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Anonymous Poster #1

Three-Phase Problems

01/22/2017 6:16 AM

Hi, Say in a three phase system, in R phase we have a line current 0f 5 Amps, in Y phase of 6 amps, and in B phase a line current of 4 Amps. But it is visible that the load is not balanced in three phases. How to go for load balancing? Regards,

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#1

Re: three phase problems.

01/22/2017 7:25 AM

what are The loads on each of the phases..list them out and look to see what if any can be moved .. ideal the 6 needs to have part sent to the 4 to get as close to 5 as possible.

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Anonymous Poster #1
#25
In reply to #1

Re: three phase problems.

01/23/2017 9:20 AM

load moving is okay......but the loads are (2.A + 2.A=4A; 2.5A+2.5A=5 A; 3A+3A=6A), so, how can we move 1A to R phase to make it 5 A? because the loads are 2*3A in B phase ie it can only be moved in 3 Amps.

My further concern is using RCCB/RCBO here (or in similar cases of current imbalance in three phases), if there is a current difference of more than 30 mA (sensitivity), because there is more current difference already than sensitivity.

Regards,

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#26
In reply to #25

Re: three phase problems.

01/23/2017 9:57 AM

Option A: switch something off.

Option B: change earth leakage unit (RCD) to standard 60ma.

Option C: Call 0044 2722 3124 and tell someone who cares.

Option D: add some loads to the lower unbalanced phases.

Option E: moving loads. (https://www.olx.com.pk/karachi/movers-packers/)

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#27
In reply to #25

Re: three phase problems.

01/23/2017 10:22 AM

That description sound like you are using three single phase Ground Fault Interrupters on a three phase power application rather than a correctly designed three phase GFI device.

Use the correct devices for the job and the balancing issue won't be a problem being you will never ever get a three phase load to stay balanced to within +- 30 mA no matter how hard you try. Basic system voltage variation over time will guarantee it to be impossible.

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#28
In reply to #25

Re: three phase problems.

01/23/2017 2:52 PM

3 + 2 = 5

3 + 2 = 5

2.5 + 2.5 = 5

What is the problem?

RCBO takes all 4 wires, 3 phases + neutral through its 30 mA operating detector, it operates on the residual current. Normally, any current going out through one wire of four is balanced by current coming back in another wire(s) - look up "Kirchoff's Law" - there is only a residual if current goes back to source via ground or another circuit.

It does not matter if the 3 line currents are not balanced - any unbalance is cancelled by the neutral current coming back in the opposite direction.

https://en.wikipedia.org/wiki/Residual-current_device

There is some residual in a healthy AC circuit if there is unbalanced capacitance to ground but most circuits are OK - could be problem with circuit with equipment with lots of Electro-Magnetic Interference [EMI] filter capacitors to ground.

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#31
In reply to #25

Re: three phase problems.

01/24/2017 3:01 AM

<...using RCCB/RCBO here (or in similar cases of current imbalance in three phases), if there is a current difference of more than 30 mA (sensitivity), because there is more current difference already than sensitivity...>

Nonsense. That is not what a <...RCCB/RCBO...> is for. The device is not there to deal with imbalance, as imbalance is normal; what it does is establish that the vector sum of all currents passing through it does not exceed its trip setting. So the neutral conductor must pass through the device as well. The vector sum is inferred as the current passing to the earth conductor, possibly via a human being. If that current exceeds its trip setting, in this case <...30mA...>, it operates to disconnect the fault.

Imbalance is normal, regular, and expected; it does not constitute a fault nor anything worth tripping over, and certainly not at <...30mA...>.

If in doubt, consult a qualified local Electrician.

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#32
In reply to #25

Re: three phase problems.

01/24/2017 3:07 AM

So swap a 3A load with a 2A load instead.

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#2

Re: three phase problems.

01/22/2017 8:03 AM

Why do you feel such a small load needs to be precisely balanced to begin with?

Are you saurevbabu from the other thread regarding trying ballace a tiny three phase load for no as yet justified reason figuring that if you ask again but anonymously this time you will get a answer you want and not what you got last time?

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Anonymous Poster #1
#3
In reply to #2

Re: three phase problems.

01/22/2017 8:09 AM

as a matter of fact, in my earlier workplace, there used to be a problem of melting of cables. They say - it is because of three phase current imbalance. Thats why I wanted to analyze from the beginning.

Regards,

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Anonymous Poster #2
#5
In reply to #3

Re: three phase problems.

01/22/2017 11:12 AM

Well, if your 'cables' here were single strands of #32 wire, you might have a problem...

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#13
In reply to #3

Re: three phase problems.

01/22/2017 2:39 PM

Sounds more like an undersized Neutral cable melting due to current imbalance or excessive harmonic current in the Neutral cable.

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#14
In reply to #13

Re: three phase problems.

01/22/2017 5:07 PM

Or it could be it was from using Cat 5 ethernet cable as a primary power feeder like the OP must doing to have concern over a 1 - 2 amp phase to phase current imbalance.

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#18
In reply to #3

Re: three phase problems.

01/23/2017 5:08 AM

That's nonsense. The only reason a cable melts is because the overcurrent protection arrangements are inadequate.

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#19
In reply to #18

Re: three phase problems.

01/23/2017 5:26 AM

Earthed Neutrals do not have fuses. BS7671 clause 132.14.2

Maybe their problem is reduced CSA neutral conductors in 3Ø, without reduced current.

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#21
In reply to #19

Re: three phase problems.

01/23/2017 7:30 AM

Good guess. Whatever is causing a cable burn at 5-8A remains a curiosity, though.

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#23
In reply to #21

Re: three phase problems.

01/23/2017 8:05 AM

Curiosity solved. Wet string burns when dry.

