Question { Birla, 44771 }

How do you maintain the earthing resistance?

Answer

Better to use stainless steel bolts nuts and washers since
GI ones tend to deteriorate and get corroded after a long
time and it becomes difficult to open the bonding,
connection for taking measurements.
Simple method is pour lot of salt and water around the
earth electrode and surely you will get a very good value
of earth resistance instantly.

Question { Hindustan Zinc, 26892 }

how to calculate no. of earthing pits and their diamensions?
no. of lightning arrester rods which are required to
protect a building of 35 mtr and we calculate?

Answer

The number of earthing pits depends on the soil resistance.
We may require earth resistance of 1ohm or 3 ohms at the
most. To get this we will have to install at least 2 pits
and as per parallel formula i.e. 1/r1+1/r2+1/r3= 1/R. We
have to keep on increasing the number of earth pits till we
get the required value for R=1 or 3 as per requirement.

Question { Wheels India, 18234 }

What power factor maintain you maintain at u r organisation

Answer

We maintain 0.8 and above since we are penalised by KEB for
low pf. we generally maintain close to 0.9 by use of power
capacitors. Too high i.e. above .95 is too costly to
install and also the problem is that if power factor is
close to 1 then there are chances of high voltage affects
motors and we suffer more motor burning at our plant.

Question { Wheels India, 18234 }

What power factor maintain you maintain at u r organisation

Answer

Well so many answers to this simple question. The real
answer as per my experience is 0.85 to 0.9. The minimum
requirement by the Electricity boards is 0.8 to 0.85 or
they will charge a fine. But for industry 0.85 to 0.9 is
optimum and above this you will have to use large capacity
of Capacitor banks which is not practical. Also if
capacitors are too many than this will give leading PF and
this is not good since this leads to higher voltage and
many times has resulted in motor burning or failures, and
permature failure of electical equipment.

Question { 5286 }

how to read AWG n how to convert AWG reading into mm?

Answer

American Wire Gauge
(AWG) Diameter
(inches) Diameter
(mm) Cross Sectional Area
(mm2)
0000 0.46 11.68 107.16
000 0.4096 10.40 84.97
00 0.3648 9.27 67.40
0 0.3249 8.25 53.46
1 0.2893 7.35 42.39
2 0.2576 6.54 33.61
3 0.2294 5.83 26.65
4 0.2043 5.19 21.14
5 0.1819 4.62 16.76
6 0.162 4.11 13.29
7 0.1443 3.67 10.55
8 0.1285 3.26 8.36
9 0.1144 2.91 6.63
10 0.1019 2.59 5.26
11 0.0907 2.30 4.17
12 0.0808 2.05 3.31
13 0.072 1.83 2.63
14 0.0641 1.63 2.08
15 0.0571 1.45 1.65
16 0.0508 1.29 1.31
17 0.0453 1.15 1.04
18 0.0403 1.02 0.82
19 0.0359 0.91 0.65
20 0.032 0.81 0.52
21 0.0285 0.72 0.41
22 0.0254 0.65 0.33
23 0.0226 0.57 0.26
24 0.0201 0.51 0.20
25 0.0179 0.45 0.16
26 0.0159 0.40 0.13

Question { Kuwait Oil Company, 14266 }

Hod do u calculate cable size for underground laying and
the factors effecting/influencing it??

Answer

In case of laying of single core cables then trefoil
formation should be adopted. i.e. R Y and B phases should
be clamped with alluminium clamp only or Stainless steel
clamp to avoid induction.

Derating factors are also to be considered which depends on
the distance of cable laid from each other. If cables are
touching one another incase of multicore cables then
derating factor will be about 0.5 to 0.6 or so.

Question { 4802 }

According to general circuit diagram of 1-Phase t/f , why
the primary is not get short circuited ,even though the
supply terminals both are directly connected through
primary winding?

Answer

To understand this correctly take the example of a simple
bell in your home. It has only 1 winding and due to this
the solenoid acts and rings the bell. But the number of
turns should be correct or if less then shortciruit may
occur and if more then it will not work the bell.

In general the formula V secondary/ V primary = Turns
secondary / turns primary. i.e. Vs.Tp = Vp.Ts

Question { 4895 }

What is the Derating factor of power cables when exposed to
direct sunlight in (both PVC & XLPE).

Answer

Generally all cables are rated for ambient temperature. Now
temperature varies from place to place and hence we have
tables showing cables in air and cables buried underground.
Then there is cables spaced between one another which is
more important aspect.
Cable current carrying capacity should also be based on our
own judgement. The table only is a guide. Its just to help
us get the correct size cable for your requirments. So all
these factors help us in deciding the best size cable for
our load. Distance is also an important consideration due
to more voltage drop.

