MCCB Types C and D?

Protective devices means MCB or MCCB, therefore I selected MCCB's rather since 399 A is a higher load than 100 A. When a load current is higher than a 100 A, then MCCB is to be used. That's why I am using MCCB. Otherwise, the regulation says MCB rather. So when the load current for a 3 phase motor would come up to 399 A, my next standard size breaker would be selected 400 A. How is that?

Tornado writes:
>Motor wiring is sized at 1.25 x FLC;
It isn't the case in Abu Dhabi, UAE though.

Then what is the case in Abu Dhabi? In an international forum such as this, weird local schemes must be explained.

Tornado writes after a long time:

>In an international forum such as this, weird local schemes must be explained.

Read the followings. It must not be same but identical. This is the best code thus far that I have come across. It is based on British Standards, I think.

Abu Dhabi old regulations (2007-09) has it,which is still valid and good:

"To ensure protection against overload, Circuit conductors must be sized taking into account the time-current characteristic of the Protective Device.

[Note: in order to provide adequate protection against overload the Protective Device nominal rating must be more than the maximum rating of the Circuit for which overload protection is required, ...]"

Tornado writes:

>Motor wiring is sized at 1.25 x FLC; inverse-time circuit breakers can be sized at 2.5 x >FLC. (This is in the U.S. NEC, but may differ in your jurisdiction).

My apology in advance. As per regulations, we OUGHT to protect the cable. Then why the MCCB can be sized 2.5 times FLA? I must explain myself again. The full-load current (FLA) comes up to be 399 A. I would like to size my MCCB for the following protections:1) Overload 2) Short-circuit So the normal load current is 399 A. My MCCB size is the next standard CB (higher than FLA) = 400 A. Why would I need 2.5 times?

> It also may depend on >starting method, and may need consideration of time-to-trip curves for proposed CB >selections.

Agree in here. During motor initial stage of starting, it draws a higher current than the normal FLA? Right. That is known as starting current which results in a voltage drop. We use starters to keep the starting currents within the limits. For e.g. the regulation has a chart which states that a motor of rating between 37.3 KW to 112 KW would have max. permissible starting current of 1.5 times FLA. In our case: FLA = 399 A; MCCB = 400 A; Max P Starting C almost = 1.5 x 399 A = 598.5 A or approx. 600 A. But I am shooting to use the type C (MCCB if it is there in the market) which should have a starting current = 10 x FLA = 10 x 399.3 amost = 4000 A. Shouldn't that handle the starting issue when in fact one use a starter with that capacity? I mean if you read above the max. permissible starting current issue of 600 A?I don't find the answer thus far that I am hoping or looking for. It isn't me but that fellow in the other thread would need it. Some one down below talks about breaking current of MCCBs. Well, MCBs has a breaking current too, but in fact lower than MCCBs. That would be in kA rather. It would depend how it would be cascaded? If the motor is directly connected to the LV panel (45-50 kA), then it should have a MCCB (breaker) with 35 kA breaking capacity or so...

The motor is (usually) protected by an overload device (relay, heaters, etc.) In case of "small" overcurrents, this device also protects the wiring. The CB is principally for short circuits, ground faults, or other "large" overcurrents.

Appreciate your concern abasins,

The overcurrent due to inrush current is not considered. This magnetizing currend will be 10 to 13 times Full Load Current of the motor. This maximum magnetizing current exists only in the initial cycle (within 20 milliseconds - since UAE frequency is 50Hz) and reduces into the starting current then to the normal load current. The normal instantaneous tripping time of 400A MCCB is 10 ms to 1 ms (for the breaker with 5 to 10 In). If the peak value of the current also is considered for the inrush, it will be increased by another factor of 1.4. The 400A MCCB can not withstand this instantaneous current and hence it will be interrupted definitely.

Another point to be considered is that the 400A rated capacity of the breaker is at standard lab temperature. De-rating should be considered for grouping factor or in a closed cabinet and for ambient temperature. Eventhough the electrical rooms in the UAE are air conditioned, the breakers are derated to 45C for coordination reasons.

Musthafa writes:

[Appreciate your concern abasins,

[The overcurrent due to inrush current is not considered. This magnetizing current will [be 10 to 13 times Full Load Current of the motor. This maximum magnetizing current [exists only in the initial cycle (within 20 milliseconds - since UAE frequency is 50Hz) [and reduces into the starting current then to the normal load current. The normal [instantaneous tripping time of 400A MCCB is 10 ms to 1 ms (for the breaker with 5 to [10 In). If the peak value of the current also is considered for the inrush, it will be [increased by another factor of 1.4. The 400A MCCB can not withstand this [instantaneous current and hence it will be interrupted definitely.

