[Spir Georges GHALI]

@milan vekariya said:
In any (2 OR 4 pole)rccb out put side neutral wire contact with earth, then rccb become trip(fault) neccesory. Remove neutral wire from earth contact. It is helthy (safe)condition.

Dear ;

The Neutral point in the output side of any RCCB or RCBO or ELCB shouldn’t be connected the the Earth point.

[Spir Georges GHALI]

@sinnadurai sripadman said:
ABB has made MCB with high breaking capacity like 25 or 50kA unlike 10kA,I remember reading in a website,you can get more information from manufacturer’s publications from ABB,Hager etc

Dear ;

All serious manufactures like ” Schneider Electric, hager, ABB, … ” have MCBs with ” up to ” 50kA ” as Breaking Capacity, but especially for the ” Breaking Capacity ” we should know the following :

1-  The MCB should be tested according to ” IEC 60898  or  IEC 60898-1 “, and also can be tested according to ” IEC 60947-2 “.

2- The Breaking Capacity’s value should be written as follow :

      – According to ” IEC 60898  or  IEC 60898-1 ” : in ” Amp ” like ” 3000A  or  6000A ” and absolutely in a      

      rectangular.

      – According to ” IEC 60947-2 ” : in ” kA ” like ” 6kA  or  10kA “

3- According to ” IEC 60898  or  IEC 60898-1 “, the Breaking Capacity’s value of any MCB should be written on the front face, and the value according to ” IEC 60947-2 “, if tested, can be written on the side.

4- Normally, the values according to ” IEC 60947-2 ” are bigger than those according to ” IEC 60898  or  IEC 60898-1 “, because the sequences of testes are different.

[Spir Georges GHALI]

@mohamed sleem said:
If we are doing TN-C-S system and the cable protection is a breaker, would the 1x300m2 be acceptable with a feeder 8 sets of (4x1c 630mm2)? & if all of the sets installed in group the 8x1c300m2 will be ok?

Thank you,

Dear Mr. Mohamed ;

First at all, if you use ” TN-C-S ” as Earthing System , that means at the 1st level or maybe at the 2nd level also you have ” TN-C ” and ” PEN “, for that it will be better to do as follow :

– Check the Single-phase loads if they are distributed on 3 phases ( even if there’s some difference ).

– Check if the ” multiple third Harmonics ” are existed, if yes, you should know the percentage of each one.

Then, according to the results of these checkings, we can decide the section of ” PEN ” as follow :

– If the Single-phase loads are well distributed, and the percentages of the ” multiple third Harmonics ” are small and/or the total current of them is small, we can assume that ” 1x300m² ” or ” 2x240m² ” are enough, then we should calculate the Earth Fault Current for one of these 2 sections, and check if the chosen section can withstand the ” Thermal Stress ” of the Earth Fault Current during at least 1 or 2 sec ( certainly not more than 5 sec ), it not we should oversize the PEN’s section to ” 3x240m² ” or ” 2x300m² ” and repeat the procedure.  

– If the percentages of the ” multiple third Harmonics ” aren’t small, it will be better to know the total current of them either by calculation or from the catalogues of the equipment generating these Harmonics, then we can use the same procedure that we use to define the Phases’ sections to define the section of ” PEN “, do the calculation of the  Earth Fault Current, then check if the PEN cable can withstand the Thermal Stress. 

By the way, and for ” TN-C ” system, please note that in all case the setting of the ” Magnetic  or  Short-time Protection ” should be smaller than the Earth Fault Current value by ” ≥ 20% “.

[Spir Georges GHALI]

Dear Mr. Nasir ;

Refer to your 2^nd Topic about the Circuit Breaker, and as we talk about “ Low Voltage Circuit Breakers ”, please let me clarify again some points, that are :

1- It’s written “ To prevent this larger amount of current from entering the circuit ahead of its path ” !!! ???  We know all that any current “ Load Current  or  Overload Current  or  Short Circuit Current ” flows in the circuit and go ahead to the source, because the circuit should be completed from the fault point till the source to have the fault current, so, how the Circuit Breaker can prevent the Short Circuit Current going ahead ( entering the circuit ahead ) ?  

