Question { Intergulf, 141932 }

what is the difference between the HT Cable and LT Cable ?

Answer

For LT cable

1. Use : Up to 690V

2. Insulation : PVC
3. Core(Neutral) : Yes
4. Insulation Grading : 1100 V

For HT cable

1. Use : Above 690
2. Insulation : XLPE
3. Core(Neutral) : No
4. Insulation Grading : Above 3.3kV, 11KV..etc

( Please note that now 690 V using in VFD application with
LV cable and 440 V for DOT motor starter)

Question { Adani, 5641 }

whycircuit breaker is not connected at the generator end
where the capacity of the generator is more say 60MW, 100MW
etc.

Answer

Due to following points
1)Transient Recovery Voltage (TRV)
2)Higher X/R value
3) Delayed current Zero
circuit breakers was not used after above 60 MW generators.

However please note that now full line of IEEE C37.013
qualified medium-voltage vacuum generator circuit breakers
for voltages up to 24 kV and generators up to 200 MW for
both indoor and outdoor applications are available in
market.

Please note that Circuit breakers applied to generator
switching applications are subject to conditions quite
different than those of a normal distribution circuit
breaker used in industrial, commerical and utility systems.
Distribution circuit breakers, whether of the outdoor
substation type or of the drawout type for use in metal-
clad switchgear, conform to IEEE C37.04 (basis of ratings),
IEEE C37.06 (preferred ratings), IEEE C37.09 (testing) and
IEEE C37.10 (application.)

Question { 3601 }

why in an electrical lab a bulb connected to supply goes off
slowly when the main supply is switched off?

Answer

Generally capacitors are connected across power supply for
power factor improvement and as a Harmonic filter. As we
know that capacitor having power storing property so when
power supply switch off all power stored by capacitors
discharge through connected load. That's why bulb goes off
slowly.

Question { 12504 }

Why HRG and LRG used beside transformer, what is the purpose

Answer

Resistance Grounding falls into two categories: Low
Resistance and High Resistance

1) In Low Resistance Grounded (LRG) Systems the current is
limited to 25 amps or more. Generally the range is from 25
to 600 amps, although in some systems it may be even
greater.

2) In High Resistance Grounded (HRG) Systems the current
is limited to 10 amps or less.

A) On systems with a voltage of 1000 volts or less (phase-
to-phase) grounding resistors are not used. Normally these
system use a solidly grounded configuration with no
intentional impedance being introduced into the system.

B) Resistance Grounding is recommended on Medium Voltage
Systems from 1000 volts to 15,000 volts phase-to phase.

C) Because of the cost, Resistance Grounding is not usually
used on systems above 15,000 volts phase-to-phase.
Additionally, the use of a solidly grounded system allows
for use of equipment which is insulated for the phase-to-
neutral voltage of the system.

D) When a system has protective relays which will trip the
circuit if a ground fault occurs, a grounding resistor with
a 10 Second Rating is often specified, because the relays
will trip the system in less than 10 seconds. However, One
Minute or Ten Minute ratings are sometimes used for an
extra margin of safety, even though the cost will be
greater.

Question { 12504 }

Why HRG and LRG used beside transformer, what is the purpose

Answer

Resistance Grounding falls into two categories: Low
Resistance and High Resistance

1) In Low Resistance Grounded (LRG) Systems the current is
limited to 25 amps or more. Generally the range is from 25
to 600 amps, although in some systems it may be even
greater.
2) In High Resistance Grounded (HRG) Systems the current
is limited to 10 amps or less.
------------
A) On systems with a voltage of 1000 volts or less (phase-
to-phase) grounding resistors are not used. Normally these
system use a solidly grounded configuration with no
intentional impedance being introduced into the system.
B) Resistance Grounding is recommended on Medium Voltage
Systems from 1000 volts to 15,000 volts phase-tophase.
C) Because of the cost, Resistance Grounding is not usually
used on systems above 15,000 volts phase-to-phase.
Additionally, the use of a solidly grounded system allows
for use of equipment which is insulated for the
phase-to-neutral voltage of the system.
·D) When a system has protective relays which will trip the
circuit if a ground fault occurs, a grounding resistor with
a 10 Second Rating is often specified, because the relays
will trip the system in less than 10 seconds.However, One
Minute or Ten Minute ratings are sometimes used for an
extra margin of safety, even though the cost will be
greater.

Question { ETA, 8777 }

What is the difference between LT & HT Cable

Answer

LT HT
1. Use : Up to 690V Above 690

2. Insulation : PVC XLPE
3. Core(Neutral) : Yes No
4. Insulation Grading : 1100 V Above
3.3kV, 11KV..etc

( Please note that now 690 V using in VFD application with
LV cable and 440 V for DOT motor starter)

Question { 13201 }

Q- Why Phase tester is only used for checking AC not DC ?

Answer

A mains tester works because the potential difference
between the live and Earth makes a very small
(imperceptible) current flow through the tester and our
body to Earth. This current is just enough to light the
lamp / Neon. When you stick it in the Neutral socket, the
potential difference (just a volt or two, usually) is not
enough to give sufficient current to light the neon.When
the tester light glows, it mean electrons flow like,from
live terminal > tester > body > floor & walls of building >
inside the earth>goes back to the generator, via earth or
neutral conductor (Please note that somewhere, Neutral and
Earth are connected together so we have, in effect, a huge
loop of circuit.)

Above description is for AC supply only ( path complete
thorough earth or nutral to generator neutral), however
please note the fhilosophy is not applicable for DC supply
where - V is in floting condition (not grounded or earthed)
so due to un completation of that circuit tester light will
not glow.

Question { 4479 }

Q- In the process of checking AC through Phase Tester our
body is the part of circuit but we don't recevie Shock, Why ?

Answer

A mains tester works because the potential difference
between the live and Earth makes a very small
(imperceptible) current flow through the tester and our
body to Earth. This current is just enough to light the
lamp / Neon. When you stick it in the Neutral socket, the
potential difference (just a volt or two, usually) is not
enough to give sufficient current to light the neon.When
the tester light glows, it mean electrons flow like,from
live terminal > tester > body > floor & walls of building >
inside the earth>goes back to the generator, via earth or
neutral conductor (Please note that somewhere, Neutral and
Earth are connected together so we have, in effect, a huge
loop of circuit.)

Above description is for AC supply only ( path complete
thorough earth or nutral to generator neutral), however
please note the fhilosophy is not applicable for DC supply
where - V is in floting condition (not grounded or earthed)
so due to un completation of that circuit tester light will
not glow.