[Another point to be considered is that the 400A rated capacity of the breaker is at [standard lab temperature. De-rating should be considered for grouping factor or in a [closed cabinet and for ambient temperature. Eventhough the electrical rooms in the [UAE are air conditioned, the breakers are derated to 45C for coordination reasons.

I thank Muthafa for his generous and kind response. Can you read the following and then conclude about why won't the The 400A MCCB withstand this instantaneous current and hence it will be interrupted definitely? The Instantaneous Adjustable trip (breakers) are designed for such. It would trip only when the over current exceeds the maximum set value?

Some more points that would be useful for the topic under discussion:

Cut 'n paste .1:

The instantaneous or "magnetic" pickup was made adjustable, so it could be set above the motor inrush. This was a significant improvement over the fixed instantaneous thermal-magnetic breaker.

<Instantaneous adjustable trip (breaker)>

Cut 'n paste .2:

Circuit Breaker Curves

The following curve illustrates a typical thermal magnetic molded case circuit breaker curve with an overload region and an instantaneous trip region (two instantaneous trip settings are shown). Circuit breaker time-current characteristic curves are read similar to fuse curves. The horizontal axis represents the current, and the vertical axis represents the time at which the breaker interrupts the circuit. When using molded case circuit breakers of this type, there are four basic curve considerations that must be understood. These are: 1. Overload Region 2. Instantaneous Region 3. Unlatching Time 4. Interrupting Rating 1. Overload Region: The opening of a molded case circuit breaker in the overload region is generally accomplished by a thermal element, while a magnetic coil is generally used on power breakers. Electronic sensing breakers will utilize CTs. As can be seen, the overload region has a wide tolerance band, which means the breaker should open within that area for a particular overload current. 2. Instantaneous Region: The instantaneous trip (I.T.) setting indicates the multiple of the full load rating at which the circuit breaker will open as quickly as possible. The instantaneous region is represented in the following curve and is shown to be adjustable from 5x to 10x the breaker rating. When the breaker coil senses an overcurrent in the instantaneous region, it releases the latch which holds the contacts closed. The unlatching time is represented by the curve labeled "average unlatching time for instantaneous tripping." After unlatching, the overcurrent is not halted until the breaker contacts are mechanically separated and the arc is extinguished. Consequently, the final overcurrent termination can vary over a wide range of time, as is indicated by the wide band between the unlatching time curve and the maximum interrupting time curve. The instantaneous trip setting for larger molded case and power breakers can usually be adjusted by an external dial. Two instantaneous trip settings for a 400Abreaker are shown. The instantaneous trip region, drawn with the solid line, represents an I.T. = 5x, or five times 400A= 2000A. At this setting, the circuit breaker will trip instantaneously on currents of approximately 2000Aor more. The ± 25% band represents the area in which it is uncertain whether the overload trip or the instantaneous trip will operate to clear the overcurrent. The dashed portion represents the same 400Abreaker with an I.T. = 10x, or 10 times 400A= 4000A. At this setting the overload trip will operate up to approximately 4000 amps (±10%). Overcurrents greater than 4000A(±10%) would be cleared by the instantaneous trip. The I.T. of a circuit breaker is typically set at its lowest setting when shipped from the factory.

3. Cut 'n paste:

As current increases the magnetic field strength increases, which aids in opening the contacts. This circuit breaker has an adjustable instantaneous trip point from 900 A to 2000 A, which is 4.5 to 10 times the 200 A trip unit rating. If the trip point adjustment is set to minimum (900 A), and a fault current of 900 amps or greater occurs, the breaker will trip within 1 cycle (16.8 ms). If the trip point setting is set to maximum (2000 A), and a fault current of 900 amps occurs, the breaker will trip between approximately 12 and 55 seconds.

A greater fault current will cause the breaker to trip faster.

Detailed provided by OP is self explanatory. OP can look at the curve which is referred in the post. I will add a small math here:

Full load current is 399A. The instantaneous value of this will be 564 (1.414x399A). Now the transient inrush during starting will be 7332 (13x564). If OP looks at the curve, the unlatching time is between 3 & 4 milliseconds.

BTW, why OP is so stringent to use MCCB as an over load protection?. Even though soft start is used as starting method, the breaker is under thermal stress because of the current is at its threshold of 399A.

Musthafa,

Regarding the math and then my concern is to withstand the initial starting current since MCCB is the first protective device to let through the current. Please read on as I cut 'n paste and explain? :)

< cut and paste from CR4 as mentioned above and below?

{

ii) MCCB + Contactor + Overload Relay

While selecting an MCCB for motor application, it MUST be ensured that the Instantaneous Release setting in the MCCB is set to a value higher than the highest anticipated magnetising inrush current, while switching-on the motor.

How to calculate this value?