Question { 22519 }

In Siemens S7-300 PLC, what is funtion and funtion block ?
What is the difference between them ? When to use function
and when to use function block

Answer

support.automation.siemens.com

you may approach them for your answer if it is related to
specific equipment used by yourself.

vi S7 Standard Functions Part 2 C79000-G7076-C113-02 The
following functions and function blocks are described in
this manual:
Function or Function Block
Number
Page
oftware Timer On Delay—Retentive (TONR) FC80 5-2 Indirect
Block Move (IBLKMOV) FC81 4-2 Reset Range of Outputs (RSET)
FC82 1-2 Set Range of Outputs (SET) FC83 1-6 Add to Table
(ATT) FC84 2-2 First In/First Out Unload Table (FIFO) FC85
2-4 Table Find (TBL_FIND) FC86 2-6 Last In/First Out Unload
Table (LIFO) FC87 2-9 Table (TBL) FC88 2-11 Move Table to
Word (TBL_WRD) FC89 2-13 Word Shift Register (WSR) FC90 3-2
Word to Table (WRD_TBL) FC91 2-15 Bit Shift Register (SHRB)
FC92 3-4 Seven Segment Decoder (SEG) FC93 6-2 ASCII to Hex
(ATH) FC94 6-4 Hex to ASCII (HTA) FC95 6-6 Encode Binary
Position (ENCO) FC96 6-8 Decode Binary Position (DECO) FC97
6-9 Ten’s Complement (BCDCPL) FC98 6-10 Sum Number of Bits
(BITSUM) FC99 6-11 Reset Range of Immediate Outputs (RSETI)
FC100 1-4 Set Range of Immediate Outputs (SETI) FC101 1-8
Standard Deviation (DEV) FC102 7-2 Correlated Data Table
(CDT) FC103 2-18 Table to Table (TBL_TBL) FC104 2-21
Scaling Values (SCALE) FC105 6-12 Unscaling Values
(UNSCALE) FC106 6-14 Lead/Lag Algorithm (LEAD_LAG) FB80 6-
16 Discrete Control Alarm Timer (DCAT) FB81 5-4 Motor
Control Alarm Timer (MCAT) FB82 5-7 Index Matrix Compare
(IMC) FB83 8-2 Scan Matrix Compare (SMC) FB84 8-6 Event
Maskable Drum (DRUM) FB85 5-10 Pack Data (PACK) FB86 4-4

Question { 3209 }

How over load relay works.

Answer

An over load relay acts on thermal bi metal principle and
whenever current exceeds a set value then the bimettalic strip
bends and trips a lever which in turn trips the circuit.

Question { 17037 }

If you see the HRC fuse name plate. Its shown ampere rating
shown which is in many KA (like 80KA and so on). any body
explain what is meaning of this value?

Answer

I understand your predicament. But you will only have to
see the 100A which is written in your case and this is the
capacity of the fuse.

80KA is the rupturing capacity of the fuse which is not for
you but for system designers who want to know these things.

Question { 15018 }

if we supply DC power to AC transformer then what its result?

Answer

Real electrical engineers or electricians will not take
such risks. Most electrical equipment is very costly and as
electrical personnel it is my sincere request not to play
with electrical equipment but respect it and take all
correct steps to ensure that proper tests are done before
any switching on is done. My one policy is an electrician
can check 10times but mistake takes place only once. BOOM.

Question { 8029 }

how can we calculate that how much capacitor is needed for
a system to increase the pf?

Answer

There is even no need to calculate. We have these charts which give the exact kvar required to be installed if we only know the load in KW and present pF and PF required.

Question { 4886 }

can we check the resistance of earthing electrode while it
is connected with the neutral of UPS system

Answer

No!! You will have to disconnect the connection which is
usually bolted to the earth electrode.

Then with earth megger you will have to test the earth
electrode independently.

Question { Meinhardt, 4017 }

what is the advantage of Cathodic eathing system and what
is the basic principle and how itis working, please explain
clearly .

Answer

cathodic protection systems are two types one is ICCP i.e.
Impressed Current Cathodic protection and the other is
sacrificial Anode Protection system.
Sacrificial anode ytpe cathodic protectionsystems provide
cathodic current by galvanic corrosion. The curent is
generated by metalically connecting the structure to be
protected to a metal/alloy that is electrochemically more
active than the material to be protected. (Both the
structure and the anode must be in contact with the
electrolyte.) Current discharges from the expendable anode,
to the electorlyte and onto the structrue to be protected.
Special backfill mateiral can also be used.
contact williamlobo@rediffmail.com for more details.