Electrical Construction Works.

Hi

1) isolator is only switching device that has no over current protection device associated with it, where as MCCB is also a switching device but provided with internal over current protection devices,

The standard of both devices are different: Isolator : IEC 60937 - 3,

MCCB IEC 60947 - 2, Test of both of the devices are also so different,

2) As far as the constuction is concern, the size of isolator is very compact compared to MCCB, for same current rating the frame size of isolator is less,

3) if current is to be distributed equally in all three phases, 18/3 means each phase is loaded with 6 nos. of MCB, i.e. 6 x 20 = 120 A in addition to safely factor of 20 % for over loading and other 5 to 10 % for futur derating,

Best regards

divyesh patel

Hi,

1. The Isolator is nothing but Mannually operated high current rating switch, there is no any automatic tripping mechanism.

2. The MCCB is the breaker which can be operated by mannually while switching operation required. It has many feautures as follows,

1. Automatic Tripping for Over load current,

2. Presetable value for load current

3. measures the Shortcircuit Level above 10KA

4. Availble with both Electromagnetic Release and Microprocessor based release

5.Presetable time for release and etc...

3. The Incomer breaker(MCB) size should be More than 100A 4P MCB (OR 125A 4P MCB) rating. And the additional safety factor is Short Circuit Level at that DB location ( should be less than 10KA)

By,

R.Raviraja Pandian

They both do what there name suggests I would suggest if you are going to mess around with something potentially lethal (electricity) without any understanding of what you are doing that you get some help and advice from a good electrical engineer. It will cost you a few dollars but it will insure that you stay alive

dear sir

perfect answer i think that the person who ask this question has no idea about

electrical power projects and he need online help

Sir,

1) Others have all said that the MCCB has overcurrent protection and other forms of protection, while the isolator is a switch without these protections. In addition to this, the isolator is usually rated to carry its specified current, but not to open under load (not to break this current). It is therefore used to disconnect the voltage from your equipment after the equipment has already been turned off, so you can work on that equipment.

2) Knowing this, I believe you can determine where the isolator should be located in relationship to the MCCB and the rest of the equipment.

3) I disagree with other posts on the sizing of the MCCB. The individual branch circuits are provided with CB's sized to protect the wiring and the devices connected to the wiring. They must be sized to convey the required amount of current. However, the majority of applications have a total load significantly less than the sum of the installed CB's. That is why codes usually have tables and sections to estimate and predict the total load for a particular type of installation. Without knowing the type of occupancy being served and the sizes and types of electrical loads within this occupancy, I would be unable to suggest the proper size of the MCCB. You could, for example, choose to have one circuit supplying each room, or one circuit supplying only portions of a room. This could give you many circuits which are lightly loaded and a total load far below the sum of the CB's. It is true that a 25% increase is a good safety margin (that is 18/3*20*1.25 = 150A), but that is probably a very expensive oversizing.

Were these school questions?

--JMM

Hi.
The isolator is only a device that can be used to switch off the power manually.. In the case of maintenance or whatever, you can simply switch it off whenever you want.. However, The MCCB is used so that it has the ability to switch off the power in the case of emergency such as: short circuit, overload..etc...
In the electrical installation, we DO care that the size of the cable must be suitable to the rate of the MCCB. If we are using a 4C X 300mm², then we have to use a MCCB of the rate 400A.. In Dubai Electricity and Water Authority "DEWA" if you are using a MCCB of less rate than 400 in this example, then it is not accepted.. Again the cable size must be suitable for the MCCB amperage rate!!
In the same time, if we have a long cable where the voltage drop is more than 3%, in this case no problem in using a cable size more than the one suitable to MCCB. i.e. You can use a cable of a size 185mm2 or 240mm² "to avoid voltage drop" with an MCCB of a rate 250A as an example only...
We put the MCCB in the LV panel side, where the cable goes to the Sub Main Distribution Broad "SMDB" and be glanded there in the SMDB case and then connected to the isolator..
Sometimes we do not care about what is written on the isolator, I mean it's amperage rate!! if it is a little bit smaller than the rate of the amperage of MCCB in the LV panel side... But you know, it must be a matter of something logic, as we can not terminate a 300mm² cable into an isolator of 200A in rate!!!!!
I hope I have given you a little idea about Isolators and MCCBs...

Rami Bahlq

"Sebastia..You are always in mind"