Typical Magnitude of Staring Current (I_st): 6 to 8 times the motor rated current (I_rM)

Tolerance for Locked Rotor Current as per IS 325 (Table-1): + 20%.

So, the maximum anticipated starting current: 1.2 x 8 = 9.6 I_rM

Transient inrush current (peak): √2 x 9.6 I_rM = 13.57I_rM

Also, vide IS/IEC 60947-2, Clause 7.2.1.2.4, there is a tolerance of +/- 20% in the values of current for tripping of the circuit breaker by short-circuit releases. This means that, even with the -20% tolerance, the MCCB shall not trip for the above magnitude of the magnetising inrush current. This also means that the effective setting of the instantaneous release setting in the MCCB should be above 13.57 / 0.8 = 16.96 I_rM; But, this is peak value. Whereas the instantaneous setting in the MCCB is in RMS value. Hence, the above value has to be converted to RMS value; (i.e.) 16.96 / 1.414 = 11.99 I_rM; Thus, the instantaneous setting in the MCCB has to be above 11.99 I_rM or say, it should be 12 I_rM. Assuming that the rated operational current (I_n) of the MCCB is the same as that of the rated full load motor current (I_rM), then, the instantaneous setting in the MCCB shall be set to 12 I_n.

}

From above and even previous posting under the thread here:

For 400 A MCCB = 10 x In = 10 x 400 = 4000 A

OR: 12 x In = 12 x 400 = 4800 A

It would show the maximum capacity?

As I have posted above:

For 6 x In = 6 x 400 = 2400

For starting current = 5 x 399 = 1995A, it shall not trip. If it is set at lower I.T, then it would trip on 2400 A or above. But if it is adjusted on upper 4000 A, then the I.T would occur when it reaches that level or above. It is shown that it would withstand the initial starting current and won't trip since it is the first protection device. It must allow the required current for the rest electrical circuit (along the motor) to properly function. The MCCB is the first protective device in the circuit. And it is to protect the cable and NOT the motor. The issue of starting current and its reduction as I quote the regulation, "The motor starting current and resulting Voltage drop is reduced when motor starters employing current limiting starting equipment are used." So, I am not concern much about that equipment except to know what I know already i.e. the peak value.

abasins:

<if it is set at lower I.T> You mean the Interruption Time?

Is the MCCB you are referring with selectable time set? If so, it can override the tripping condition by suitably selecting the timing. Again because of thermal stress by the load current of 399 A, it is not recommended. If you are confident about it, try it.

Musthafa,

I thank you for carrying out meaningful discussion. I.T means Instantaneous trip. Again, you might be very right about the thermal stress but the breaker manufacturer must guarantee that. If you are talking about something else, like my concerns would be (which I am shooting for sizing my breaker) to worry about the temp. rise when current increase in the cable. Due to that heat, the breaker must trip. The cable I would select or size should be rated for 399 A. Beyond that, I would like to protect my cable from overheating and possible damaging, thus the right size MCCB to trip.

Abasins,

We are advised by our forefathers to select the cable for 1.25 times the load current. This is concluded by them form their experiences in vast area. You are just focusing the normal load current FLA of your motor. Normally, you have to select next higher size; how higher is 400 from 399? Not only that, you need to assume a service factor to the motor as 1.1. Then a prolonged positive voltage variation in the distribution system. Again, the star/delta starters in common usage are open transition. During star to delta changing over time, some time the instantaneous peak between the voltage of delta supply to the stator and the generated voltage of the rotor (cause, during transition period rotor is working as generator) will be much higher than the system 400 V. All these factors leads to increase the current beyond the normal load current.

OK, selecting the CABLE but not the breaker? These are two different things. One would depend on the other...Hahahahaha....I cannot argue with what your forefathers taught you Vs. Abu Dhabi Wiring Regulations...I would follow the regulation until...Just a joke Musthafa and by God no implications as it is truly a slang in my culture, the Pukhtun (Pakistani one)...It says that the animosity of camel is expensive...Likely to the end??? Why is that? Another saying is that the tale seems longer as the fart of a camel? OR, your tale is so long which amounts to be a fart of a camel? Why is that? My apology in advance as enjoyed your humor...

If the cable can be selected for 1.25 x In (=500A),why the breaker can not be 500A, since it is is used for cable circuit protection only. Does the utility provider, ADDC not allow you to use 500A MCCB with 2 nos 150sq.mm cable as feeder ?

