IBX5980432E7F390 Electrical Test - BLOG PELAUT

Electrical Test



Electrical safety
Before working on electrical installations on board ship, one should be aware of basic safety precautions and that regulations exist about the construction, installation, operation and maintenance of electrical equipment so that danger is eliminated as far as possible. Minimum standards of safety are issued by various International Bodies and National Governments. Ships’ staff must operate equipment in a safe manner and maintain it in a safe condition at all times. Failure to do so will cause danger with possible disastrous consequence.
(a) Which bodies?
(b) Which safe manners (DO's and DON’T's )are considered when working with electrical equipment?
Electrical safety
(a) International (e.g. SOLAS), national and international standards associations (e.g. BSS and JEC), learned societies (e.g. IEE), classification societies (e.g. Lloyds), etc. Where danger arises it is usually due to accident, neglect or some other contravention of the regulations.
(b) Ships’ staff should keep in mind an essential list of DO’s and DONT’s when working with electrical equipment.
DO get to know the ship’s electrical system and equipment. Study ships’ diagrams to pinpoint the location of switches and protection devices supplying distribution boards and essential items of equipment. Write down this information in a note book. Note the normal indications on switchboard instruments so that abnormal operation can be quickly detected.
DO operate equipment according to manufacturers’ recommendations.
DO maintain equipment according to manufacturers’ recommendation, or shipowners maintenance procedures.
DO ensure that all guards, covers and doors are securely fitted and that all bolts and fixings are fitted and tight.
DO inform the Officer of the Watch before shutting down equipment for maintenance.
Do switch off and lock off supplies, remove fuses, and display warning notices before removing covers of equipment for maintenance.
Do confirm that circuits are dead (by using a voltage tester) before touching conductors and terminals.
DON'T touch live conductors under any pretext. DON'T touch rotating parts.
DON'T leave live conductors or rotating parts exposed. DON'T overload equipment.
DON'T neglect or abuse equipment.
You should think 'safety' at all times and so develop a safety conscious attitude. This may well save your life and the lives of others. Most accidents occur due to a momentary loss of concentration or attempts to short-circuit standard safety procedures. DO NOT let this happen to YOU.

What are you to do if difficulties arise in locating a fault on an item of equipment and only a wiring diagram is available?
It may well save time and trouble to convert the wiring diagram into a much simpler and more useful circuit diagram. When converting a wiring diagram into a circuit diagram certain basic
rules and Conventions should be followed:
1. Every sequence should be drawn from left to right and from top to bottom (where possible).
2. Each stage should be in order of occurrence from left to right.
3. All Contacts and components which are in series should be drawn in a straight line (where possible) with the component they control.
4. All Contacts and components which are in parallel should be drawn side by side and at the same level to emphasise their parallel function.
5. All major components operating at bus-bar voltage should be drawn at the same level (or aligned horizontally) to help identify the required components quickly.
6. All contacts should be shown ‘open’ or ‘closed’ as in their NORMAL or un-energised condition.
Electric shock
Nearly everyone has experienced an electric shock at some time. At best it is an unpleasant experience, at worst it is fatal. Anyone who has access to live electrical equipment must be fully aware of first aid and safety procedures related to electric shock as described in relevant safety acts. Copies of these safety procedures should be displayed on board ship. Electric shock is due to the flow of current through your body. This is often from hand to hand or from hand to foot. A shock current as low as l5mA ac or dc may be fatal. Obviously the size of shock-current is related to the applied voltage and your body resistance. Unfortunately, your body resistance goes down as the applied voltage goes up. This means that the shock current is further increased at high voltages. The size of your body resistance also depends on other factors such as your state of health, the degree of contact with live wires and the perspiration or dampness on your skin. Typical dry full contact body resistance is about 5000♎ at 25V falling to about 2000♎ at 250V.
Voltages of about 60V and below are regarded as reasonably safe for portable handtools. This is why special step-down isolating transformers are used with portable tools and hand lamps. These transformers supply the tool or lamp at 110V ac but because the secondary winding is centre-tapped to earth, the maximum shock voltage to earth is 55V. Electric shock is often accompanied by falling, which may cause additional physical injury and require first aid action. If the shock victim is unconscious, resuscitation must take priority over first aid methods. Check the resuscitation techniques displayed on the electric shock posters displayed on board.

What would the equivalent shock current levels be at 25V and 250V?
5mA and 125mA.

Which main 5 testing operations are carried out on board ships, and which instruments are needed for them?
1. Insulation Resistance; Megger tester.
2. Circuit Continuity; Low Resistance meter.
3. Component Resistance; Universal or Resistance meter.
4. Voltage; Universal or Voltage meter.
5. Current; Universal or Amps meter.

Why should the measurement of the insulation resistance of a machine ideally be made while the machine is hot?
Insulation becomes more 'leaky' (its IR value falls) at high temperatures. So testing while hot shows the realistic IR value at, or near, working temperature. Insulation resistance can vary considerably  with changing atmospheric conditions. A single reading gives little information. However, the regular recording of insulation resistance readings may show a downward trend
which indicates impending trouble which can be remedied by preventive maintenance.

What should a clampmeter indicate if clipped around a 3-core cable which is known to be carrying 100A ac to a motor?
Zero. This is because the clampmeter monitors the magnetic flux around the cable which is produced by the current. In a balanced 3-core (or 2-core for that matter) cable, the net flux is zero - hence no indication. This is why the clampmeter is only connected around one conductor.

A 10A motor operates from a 220V insulated system. The supply cables have a total impedance of 0.01. If (a) an open-circuit fault, (b) an earth fault and (c) a short-circuit fault occurred, what circuit current would flow in each case.
(a) The open-circuit fault has infinite impedance and, since I = V/Z, then I = 220/♌= ZERO
(b) The earth fault has NO effect on the circuit current I = 10A (This is an INSULATED system)
(c) The short-circuit fault impedance is only the 0.01 ♎ of the cables. Since I = V/Z,
I = 220/0.01♎= 22 000 A.

What would be the ohmic value of an NER (Neutral Earthing Resistor) to limit the earth fault current to the full load rating of a 2MW, 0.8pf, 3.3kV, 3-phase generator?
In a 3-phase system, Power = ✔️3.VL.1L ,cos Φ where VL is line voltage (3.3kv), 1L is the line current and cosΦ is the power factor. The generator full load current is:
IL = 2000,000W/✔️3.3300.0.8 = 437A

Under E/F conditions a phase voltage of 3300/✔️3 = 1905V drives the fault current through the NER. Its ohmic value has to be 1905V/437A = 4.4♎

If your ship is designed for 60Hz at 440V; what value should the shore supply voltage be if operating at 50Hz?
Supply voltage should be reduced to about 380V.

Suggest reasons why protection equipment is essential in an electrical distribution system.
(a) To disconnect and isolate faulty equipment in order to maintain the power supply to the remaining healthy circuits in the system.
(b) To prevent damage to equipment from the thermal and magnetic forces that occur during short circuit and overload faults.
(c) To protect personnel from electric shock.

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