2- It’s written “The working of a circuit breaker is more or less similar to a fuse ” !!! ??? : if you talk about protection issue “ Yes ” that’s right, but if that’s about working way, sorry to say that’s not right, because we know all that the working way of a Circuit Breaker is completely different than the Fuse as the Fuse-link in any Fuse will melt when the current flows during a predefined time is bigger than a predefined value, but in a Circuit Breaker no piece will melt.

3- Most of the Circuit Breakers ( 95% or even more ) contain 2 kind of Protections, that are :

• “ Thermal Overload Release ” : used to protect the electrical circuit against the “ Overload Faults ” can happen in an electrically undamaged circuit, that means those faults are provoked by the connected equipment or loads ( IEC 60947-1 / definition 2.1.7 ). Noting that the Tripping Order of this protection is “ Delayed ( Inverse-time ) ”.

• “ Magnetic  Release ” : that’s used to protect the electrical circuit against the “ Short Circuit Faults ” can happen in any part of an electrical circuit “ components or connected equipment or loads ” ( short circuit between 2 or more live conductors ) ( IEC 60947-1 / definition 2.1.5 ). Noting that the Tripping Order of this protection is “ Instantly or Delayed ( up to 600 ms ) ”.

Noting that the Operation way of those 2 Protections Releases is totally different.

4- It’s written “ When the current decreases back, the electromagnet is de energized and the contact is maintained again and the circuit operations safely ” !!! ???  There are in this sentence 3 points that :

• We know all that the Fault Current doesn’t decrease by itself, except the Over-load’s Current in the case of without tripping and when the load re-becomes normal, but the short circuit current doesn’t, never ever, decrease without doing the necessary maintaining of the damage circuit.

• In the standard operating of any Circuit Breaker when it trips according to an order of Magnetic Release, the electromagnet is immediately de-energized as there’s no more flowing current to energize it.

• If you mean that the contacts will close again, sorry to say that’s not right, because in the standard operating of any Circuit Breaker when it trips the contacts doesn’t reclose by themselves, except if the Circuit Breaker is “ Electrically Operated ” ( equipped with Electric Motor, Closing & Opining relays, with a Control System ). Even more, we shouldn’t re-close any Circuit Breaker if the tripping was provoked by a short circuit current, because the Making Short Circuit Current will be, at the first cycles ( 5 to 10 cycles ) of current, bigger than the short circuit current.

[Spir Georges GHALI]

@shashank said:
why should the PEN cable be

firstly connected to the earth point at Load, then to the neutral
point?

Dear ;

Ac the ” Protection of person from the Electric chocks ” is the main goal to have an Earthing point in any electrical equipment, the PEN should be connected first

[Spir Georges GHALI]

Dears;

Refer to the Topic of Mr. Mill, I would like to mention the following points:

– The number of Harmonic is called “ Order ” ( Harmonic order 3 is the Third Harmonic “ 150 Hz ).

– The “ Soft Starter ” generates the Harmonics only in the starting & stopping steps, and as those steps are transient ( a few seconds ), so, in normal running no Harmonics are generated, and it’s considered that the “ Soft Starter ” are not Harmonics generator.

– The Elevator generates harmonics only if it’s controlled by VSD.

 

By the way, I want to mention that :

– “ IEC 61000-2-2  &  2-4 ” defined the maximum accepted level percentages of Voltage Harmonics in the LV Networks, either for Distribution Networks, or for privet networks connected to the utilities networks.

– “ IEC 61000-3-2  &  3-12 ” defined the maximum percentages of Current Harmonics that can be generated by electronic equipment, and even the maximum value of “ THD-I ”.

