cables protection

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danielb

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Hi

My name is Daniel and I have a question for you

Please, can you give me guidelines how to protect the cables, from overload and short circuit, by mean a circuit breaker, in accordance with NEC and IEEE

std242-2001

Thank you in advance for your kind attention

Kind regards

Daniel
 
That's a short question with a very long answer. To start with, it depends on the type of load, the type of cable routing (to the extend that it impacts ampacity), the type of facility, and the possible existence of special rules published by the local authorities.

What is it that you are trying to do?
 
explanation

explanation

Hi and thanks for your prompt replay

I Know the kind of load, the current and the size of the cable.
How can I choise the suitable circuit breaker for overload and short circuit protection ?

Kind regards

Daniel
 
I do not need to protect a particular plant with defined characteristics. Therefore the following is only a general request :
I just would like to know which basic prescriptions shall be complied with when choosing a moulded-case circuit-breaker in order to protect a cable (supplying a generic load) against overload and short-circuit.
Once the current absorbed by the load and the cross-sectional area of the cable are known, how shall I set the protection functions of the moulded-case circuit-breaker in order to protect the cable also in case of overload and short-circuit?
Which is the reference Standard (IEEE) to get the proper co-ordination between molded-case circuit-breaker and cable?

Kind Regards

Daniel
 
In general the rating of the OCPD (overcurrent protective device) must be equal to or less than the rated ampacity of the conductor.
don
 
how shall I set the protection functions of the moulded-case circuit-breaker [/QUOTE said:
the circuit breakers are already set when you buy them "genaric sizes".You buy the correct size breaker to match the wire and load to be protected.
Click to expand...
 
Daniel,

It sounds like you are trying to perform a coordination study.

In the US most molded case cirbuit breakers are designed to UL489 standards and not to IEEE. A UL489 circuit breaker will always protect a UL listed conductor when it does not exceed the limits imposed by the NEC (which were mentioned by Don).
 
don_resqcapt19 said:
In general the rating of the OCPD (overcurrent protective device) must be equal to or less than the rated ampacity of the conductor.
don
Click to expand...


OK but I think that this requirement is only for overload;

To not damage a cable in case of short-circuit what I have to do ?

Kind regards
 
Daniel,
In the NEC "overcurrent" protection includes, short circuit protection, ground fault protection and overload protection. Again in general an OCPD sized at or below the conductors ampacity as shown in Table 310.16 will protect the conductor from damage in the event of a short circuit.
Don
 
A standard MCCB with thermal trip units for LT overcurrent conditions will have a build-it magnetic Instantaneous (I) function for protection against short-circuits. The settings for the I function may or may not be adjustable base on the manufacturuer, rating, type, etc. of MCCB. Alternately, there are several larger MCCB's that will have electronic trip units with LT, ST, I, GF, I^2T fucntions to better coordinate with upstream and downstream OCPD.

That's where the coordination studies come in to determine the most beneficial settings based on the nature of the load, where the load being protected is in the distribution system, the available short circuit current available at that point in the system, device ratings, and NEC requirements etc.

Every system and design is different and the selection of an OCPD must take into account the many factors required by NEC and best practices to make the system inherently safe (NEC) and functional for the given purspose of the system.
 
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