Musthafa writes:
>If the cable can be selected for 1.25 x In (=500A),why the breaker can not be 500A,
Musthafa, you may be right in your ways, but "In" is the current the breaker is rated with. You cannot use it twice "In" i.e once 400 A & then 500 A. I don't use or buy that formula, even it might come closer to the my results though. Remember, the I (load) = 399 A; and we selected the next standard size breaker = 400 A. Now having said that, you can certainly select a 500 A MCCB if you think you would like. (1) First of all, there isn't a standard 500 A MCCB here in UAE, that I know of. (2) After the MCCB 400 A size, the next standard size could be 630 A. You may select a 630 A instead.
>since it is is used for cable circuit protection only. Does the utility provider, ADDC not >allow you to use 500A MCCB with 2 nos 150sq.mm cable as feeder ?
They may allow it. I don't know about that since I never worked for ADDC. I have only passed their competency exam to be a qualified electrical engineer. It must be filled by Arab engineers :) I could explain further but at this time I would refrain from two cables MCCB etc. Let me paste the part of regulations if I could find about sizing diff...

First of all, my congratulation in passing ADDC competency exam.

630A breaker with adjustable (80 to 100%) thermal setting is available.

BTW, jokes are good for health; and remembering old proverbs always enlighten your ways.

An MCCB is not a motor protector.

MCCBs protect the conductors and should be sized in accordance with the conductor sizing.

Conductor sizing is selected according to the load, acceptable voltage drop on load and fault current levels.

Wal writes:

>An MCCB is not a motor protector.

Yes. I said it is a Protective Device only.

>MCCBs protect the conductors and should be sized in accordance with the >conductor sizing.

>Conductor sizing is selected according to the load, acceptable voltage drop on >load and fault current levels.

It isn't the case when you write according thee load in Abu Dhabi, UAE though.

Actually, that motor problem in that other thread I am trying to post. The problem with 210 KW etc. Hahahaha...

So the load current comes out to be 399 A and my selection of MCCB would be 400 A. To make it more simpler, I must add to it by saying that the starting current for type C would get to be = 10 x FLA = 10 x 399.3 amost = 4000 A

I am still waiting on ....Hehehehe...

kvsridhar wites:
>An MCCB or an MCB (i can't decide which one you are talking about, since B/C/D curves >refer to MCBs, not MCCBs) is mainly meant
The following link has it. The CB 250 A cannot be MCB? Right? 250 A must be MCCB rather...Read on...
Magnetic Tripping Characteristics

http://static.schneider-Thermal-magnetic circuit breakers 250 A frame and larger have an instantaneous magnetic trip which in most cases is adjustable from 5 to 10 times the circuit breaker's ampere rating. A single magnetic adjustment on the face of each circuit breaker sets the limits of the magnetic trip mechanism, which simultaneously adjusts all poles of the two or three pole breaker to the same magnetic trip level.
telectric.us/docs/Circuit%20Protection/Molded%20Case%20Circuit%20Breakers/0100-400%20A%20Frame%20FA-LA/FA-FC-FH/0600DB0105.pdf

i have not heard an MCCB characteristic refeered to as B, C or D though it may follow the same curves. Maybe in your area it is the pratice. Not important, it is an inconsequential point.

The main point is that the motor needs a motor-protection relay, while the whole circuit needs sc protection by an MCCB.

Wal writes:

>An MCCB is not a motor protector.

Wal, it is to protect the cable. I know that...:)

>MCCBs protect the conductors and should be sized in accordance with the >conductor sizing.

>Conductor sizing is selected according to the load, acceptable voltage drop on load >and fault current levels.

Remember I quoted from the regualtions the other day. So, here we go again:

The CROSS-SECTION of cables MUST be slected according to the EXPECTED LOAD, Voltage drop, ambient TEMP and ....

You should be clear that if rated current is 399 A then whether its motor application or any other load application you may not go for 400A MCCB as it can't be able sustain high inrush current.

Also there is no specifications of C or D type in MCCB.

In MCCB there is breaking current which will be decided after calculating fault current.

You Might getting confused between MCB and MCCB.

The design technique to be applied is called 'co-ordination', and the methodology is all in British Standard 7671.

The usual approach is to use fuses to protect the cable, and a motor overload device to protect the motor.

If you want to know more details about MCB'S then visit at link below:-

http://electrialstandards.blogspot.in/2014/04/mcb-characteristics-and-applications-of.html

a) Somebody referred if 500A then thermal adjustment can be done 0.8-1.0 In (Set at 0,8 In of 630A MCCB), however what we have been taught is that for a motor feeder, MCCB has to be used with only S/C Protection (There should not be any overload protection in MCCB) and Overload relay to be used for Overload protection.
b) When it comes to star delta, yes, issues regarding changeover form star to delta comes in picture when both voltages applied in delta mode and induced EMF of motor may come together and picture can be more damaging
c) Swithgear Manufacturer's publish Type 2 Charts (Proven, Tested with Third Party Certification) and have made life easy for users, why debate on that. Just select HP Rating of Motor (IE1 or IE2) and refer type 2 charts for Selection of Switchgear Components.