Question { Juniper Networks, 23397 }

how many earthing required for ups, what should be the
earth voltages

Answer

Three Earthings are required.

1. DC side Earthing to avoid the distortion in the line.
2. Equipment Earthing
3. Electronic Earthing for the cards which works in low
voltage.

Where the internal convertor will have the Trafo, that will
be connected for the common earthing.

Both should not common, if Electronic items will get affect
when Ground current flows more than a limit.

Question { 5749 }

How to calculate cable size in sq units based on motor rating
is KW

Answer

Usualy in large industries the cable size is calculated,
for 1A of current its 1 sq mm of aluminium cable and 0.5sq
mm of copper cable.

Ex: In case 600A : 300 sqmm or 2R x 150 sqmm cable to be
preferable. in same case iys 150sqmm of copper cable is
enough.

Prabhas

Question { 5126 }

While testing the 33KV Busbar by using Hipot kit, what
should be the test
voltage we have to apply?

Answer

You should apply the voltage step by step till the double
voltage capacity of the cable. this is during only in the
1st time of test for 2nd time onwards 1.8% is preferble.

Ex:33KV: You apply till 65KV

Question { 6197 }

Explain how you can give the overload setting to any Motor
Protection
Relay for a HT motors with formula and example?

Answer

Motor current rating is calculated by using the formula
I= P/1.73*V*PF

Normany it is 1.8times of power of the motor will be their
in 420v 3 phase motor.

Ex: 500Kv HT motor 11kv voltage
is Approx. 32A is the rated current.
so 32.5 or 33A is OL Setting.