Some electrical definitions

Q. What is megger?

           It is a portable instrument used for measuring insulation resistance of a circuit.

Phase sequence indicator: It is an instrument which indicates the order in which the conductors in a poly phase system reach their maximum current or voltage.

Pharos: A quality which varies simusodially when represented as a complex number.

Rheostat: A resistor whose resistance can be varied whilst connected to a circuit.

Transistor: An active semiconductor device which has 3 or more terminals.

Eddy current : A localized electric current which is produced due to varying magnetic field.

Impedance: It is the ratio of simusodial voltage and current when both are expelled at RMS terms.

Inductance: The property of an electric circuit that causes an e.m.f to be generated by a change in the current flowing.

Reactance:  The non –resistive component of impedance in an a.c current arising from inductance and capacitance.

RMS: It is the square root of mean value of the square of all the instantaneous values taken over a complete cycle.

Phase sequence: The order in which the voltage in the 3 phases or reach their maximum positive values is called phase sequence.

Slip: The speed of the rotor is relative to that of the rotating flux is term as slip.

Trickle Charge: To maintain a battery which is not in use in a fully charged condition. A small current is supplied which is equal to the internal losses of the battery. 

Short circuit: The circuit in which live or positive wire & negative or neutral wire meet without consuming device is called short circuit.

Fault detected by - resistance test 

Earth fault: Due to break of insulation allowing the conductor to touch the earthed metal.

Fault detected by - earth fault indicators lamp 

Preferential trip: If a generator is overload, then preference relay set an alarm and acts to trip selected non essential load that is

- first trip air condition & ventilation after 5 secs,

- 2nd trip refrigeration cargo plant after 10 secs,

- 3rd trip deck equipments after 15 secs.

It also monitors generator over current & reverse power. 

Reverse Power Trip: A relay which monitors the direction of power flow from a generator to the switchboard. In the event of a reverse power flow it will trip & disconnect the main circuit breaker.

Causes of reverse power trip:

1) two generator running, loss of excitation in one.

2) two generator running, load is shifted to other one but breaker was not put off.

3) circuit breaker of idle generator is put on adverently.

4) prime mover failure 

Single Phase: It means loss of one phase in three phase supply.

- running motor will run with fault

- stopped motor will not start

- other phase will take over current & overheated. 

Under Voltage Trip: closure by mistake of an alternator breaker when the machine is dead is prevented by an under Voltage trip. 

AVR: Automatic Voltage Regulator which is used in conjunction with generator for controlling the terminal voltage to give a steady voltage under varrying load. 

Diode: It is a semiconductor device with two terminals, typically allowing the flow of current in one direction only (electrical non return v/v)

Rectifier: An electrical device converting an a.c current into d.c current by allowing to flow in one direction only. 

Capacitor: A device used to store electrical charge consists of one or more pair of conductor separated by an insulator. 

Ingress Protection: The motor enclosure protection is defined by the ingress protection code where a two figure number is used to indicate the degree of protection against the ingress of solids & liquids.

Motor Protection: Three method of motor protection:

1) overload trip

2) overheat protection

3) bi-metal strip

Circuit Breaker safety:

1) safety interlock

2) under voltage trip

3) separate arching contact are fitted which are designed to open after & close before main contact. 

Key Lamp Method: When we give load to an alternator by emergency synchronizing lamp, there are three lamps. One is connected across the same phase, other two is cross connected. When a incoming machine is synchronized the lamp which is connected across the same phase will goes dim & the opposite lamp will show equal brightness, then we close the circuit breaker. This method is called key Lamp Method. 

Difference between fuse & relay:

Fuse	Relay
1) one time use	1) can be used many times
2) it can be used in alternator distribution system	2) it only be used for motor & alternator protection.
3) if voltage increase, fuse will burn	3) timing & current can be set
4) it can be used as backup of O/C relay	4) it can be used as first O/C protection of motor in alternator

Earthed Neutral:

1) neutral point of the alternator earthed to the hull.

2) higher fault current flow

3) equipment running with fault must be disconnected

4) fault current high enough to operate the trip

5) over voltages are lowered

Insulated Neutral:

1) neutral point of the alternators insulated.

2) very little current will flow through an earth fault

3) in essential electrical motor can be kept running until stoppage for repair is convenient.

4) over voltages are higher

 

Residual magnetism: The magnetism retained by certain materials after the magnetizing force has been removed. 

E.M.F: when current is supplied in the terminal potential difference is called e.m.f.

Back e.m.f: when current flow from the main supply through the armature conductors of a D.C motor produced e.m.f. This induced e.m.f try to rotate the armature in opposite direction is termed e.m.f. (this must be always less than the terminal voltage)