FAQ Orals (Electrical)

What Minimum Current Can Be Regarded As Fatal?
A Shock Current As Low As 15mA Ac Or Dc Can Be Fatal.
 
What Is The Function Of A Fuse?
The Function Of A Fuse Is To Give Short-circuit Protection And Also Overload Protection And Operates In Milliseconds
 
What Would You Consider A Minimum Insulation Reading Resistance?
Insulation Resistance Must Be Kept Above At Least 1MW, The Higher The Insulation Resistance The Better.  Companies Have Their Own Regulations, But Most Companies Only Allow Readings Above 5MOhms.
  
How Can You Reverse The Direction Of A Motor?
By Swapping Any Two Supply Line Connections To The Motor.
 
Give Reasons Why Protection Equipment Is Essential In An Electrical Distribution System.
To Disconnect And Isolate Faulty Equipment In Order To Maintain The Power Supply To The Remaining Healthy Circuits In The System.
To Prevent Damage To Equipment From The Thermal And Magnetic Forces That Occur During Short Circuit And Overload Faults.
To Protect Personnel From Electric Shock.
 
 Explain Regulations Regarding Power Supplies For Steering Gears.
The Electrical Power Supply Must Be From Two Widely Separate Supplies I.e. One From The Main Switchboard And The Other From The Emergency Board. 
Electrical Leads And Fuses Are To Allow 100% Overload, Giving Only Short Circuit Protection.
If Failure Does Occur On One System, The Stand-by System Will Cut In Within 45 Seconds.
The Steering Motors Will Have Sequential Starting.
 
 What Is The Normal Reading On And Insulation Test Meter?
Infinity  Is A Normal Reading, But Readings Should Be Kept Above 5 Meg-ohms..
 
When Would A Star/Delta Starter Be Used On A Motor?
Where A Slow Moving High Inertia Load Is Involved The Starting Time Must Be Considered Because Of The Heating Effect Of The Starting Current And In This Case A Star/Delta Starter Would Be Used.
The Star/Delta Starter First Connects The Starter Windings In Star And When Running Changes Over To Delta.  The Star Connection Results In About 58% Line Voltage Being Applied To Each Phase With Therefore A Reduction In Starting Current.  The Starting Torque Is Also Reduced To About 1/3 Of Its Direct On-line Valve.
 
How Would You Go About Finding An Earth Fault In The System?
Finding An Earth Fault Would Be By The Process Of Elimination I.e. Circuit Breakers Would Be Opened And Closed Until Earth Fault Disappeared, Taking Care Which Breakers Were Being Opened And Closed As They Maybe Supply Essential Loads At The Time.  It Would Be Good Practice To Start With Places Such As The Galley And Laundry Where Faults Are Common.

What Is Single Phasing Of A Motor?
Single Phasing Is Where One Of The 3-phase's Supplying The Motor Becomes Disconnected.  The Motor Will Continue To Run If This Happens And Can Result In Motor Burnout.  The Effect Of Single Phasing Is To Increase The Current In The Two Remaining Lines And Cause The Motor To Become Very Noisy Due To Uneven Torque Produced.
 
 What Are The Causes Of Single Phasing?
Single Phasing Can Be Caused When One Of The Three Back Up Fuses Blows Or If One Of The Contactor Contacts Is In Open Circuit.
 
What Is Meant By Protection Discrimination?
This Is The Ability Of A Protection System To Disconnect Only Faulty Circuits And To Maintain The Electrical Supplies To Healthy Ones.
Discrimination Is Achieved By Co-ordinating The Current Ratings And The Time Settings Of The Fuses And Over-current Relays Used Between The Generator And The Load.
The Devices Nearest The Load Having The Lowest Current Rating And Shortest Operating Time;  Those Nearest The Generator Having The Highest Current Rating And Longest Operating Time.
I.e.  A Motor Fault On A Centrifugal Pump Would Not Cause The Generator Breaker To Trip.
 
Describe How An A C Motor Works?
Most A C Motors On Ships Are Of The Induction Or Squirrel Cage Motor Types, The Principle Of Operation For Which Is As Follows:  The Starter Is Made Up Of Three Separately Phased Windings, To Which A 3-phase Supply Is Connected.  The Rotor Has A Series Of Copper Conductors Along Its Axis, Which Are Joined By Rings At The Ends To Form A Cage.  When The Motor Is Started, The Rotating Magnetic Field Induces An E M F In The Cage And Thus A Current Flow.  The Current Carrying Conductor In A Magnetic Field Produces The Motor Effect, Which Turns The Rotor.  The Motor Speed Builds Up To A Value Just Less Than The Speed Of Rotation Of The Magnetic Field.  The Motor Speed Depends Upon The E M F Induced In The Rotor And This Depends Upon The Difference In Speed Between The Conductors And The Magnetic Field.
A Number Of Different Fixed Speeds Are Possible By Changing The Number Of Poles (Conductors).
 
What Is Meant By The Term, Type Ex'e'?
Ex'e' = Increased Safety Equipment.
Increased Safety Equipment Is Based Primarily On The Elimination Of  "open Sparking", I.e. Equipment That Does Not Have Contacts.
 
What Is Meant By The Term, Type Ex'i'?
Ex'i' = Intrinsically Safe.
Equipment With Circuits That Do Not Generate Enough Heat To Ignite A Gas.  This Generally Means Limiting Circuits To Less Than 30V, 50mA.
 
What Is Meant By The Term, Type Ex'p'?
Ex'p' = Pressurisation.
This Is Where Equipment Is Pressurised To Prevent Any Gases Entering Equipment.

What Is Meant By The Term, Type Ex'd'?
Ex'd' = Flame Proof Enclosure.
This Is An Enclosure That Will Withstand An Internal Explosion Of Flammable Gas Within Itself And Dissipate Heat From That Explosion Before Reaching Outside Atmosphere, Which Could Ignite The Gas In Surrounding Atmosphere, I.e. A Ship's Battery Locker Could Be Classed As Ex'd'.

 Why Do You Require Earth Lamps On 220V Distribution Board When You Have Them On The Main Switchboard?
You Require Earth Lamps On The 220V Distribution Board As Well As The Main Board, Due To The Air Gap In Step Down Transformer, I.e. 440V Earth Lamps Can Not Detect Earth On 220V Systems Due To The Gap.
 
What Would You Do If You Found An Engineer In Electric Shock?
First Raise The Alarm Then Isolate Or Switch Off The Current.
If This Is Not Possible, Pull Or Push The Victim Away From The Source, Taking Care Not To Make Electrical Contact With The Victim Or The Electrical Source, I.e. Use Something Non-conductive.
Once The Victim Has Been Removed From The Electrical Source, If The Breathing Is Feeble Or Ceased, Artificial Respiration Should Be Started At Once.  Delay In Starting Artificial Respiration Can Prove Fatal.
Note:  If Artificial Respiration Is Started At Once, 70% Of The Victims Recover.  If There Is 3 (three) Minutes Delay, Only 20% Of Victims Recover.