Label instructions: Keep string wet to conduct. Do not over load as any drips may cause electrical leakage. In case of over-soaking use warm water to reduce shock from coldness. Do not touch energised wet string unless it is dry.

Caution: when dry, the string is a line isolator. To check balanced load use calibrated spirit level. Do not tilt head. Do not puncture bubble. Get an adult to help you.

Transmission line: see rope instruction.

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#29
In reply to #21

Re: three phase problems.

01/23/2017 7:54 PM

OP never said this low current application has a problem, only an application at a previous job had a problem.

as a matter of fact, in my earlier workplace, there used to be a problem of melting of cables. They say - it is because of three phase current imbalance. Thats why I wanted to analyze from the beginning.

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#33
In reply to #29

Re: Three Phase Problems

01/24/2017 3:09 AM

<...They say...>

Clearly, <...They...> are wrong.

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#24
In reply to #3

Re: three phase problems.

01/23/2017 8:27 AM

I worked for 3 years at a consulting group in the electrical department. We were quite involved at that time with renovating nursing homes and hospitals. I saw melting cables (wires) in one facility I had to survey before deciding what to change to get then up to code regulations. This was a Mennonite nursing home. Now the Mennonites will not use electricity in their residences on their farms, but had to for a state approved nursing home. They also would hire only their own folks for jobs in the nursing home. So they had a maintenance man / electrician who never had worked with electricity. He had AWG 14 wire (2 mm²) on any thing up to a 50A breaker. He took a breaker and kept putting receptacle and lighting loads on it until it would open, then backed some load out. I suggest this may be what you were seeing if you had wires melting - too much load on too little wire size.

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Anonymous Poster #2
#6
In reply to #2

Re: three phase problems.

01/22/2017 11:48 AM

Because the OP cannot distinguish the subtle nuances between the reasonably-well-balanced three-phase load he has here and somebody else's outrageously ridiculously unbalanced load connected with undersized cables not spec'd for worst-case conditions?

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#7
In reply to #6

Re: three phase problems.

01/22/2017 12:46 PM

Nah, just be sure your neutral line was properly grounded. Increase of current on one line above tolerance is a sign of defect of perhaps winding coil loosing resistance or shorted along the line loop or, maybe the motor rewinding guy miscounted how many loops he has to make.

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Anonymous Poster #2
#9
In reply to #7

Re: three phase problems.

01/22/2017 1:01 PM

When you are dead, you don't know that you are dead. It's a pain only felt by others.

Same thing when you are stupid.

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Anonymous Poster #4
#16
In reply to #9

Re: three phase problems.

01/22/2017 7:10 PM

Some people though they sèem to live but are actually dead. They walk amongst us.

Oh, they've been tricked alright. Scared of death so they've died.

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#30
In reply to #9

Re: three phase problems.

01/24/2017 2:14 AM

but the ghost is still alive

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#17
In reply to #7

Re: three phase problems.

01/22/2017 8:44 PM

Harris, you have been outed! Your secrets exposed.

Tread lightly.

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#10
In reply to #2

Re: three phase problems.

01/22/2017 1:31 PM

sauravbabu and coolyaar in one week!

What have we done to deserve such torture?

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#11
In reply to #10

Re: three phase problems.

01/22/2017 2:21 PM

Both posters you mention have been members here long enough to have gotten advanced technical degrees, and yet, seem to have learned nothing, except how to come here for answers.

The latter even suggested that his superior condoned these unethical acts.

If you keep feeding a stray cat, they keep coming back for more.

Tip of the hat to AP 2 for that astute observation.

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#20
In reply to #10

Re: three phase problems.

01/23/2017 7:21 AM

Masters of the Con. Worked with many and they learn to hussle at a very early age.

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#36
In reply to #10

Re: three phase problems.

01/25/2017 12:52 PM

well i am here to keep the flow going

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#4

Re: three phase problems.

01/22/2017 9:19 AM

Reading actually helps secondly to surfing materials to read in google.

http://www.tek.com/dl/55W_28943_0_HR_Letter.pdf

http://stevenengineering.com/Tech_Support/PDFs/45RPHS.pdf

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#8

Re: Three Phase Problems

01/22/2017 1:00 PM

You should know the amperage draw of each piece of equipment you have...you should also know which breakers control what machines or other loads...sometimes the circuits are dedicated, one breaker for one machine....sometimes you can just switch the breaker positions in the box to balance a load....loads are rarely balanced all the time, because not everything is being used all the time....Some circuits require momentary starting current, that can be much higher than the running current, this should be taken into account, not only in phase balancing but also wire sizing and breaker type and sizing....

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#12

Re: Three Phase Problems

01/22/2017 2:28 PM

Already commented on similar balancing.... posts #4, #5, please refer.....

http://cr4.globalspec.com/thread/110438/Load-Balance-Problem

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Anonymous Poster #3
#15

Re: Three Phase Problems

01/22/2017 6:43 PM

What coolyaar (aka Haris Mahmood, electrical engineer from Lahore, PK) doesn't realize is that all of his postings are visible on Google. Hopefully his boss and clients will come across them someday.

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#22

Re: Three Phase Problems

01/23/2017 7:34 AM

Simple. Take any 1A single-phase load off the Y phase, and connect it to the B instead.

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#34

Re: Three-Phase Problems

01/24/2017 7:11 PM

When designed, the designer tries to allocate balancing 3 phase. But when used not always get the 3 phase balance.

Can replace four poles RCBO by four poles MCCB combination with 3 ELCB 2 poles.

If the neutral wire is out of RBCO, you'll get an error

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#35
In reply to #34

Re: Three-Phase Problems

01/25/2017 3:50 AM

Quite.

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