[Spir Georges GHALI]

@guest said:
but no load current is so small.. as we know pf is inversely proportional to current then it should have been high when no load current is small right??

 

Dear ;

 

As we know all, the ” no-load current ” of a transformer is the ” magnetizing current “, that means the current of the transformer’s impedance ” Reactance + Resistance “, and as the biggest one of those is the reactance, so the PF of no-load current is small or too small.

[Spir Georges GHALI]

Dear Mr. Steven ;

 

First, thanks for this Topic that explain the basic info about the using the Capacitors in Electric Networks.

 

By the way, I want to mention to the following points:

1- According to “ IEC 60027-1 ” the unit of Reactive Power should be written by small letters “ var  or  kvar ” and not “ VAR  or  kVAR ”.

2- I mentioned many time before, the Capacitors in any electric networks are Loads that consume Reactive Power with Leading Power Factor, but the inductive loads consume Reactive Power with Lagging Power Factor, and the results of those 2 reactive powers are the Vector summing, so, it seems that the capacitors compensate the reactive power of inductive loads.

3- The using of Capacitors to improve the PF does not affect the Active Current’s Load, but in reality, the Apparent Power needed for that load will be decreased after compensation, so, the available apparent power in the source will be increased.

[Spir Georges GHALI]

@guest said:
Will there be any change in capacity rating of a 3 phase 750 KW generator if to change from 440 VAC to 230 VAC?

Dear ;

 

First, the Power’s unit of any source ” Transformer, Generator, … ” is given in ” Apparent power – S ” and the unit is ” kVA “

 

Even that your question is not too clear, I will try to clarify that point as follow :

Assume that we have 2 sources with the same rating power of each one ” 200 kVA “, but the output voltage is different where that’s ” 400V – 3 phase ” for the 1st and ” 230V – 3 phases ” for the 2nd, and as the Power is the multiplication of the Voltage value by the current value, and the Power for both are the same, in this case the ” Nominal Current – In “  will be different that can be calculated as following :

S = 1.732 x Uo x In       →     In = S / 1.732 x Uo

so, the Nominal Currents are :

for the 1st source :

In(1) = 200 x 1000 / 1.732 x 400 ≈ 289 A

for the 2nd source :

In(2) = 200 x 1000 / 1.732 x 230 ≈ 502 A

[Spir Georges GHALI]

guest said 

why some contactor doesn’t trip even if a phase goes off

 

Dear ;

 

First, the Contactor only doesn’t contain any kind of protection, so, it’s normally do not trip if there’s a missing of a phase, but if the phase’s missing is the same that used to supply the Contactor’s coil, in this case only the Contactor will open.

 

By the way, if we want to open the Contactor when missing of any phase, we should use a ” Phase Failure Relay ” that can detect :

– Any phase’s missing

– Any phase’s undervoltage ( less than a pre-adjust level )

– Any phase’s overvoltage ( more than a pre-adjust level )

– Direction of 3 phases’ Rotation

and send an order to open the Contactor if any fault of those mentioned above appears.

[Spir Georges GHALI]

@guest said:
Can 2 machines be synchronised running at different speeds? Please explain any scenarios this would be possible.

 

Any feedback is appreciated..

 

Dear ;

 

I think you talk about 2 generators, if yes, please note the following :

1- The 2 Generators should exactly have the same specifications : Power rating, Voltages, Speeds, Speed control, AVR.

2- The best way that they are from the same Manufacture.

And even, we should install ” Load Sharing  &  Load Control “.

[Spir Georges GHALI]

Dear ;

Refer to this Topic, I would mention to the following points :

 

– About “ TN-C ” :

The following remark should be also mentioned :

– If the supply of a load is “ 3Ph + N ” and we will use only “ 1 cable of PEN ” between the network and that load, the connection of PEN should be done as follow “ the PEN cable should be firstly connected to the earth point at Load, then to the neutral point ”.

