Question { 11110 }

Do you know how to maintain a generator, if so what are
those maintenances?

Answer

First of sll it's body should be maintained clean and dust
free for proper cooling. Carbon brushes (if available)
should be checked properly for any sparking. Excitation
voltage should be correct and automatic voltage regulator
should be working perfectly. Load should not be given more
than its rating. If possible always it should be allowed to
run in better lagging power factor. If any external cooling
media is available, it should work properly.

Question { Grasim, 22507 }

Procedure for conduting vector group test on power
transformers.why it is conducted?

Answer

I also wanted to know this. But as per my knowledge, Same
phase of both primary and secondary are to be shorted
before applying voltage in the answer given by Mr.
Maheshreddy.
Please let me know am I correct?

Question { Wipro, 11944 }

What are methods to avoid the inrush current at the time of
starting of power transformer ?

Answer

We cannot avoid inrush current when any transformer is
switched on after a long period of shutdown. Only thing we
can do is minimising it by charging the transformer by the
lowest voltage (using tapp changers)and we can avoid
spurious tripping by inrush current by using only
transformer differential relays in which an inrush current
filter is used.

Question { Sify, 45326 }

Why we should use delta connection in primary and star
connection in an distribution transformer, which converts
400v a.c. voltage to 33kv.

Answer

the answer of Mr. RVG is correct. But I would like to
correct the question itself. Distribution transformer
converts voltage from 33KV to 400V, not 400V to 33KV.

Question { BSNL, 61117 }

what type of relays used in transformers?

Answer

There are many types of protection relays used in
transformers depend upon its construction, size, use etc.
They are
1. Buchholz relay for incipient faults like short ciruit,
winding temparature protection, oil temparature protection,
oil level protection, oil pressure protection, Pressure
relief valve (for oil filled transformers)
2. Differential protection
3. Thermal Protection
4. Earth fault protection
5. Over current protection
6. Over excitation protection(where OLTC is used) etc

Question { Focus, 20120 }

what is the procedure for cable sizing or on what basis we
will calculate the size of cable?????

Answer

The procedure for cable sizing is first to measure the
load in currents and voltage of load which is to be
connected in that particular cable. Then the length of
cable. Type of cable conductor whether copper or aluminium.
Medium of cable passing whether in opened cable tray (air),
closed cable tray, in open land, in cable trunch etc. The
usage of cable also to be taken care, ie like telephone
cable, control cable, power cable etc.
In an example the current rating of 10 sq. mm copper cable
in air is 40Amps. If the length of cable is shorter we can
use it and if the length of the cable more, we have to
calculate the voltage drop of that cable and may be have to
use 16 or 25 sq. mm cable instead of 10 sq mm.

Question { 21925 }

what is the use of Antipumping in HT breakers.

Answer

Anti pumping relay is used to prevent closing coil from
being burnt by the repeated closing command. Closing coil
is not designed for continuous electrical operation and
once the closing coil is energised, breaker will get on and
it will be remain energised mechanically until any trip
command is available. So continuous supply is not required
for closing coil.

Question { 24066 }

why ELCB can't work if N input of ELCB do not connect to
ground?

Answer

ELCB is used to detect earth leakage fault. Once the phase
and neutral are connected in an ELCB, the current will flow
through phase and that much current will have to return
neutral so resultanr current is zero. Once there is a
ground fault in the load side, current from phase will
directly pass through earth and it will not return through
neutral through ELCB. That means once side current is going
and not returning and hence because of this difference in
current ELCB wil trip and it will safe guard the other
circuits from faulty loads. If the neutral is not grounded,
fault current will definitely high and that full fault
current will come back through ELCB, and there will be no
difference in current.

Question { 10920 }

Will a 230VAC motor run OK at 208VAC.

Answer

Motor will run with lesser speed and torque. But if the
load is same as in the case of 230VAC, current will be more
and motor may burn.

Question { 5160 }

what is the permissible bend in the LT& HT CABLES.

Answer

12D for LV cables (upto the range of 1100V) nad 15D for
high voltage cables, where D = Diameter of cables with
insulation.