QUE-ANS:10-30 words

[1]        What are the functions of circuit breakers fuses relays?

The circuit breakers and switches are the means of controlling the flow of electric current. The fuses and the relays protect the distribution system from large fault currents.

[2]        What is system protection discrimination?

System protection discrimination is provided so the generator breaker only trips if a feeder breaker fails to do so or a bus bar fault occurs.

[3]        Why can the fault current be higher when the alternator is cold?

The fault current can be higher when the alternator is cold because the field resistance will be low then.

[4]        In practice what is the time delay from 10% full load capacity?

In practice the time delay from 10% full load currents is 20 seconds.

[5]        What is the usual short circuit condition time delay for alternator over current protection?

The usual short circuit condition time delay for alternator over current protection is 0.1 and 3 seconds, the actual setting depending on discrimination requirements.

[6]        What is the common characteristic of all over current relays?

The common characteristic of all over current relays is the bigger the current, the faster it will operate.

[7]        Why is higher viscosity oil used in marine dashpots?

Higher viscosity oil is used in marine dashpots because of the higher ambient temperatures.

[8]        By what percentage would the rating of a circuit breaker change from its free air value due to switchboard mounting?

The percentage the rating of a circuit breaker would change from its free air value due to mounting in a switchboard would be around 80 or 90%.

[9]        What is the maximum temperature allowable with rated full load current?

The maximum temperature allowable with rated full load current is 80 degrees C (i.e… 40 degrees C rise above ambient of 40 degrees C).

[10]      What determined the size of the short circuit fault current?

The size of the short circuit current is determined by the total impedance of the generators, cables and transformers in the circuit between the generator and the fault.

[11]      What apart from overload can initiate preference tripping?

Apart from overload, preference tripping may be initiated by low generator frequency.

[12]      What kind of over current protection is found in MCCB and MCBs?

The type of over current protection found in MCCB’s and MCB’s are thermal relays.

[13]      How is preference tripping tested?

Preference tripping is tested by current injection.

[14]      What can be used instead of reverse power protection?

Instead of reverse power protection, electrical interlocks or contacts which will respond to various condition such as closing of the fuel or steam admission valve.

[15]      What is the most common form of overload protection?

The most common type of overload protection is the bimetallic thermal overload relay.

[16]      What is the advantage of the thermal relay over the magnetic relay?

The advantage of a thermal relays over a magnetic relay is that they can be approximately designed to follow the motor heating curve.

[17]      What is the disadvantage of thermal relays?

The disadvantage of thermal relays is that when circumstances require a motor can not be restarted immediately until the bimetallic strips have cooled and reset.

[18]      What is used to delay the operation of a magnetic relay?

The delay in a mechanical relay can be an oil dashpot, the time lag on all oil dashpots being dependent on the viscosity of the oil used; this is further affected by temperature.

[19]      What built in motor overload protection can be provided?

Built in motor overload protection can be achieved by using thermisters.

[20]      What does this protect against primarily?

Primarily this protects against causes of motor overheating which are not reflected in the motor current i.e., blocked ventilation.

[21]      What is the advantage of the thermostat over the thermisters?

The advantage of a thermostat over a thermisters is that they are not as bulky and as expensive and they can operate on control gear without additional equipment.

[22]      What circuit protection devices are fitted on switchboards?

Circuit safety devices fitted to switchboards are; circuit breakers, fuses, reverse relays, current sensitive relays to provide protection against faults in the distribution system.

[23]      Under fault conditions what should be the last protection device to operate?

Under fault conditions, the last protection device to operate should be the generator circuit breaker.

[24]      Give two methods of tripping non vital loads to ensure essential supplies?

Two methods of tripping non vital loads to ensure essential supplies, is automatic tripping of all non vital supplies or have a timed sequence of tripping of non vital loads until the overload is reduced to an n acceptable value.

  30-60 words

[1]        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.

[2]        How is the over current protection tested?

Over current protection is tested by injecting test currents into the breaker to check their current trip levels and time lags. The test is essentially a transformer and controller rather like a welding set.

[3]        What problem can occur if the full load current is greater than the reset current?

The problem that can occur if the full load current is greater than the reset current is that the over current device may remain in a partially operated state, from this position it may creep even with normal load and trip the breaker.

[4]        How does an electronic over current relay operate?