 

– About “ TN-C-S ” :

It’s mentioned that “ Usually, the combined PEN conductor is used near the source of the system ”, but it’s better to say :

1- For the big networks where the installed power is big or too big, we use TN-C at the 1^st and maybe also at the 2^nd levels of the network, then we use TN-S.

2- By this way we can decrease the installation’s costs for these 2 levels.

3- We can’t never ever use firstly “ TN-S ” then “ TN-C ”.

 

– About “ TT ” :

Also, it’s better to add :

– As we can use many independent Earthing points at consumers side only, this system is approximately used all around the world in LV Distribution Networks to supply Housings and buildings.

 

– About “ IT ” :

We should also mention to the following :

1- If the connection of the transformer is “ ∆ ” ( no neutral point ) we can connect one phase to earth via high impedance.

2- Two main types of “ IT ” that are :

     – IT with distributed Neutral

     – IT without distributed Neutral.

3- With this Earthing system, It’s absolutely necessary to use special devices called “ Insulation Monitoring Devices ” to detect and indicate any insulation fault .

[Spir Georges GHALI]

@yudhi said:
Hi I am yudhi

I have problem with voltage drop ,
When the load at 3400 amp the voltage drop until 360 volt
I use 4000 kva step down transformer from 20 kv to 400 volt.
The cable is NYY 1x400mm , 8 cable for each phase at 400 volt
And 1x120mm at 20 kv, cos q is 0.9
Distance grom transformer to switchboard around 60 mtr

I dont know where the problem
I hope you guys can help me to solve this problem

Regards
Yudhi

 

Dear ;

First, by calculating the voltage drop of cables between the transformer and switchboard, I found that should be around “ 1% ”, noting that I assumed the arrangement of these cables is “ Trefoil ”, but let me know if the mentioned voltage drop “ ≈5% ” is only for this distance ? Or in a another place of network ?

If it’s only of this distance, you should do the following :

– Measure the output voltage of the secondary’s transformer at no-load.

– if it’s “ 400V ”, we should know the specifications of the transformer and the length of MV line to be able to calculate if this phenomena is caused by MV line.

– if the output voltage is smaller than “ 400V ”, the reasons are either the MV voltage is less “ 20kV ” (you should know this value), or the position of the “ Tap-off changer ” is not correct, in all cases, try to change this position to have around “ 400V ” or even just a little more.

– if this voltage drop is in another place of network, please let me the type, section, length, and arrangement ( if it’s Single Core ) of that cables to be able to calculate the voltage drop at that point.

[Spir Georges GHALI]

@maladili said:
The file in the link HERE

 

help in Calculation of instant short circuit tripping of circuit breaker is based on following inputs:

• Voltage(L-L)

• Voltage(L-N)

• Instantaneous Short-Circuit Triping Set

• Accuracy

• Cable Size

• Cable Material

• Actual Length of Cable

And according to what you select or choose, result of maximum length of cable for your tripping setting is calculated. Selection of fuse is based on circuit wire size input (1,5mm2 – 70mm2).

 

Dear ;

 

First, I can’t download the mentioned file.

 

The calculation of any Short Circuit Fault current value is simple by using the “  Ohm’s Law ”, where we should know, as mentioned in the Topic, the Voltage’s value and the Fault Impedance loop value, but refer to the inquiry information mentioned in the Topic I have the following comments :

A- The impedance loop value is composed of :

    1- The impedance of the source “ MV  &  Transformer ” or “ Generator ”.

    2- The impedance of all cables from the source to the point of fault, not only the final cable supplying the fault’s point.

B- In you don’t take into consideration all these impedances, the short circuit value will be certainly not correct, and you can’t adjust the protection’s setting.

[Spir Georges GHALI]

Dear ;

 

Yes, you can do it, and after this connection the transformer will be ” Auto-transformer “, but you should be careful of the current flowing in coils.

 

By the way, you know that an Auto-transformer has only one coil, to which we supply the input voltage, and from which we obtain the necessary voltage to supply the load.

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