what's the advantage of using 4 core cable over 3.5 core cable ? And then, what chang

Status
Not open for further replies.
P

phatcreators

Full Member level 5
Joined
Sep 2, 2011
Messages
315
Helped
24
Reputation
48
Reaction score
24
Trophy points
1,298
Activity points
3,458
what's the advantage of using 4 core cable over 3.5 core cable ? And then, what changes do we have to make for the neutral size & earth size cable calculation ?

thanks
 

With 4 core cable you can run one phase to its maximum load with out the other phases being used. So if you have a 3 phase supply running into a 3 bay switch board, each bay feeding a different floor in an office block, you could switch off two floors.
Frank
 
can you provide any example of this ? what do you mean by maximum load on any one phase ?

thanks
 

if your circuit is rated at 100A/phase, then using cable rated for 100A for the neutral allows you to use 100A from one phase and zero amps from the others. If all three phases were loaded to 100A, then there would be zero current in the neutral conductor. Electric motors are made without a "neutral" wire because the currents through all three phases are the same and even if a neutral wire was supplied then no current would flow through it.
Frank
 
Some general info (thumb rules)...

Neutral conductor must be at least 1/2 the size of other conductor.
It is theoraticaly explained , that algebric sum of all the phases when in balanced condition
then neutral current will be zero.

Now suppose one phase fail i.e. total unbalanced condition its neutral conductor will provide
a return path and its current will be half the load on other phases so by this
result on neutral conductor for every unbalanced condition its current will not exceed
the rated value (same as in phase conductors).

The selection of neutral conductor depends on the nature of loads, such as the load harmonics
(more than 20% loads are electronic) or the load balancing (most of the loads are 3 phase motor) etc, etc.

So, there are few general cases to consider here:
1. If the loads are almost balanced and there is no significant electronic loads, you can select 3.5 (3 + 0.5)

2. If the loads are not balanced and there is no significant electronic loads, you can select 4 (3 + 1)

3. If the loads are balanced and there is significant electronic loads, you can also select 4 (3 + 1)

4. If the loads are not balanced and there is significant electronic loads, you need to select even higher size
for neutral (3 + 1.5)

With modern equipment it is getting to be a major problem especially with IT gear.
Some cable manufacturers can now supply cable with a neutral 1.5 X phase conductor.
 
1. If the loads are almost balanced and there is no significant electronic loads, you can select 3.5 (3 + 0.5)

2. If the loads are not balanced and there is no significant electronic loads, you can select 4 (3 + 1)

3. If the loads are balanced and there is significant electronic loads, you can also select 4 (3 + 1)

4. If the loads are not balanced and there is significant electronic loads, you need to select even higher size
for neutral (3 + 1.5)

Would you give the examples of the circuitry for these given stated 4 cases ?

thanks
 


How to check & measure whether the load is balanced without any cabling ?
What are the examples of significant electronic loads ?

thanXxX
 

How to check & measure whether the load is balanced without any cabling ?
Click to expand...

Measuring without cables ??? Impossible.
We can say that 3-phase load is balanced, when load impedance in each phase has same value.
( This means that resistance and reactance in every phase has same value , i.e. currents and cos(phi)´s are equal too.)

What are the examples of significant electronic loads ?
Click to expand...

This means all the loads that generate harmonic distortion in the power grid.
for ex. electronic power supplies, motor inverters, thyristor- and triac power controllers, electronic lamp ballasts, etc,etc....
 


How to measure the load impedance in each phase ? with DMM .....

How to measure the reactance in every phase ? with DMM ......

thank You very much
 
1. If the loads are almost balanced and there is no significant electronic loads, you can select 3.5 (3 + 0.5)

2. If the loads are not balanced and there is no significant electronic loads, you can select 4 (3 + 1)

3. If the loads are balanced and there is significant electronic loads, you can also select 4 (3 + 1)

4. If the loads are not balanced and there is significant electronic loads, you need to select even higher size
for neutral (3 + 1.5)

Would you give the examples of the circuitry for these given stated 4 cases ?

thanks
 

How to measure the load impedance in each phase ? with DMM .....

How to measure the reactance in every phase ? with DMM ......

thank You very much
 

Here is one way to find some approximate values.....
This is useful only for light loads (current = few amperes )

 
Last edited:
kak111 said:
Here is one way to find some approximate values.....
This is useful only for light loads (current = few amperes )

View attachment 67277
Click to expand...

why this method & formula isn't valid for heavy loads ?
And, how to to measure load impedance & reactance in each phase for heavy loads ?

thanxx
 

what do you mean by warning that you gave in the attachment ?

without harmonics
Presume, pure real power input without imaginary (reactive) part


although you took reactance in your calculations , so what's imaginary (reactive) part are you pointing at ?

And, what is 2.load ? (measured current per phase with 2. Load) ...... Is it an another load ????

thanks yo
 

what do you mean by warning that you gave in the attachment ?
without harmonics
Presume, pure real power input without imaginary (reactive) part
Click to expand...

I made a simplified and shortened presentation
how the distortion and harmonics affect the power factor cos (phi) and
so to the result of impedance calculation, made by measuring u, i and cos(phi)
with multimeter (clamp meter)





All text...
**broken link removed**
 
Presume, pure real power input without imaginary (reactive) part
Click to expand...

Here is the picture that show the problem, when measuring impedance in
device connected to power grid.
In point A cos(phi) is not 1 and measured device is not connected to grid.

 
Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…