The electronic over current relay operates on the principal that it converts the current into a proportional voltage; this is compared with a set voltage level within a monitoring unit. The time delay is taken by the time to charge the capacitor. This sort of relay usually has separate adjustments for current trip level and for time trip.

[5]        What is the purpose of the Under Voltage Trip?

An Under Voltage Trip is fitted to all Generator Breakers.  Its main function is to Trip the Breaker when severe Voltage Dip (around 50%) occurs.  The Under Voltage Trip on a Generator Circuit Breaker also prevents it being closed when the Generator Voltage is very low or absent.

[6]        What is Reverse Power Protection for?

Generators intended to operate in Parallel must have a Reverse Power Protection Trip.  A Reverse Power Relay monitors the direction of power flowing between the Generator and the Switchboard.

If a Prime Mover Failure occurred the Generator would act as a Motor.  The Reverse Power Relay detects this fault and acts to trip the Generator Circuit Breaker.

[7]        What is the purpose of the Over Current Protection Trip?

The purpose of the Over Current Protection is to Trip the Generator in Overload situations.  The Trip is usually set at 150% Operating Capacity and has a time delay of usually 20 seconds.  This allows for short periods of Overload Currents.  It also protects against a Short Circuit in the Generator.

 

  60-100 words

[1]        State what protective devices are carried on, and peculiar to AC switchboards.

The protective devices that are fitted on AC switchboards are:

overload trip

under voltage protection

reverse power relay

Preferential trip.

For marine application, provision of protection for each phase is very rare. The most      important reason concerning the earthing of the system neutral. Normal internal generator            faults are between one phase and earth. On a typical marine system, with an insulated neutral         these faults can be tolerated until appropriate action can be taken.

[2]        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-coordinating 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.

 

[3]        How is protection discrimination achieved?

Protection discrimination is achieved by coordinating the current ratings and time delays of fuses and over current relays used between the generator and 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 the longest operating time. This will allow the protection system to disconnect only faulty circuits and maintain the electrical supply to healthy ones.

[4]        How the under voltage relay is checked and calibrated?

Under voltage protection can be electronic or electromagnetic. Checking and calibration of Generators’ under voltage relays can only be done accurately by voltage injection. A known Variable voltage is directly applied to the under voltage relay to check:

1. The voltage at which Relay pulls in (2) the voltage at which relay drops out. Generator under voltage relays have time delay to prevent false tripping during transient voltage dips (typically 15%), caused by large Motor starting currents.

[5]        Describe where overload protection arrangement associated with a shore supply connection

The overload protection arrangements associated with a shore power supply connections are:

a circuit breaker or a switch and fuses to protect the cable linking the connection box to main switchboard with data plate showing the ship’s voltage and frequency and methods of connecting the shore supply cable is displayed at the connection box.

The connection box must have suitable terminals to accept shore supply cable, including an earthing terminal to earth the ship’s hull to the shore earth.

  100-150 words

[1]        How the overload protection device is tested and set?

Overload relays and circuit breakers should be tested periodically. Bimetal elements in thermal overload relays become insensitive to heat from metal fatigue and fail to operate under overload condition. These relays should be checked for corrosion, accumulation of dirt and other foreign matter that block free movement of tripping element. Other current relays and circuit breakers may be tested by passing a specified over current through the heater or overload coil and observing the time required for it to trip. The actual tripping time should then be compared with the manufacturer’s current time characteristics. To test an overload rely, it should be disconnected from the power line and connected to low Voltage, high current power source like variable autotransformer. Current is adjusted to test Value by connecting to one of the bimetal heaters via heavy test leads. A stop watch is used to determine tripping time. Magnetic overload trips and circuit breakers may be tested in similar manner.

[2]        What is under voltage protection for generator and motor?

An under voltage release is fitted to all generator’s circuit breaker. Its main function is to trip the circuit breaker when severe voltage dip occurs. The under voltage release on a generator Circuit breaker also prevents it being closed when the generator voltage is very low or absent. The under voltage relay prevents the closure of the circuit breaker of the dead generator. Under voltage protection is also required for motor starters. The starter contactor normally provides this protection as it drops out when the supply voltage is lost or is drastically reduced. The starter circuit will not normally allow the motor to restart when the voltage supply is restored, except when special automatic restarting facilities are provided. This ensures that when the supply returns after a black out, all the motors do not start together, there by tripping the generator circuit breaker again. Essential motors may be arranged to start automatically by Sequential start with staggered time delays.