Answer / mohan rao

Lightning arrestors are always placed between line and
Gronund. Line to Ground Voltage of a 11 kV System is 6.35
kV. The lighning arrestoer rating should be above this
level but below 11 kV so that 9 kV arrestor is suitable.

Answer / d. bhowmick

All power system devices are designed with certain
tolarence limits.This is because it is always desirable
that the equipments/devices can withstand transient
voltage/current rise without any damage up to a certain
preset limit.
For an 11 KV line, the effective phase to ground voltage is
6.35 KV. That is, an LA associated with this line(between
the phases & ground)is supposed to operate when the system
voltage exceeds 6.35 KV. However, in many cases the system
voltage may momentarily rise above this value owing to
switching action or induction-that can easily be tolarated
by the associated equipments(having their individual
tolarances). Under this circumstance, it is desirable that
the LA donot unnecessarily operate especially when the rise
in voltage is within the tolance limit of the associated
equipments. To enable this, LAs with slightly higher value
of voltage rating are provided for a system operating at a
certain system voltage. Clearly, for a 6.35 KV(L-G)system
associated with a 9 KV LA, no damage is expected to be
caused if the system voltage rises by a maximum of 2.65 KV.
Beyond this, the LAs will come into action.

Answer / amit

The system voltage is always specified in RMS value or we can say that we can measure only RMS value but actually the system is subjected to peak voltage. in case of 11 kv system if the LA is provided between line to neutral, which is normally the case is, the phase value will be 11kv/sqrt(3)= 6.35kv (the phase value. but the peak value for this phase voltage will be sqrt(2)*6.35kv = 8.98kv. hence the rating of LA will be 9kv

Answer / a.umapathy

The reason is when ligtning impulse is more than 9kV the
lightning arrestor by pass the lightning impuluse so that
the 11kV system will be affected. If lightning arrestor
rating is more than the system working voltage (say 15kV)
then 11kV system will experience higher voltage which will
cause the damage in apparatus (as this designed for 11kV).
So insulation coordination of protective device must be
less than the device to be protected.

Answer / neelesh gupta

The most simple funda for calculating LA Rating-

LA Rating = Rated Line Voltage* (Square Root 2) / (Square
Root 3)

To be more precise-

LA Rating = Rated line voltage*1.414/1.732

Suggestions welcome @ neel.kewl@gmail.com

Answer / ganika

Lightning arrestors are always connected between line to
earth. Line to earth Voltage of a 11 kV System is 6.35
kV. The lightning arrestors voltage grade also same, But we are selecting the Standard, suitable, and safe apparatus availability in the market.

Answer / gagan

SAME AS ABOVE

Answer / mohan

THE L.A USED FOR 11KV SYSTEM IS 9KV BCOZ IT OPERATE AT 9KV
OR ABOVE IF WE USE L.A OF HIGHER RATING (15KV) THEN IF
VOLTAGE SURGE OF 14.5 IS PRODUCED THEN OUR LA WILL NOT
OPERATE AND PROTECTION FAIL AND HIGH VOLTAGE SURGR DAMAGE
THE SYSTEM

Answer / virender

The LA are designed to by pass the high voltage surges so
that the system voltages do not get affected and the
equipment in line are safe. We place 9kv in 11kv substation
becouse it can attract the lightning surges in the 11KV
system with low resistance and can ground quickly.

If anyone want to add more he is welcome.

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