'common'. Connect one lead of the meter onto the common terminal and touch the other meter lead onto the other terminals in turn and note the readings. The highest reading indicates the 'start' winding terminal. The other remaining terminal is the 'run' connection. Typically, the run winding is 1.5 - 6♎and the start winding is 6 - 22♎
QUESTION 43
What would be the continuity resistance of a healthy 2kW, 220V heating element?
What would be the continuity resistance of a healthy 2kW, 220V heating element?
ANSWER 43
The element current is 2000W/220V = 9.1A, and Power = 12R so R = Power/I2 = 2000/9.12 = 24.2♎
The element current is 2000W/220V = 9.1A, and Power = 12R so R = Power/I2 = 2000/9.12 = 24.2♎
QUESTION 44
An alkaline cell has an electrolyte of potassium hydroxide while a lead acid cell uses sulphuric acid. Both are diluted with distilled water. What first aid treatment would you apply should you be splashed with either electrolyte?
An alkaline cell has an electrolyte of potassium hydroxide while a lead acid cell uses sulphuric acid. Both are diluted with distilled water. What first aid treatment would you apply should you be splashed with either electrolyte?
ANSWER 44
In both cases rapidly wash eyes and skin with lots of fresh water. The electrolyte of alkaline cells causes skin burns which should be treated with boracic powder and the eyes washed out with a solution of boracic powder - one teaspoonful to a pint of water. Sulphuric acid splashes can be washed with a saline solution - two teaspoonfuls of household salt to one pint of water. For both types of battery first aid equipment should be available in the battery compartment.
In both cases rapidly wash eyes and skin with lots of fresh water. The electrolyte of alkaline cells causes skin burns which should be treated with boracic powder and the eyes washed out with a solution of boracic powder - one teaspoonful to a pint of water. Sulphuric acid splashes can be washed with a saline solution - two teaspoonfuls of household salt to one pint of water. For both types of battery first aid equipment should be available in the battery compartment.
QUESTION 45
Tanker pumprooms require two separate lighting circuits. How can the circuits be arranged so that the luminaires can only be opened up when the correct circuit has been isolated?
Tanker pumprooms require two separate lighting circuits. How can the circuits be arranged so that the luminaires can only be opened up when the correct circuit has been isolated?
ANSWER 45
Many tankers use the following arrangement: The luminaires on one of the circuits have bolts with a different size or type of head to those on the luminaires supplied by the other circuit. Typically, these would be two different types of triangular bolt head.
The keys to remove the bolts are actually the operating handles of the circuit isolators. A key can only be removed from its trapped position on the switch after the circuit has been isolated. This key can only open up those luminaires connected to the circuit which has been isolated.
Many tankers use the following arrangement: The luminaires on one of the circuits have bolts with a different size or type of head to those on the luminaires supplied by the other circuit. Typically, these would be two different types of triangular bolt head.
The keys to remove the bolts are actually the operating handles of the circuit isolators. A key can only be removed from its trapped position on the switch after the circuit has been isolated. This key can only open up those luminaires connected to the circuit which has been isolated.
QUESTION 46
A ship's battery locker is fitted with a flameproof luminaire marked Exd IIc T4. Is this luminaire certified for use in the battery room?
ANSWER 46
Yes. The hazard is hydrogen gas from the batteries which requires apparatus designed for use in apparatus gas group Tic. The ignition temperature of hydrogen is 560°C and the temperature classification of the luminaire is T4. This means that it's surface temperature will not exceed 135°C, so the temperature classification is OK.
A ship's battery locker is fitted with a flameproof luminaire marked Exd IIc T4. Is this luminaire certified for use in the battery room?
ANSWER 46
Yes. The hazard is hydrogen gas from the batteries which requires apparatus designed for use in apparatus gas group Tic. The ignition temperature of hydrogen is 560°C and the temperature classification of the luminaire is T4. This means that it's surface temperature will not exceed 135°C, so the temperature classification is OK.
QUESTION 47
Would an IR reading of 0.5 M♎ to earth be acceptable?
Would an IR reading of 0.5 M♎ to earth be acceptable?
ANSWER 47
Although a minimum of 1 M♎ is generally specified for new equipment, Lloyd's rules suggest that 0.1 M♎ is acceptable. However, most surveyors would insist on at least 1k♎/volt, i.e. 440k♎. say, 0.5 M♎ as a reasonable minimum value for a 440V generator.
Although a minimum of 1 M♎ is generally specified for new equipment, Lloyd's rules suggest that 0.1 M♎ is acceptable. However, most surveyors would insist on at least 1k♎/volt, i.e. 440k♎. say, 0.5 M♎ as a reasonable minimum value for a 440V generator.
QUESTION 48
What is the point of earthing one end of a current transformer secondary winding?
What is the point of earthing one end of a current transformer secondary winding?
ANSWER 48
Gambar...
Gambar...
Should the insulation between primary and secondary break down, the secondary circuit can be raised to full primary voltage; e.g. 440V above earth which could damage the secondary insulation with a serious risk to personnel. By earthing one end of the CT, the circuit is anchored' to zero volts. As a bonus, the earth connection will allow such a fault to be detected on the earth fault monitor.
QUESTION 49
What are the functions of EPR or butyl and PCP or CSP?
What are the functions of EPR or butyl and PCP or CSP?
ANSWER 49
EPR or butyl rubber are good electrical insulators but are not mechanically strong or resistant to oil. This is why a sheath of PCP or CSP which is stronger and has greater oil and fire resistance is fitted around the inner insulation.
EPR or butyl rubber are good electrical insulators but are not mechanically strong or resistant to oil. This is why a sheath of PCP or CSP which is stronger and has greater oil and fire resistance is fitted around the inner insulation.
QUESTION 50
Flexible cables to light fittings, power tools, etc., should be inspected for mechanical damage. In normal operation a flexible cable may be repeatedly dragged and chafed so reducing its safety. If in doubt replace flexible cables.
A copper strap or flexible earthing braid/wire is used to bond the steel frame of all electrical motors and other equipment to the ship's hull.
Flexible cables to light fittings, power tools, etc., should be inspected for mechanical damage. In normal operation a flexible cable may be repeatedly dragged and chafed so reducing its safety. If in doubt replace flexible cables.
A copper strap or flexible earthing braid/wire is used to bond the steel frame of all electrical motors and other equipment to the ship's hull.
Why?
ANSWER
Without an earth strap, a loose internal wire may touch the frame causing it to become 'live' at mains voltage with obvious danger to operators. The earth strap electrically 'anchors' the frame to the hull (zero volts) to eliminate the shock hazard.
Without an earth strap, a loose internal wire may touch the frame causing it to become 'live' at mains voltage with obvious danger to operators. The earth strap electrically 'anchors' the frame to the hull (zero volts) to eliminate the shock hazard.
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