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Breaker Curves

07/10/2013 12:22 PM

How do i calculate mcb rating for 5hp pump & the type of curve?

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#1

Re: Breaker Curves

07/10/2013 12:27 PM
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#2

Re: Breaker Curves

07/10/2013 5:32 PM

Using British Standard 7671. The MCB is there to protect the WIRING; it is the OVERLOAD that protects the motor, as has been stated in this forum countless times before <sigh>.

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#4
In reply to #2

Re: Breaker Curves

07/10/2013 11:27 PM

CB might trip for starting current,it depends on method of starting.

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#5
In reply to #4

Re: Breaker Curves

07/11/2013 3:14 AM

In that case the contents of BS7671 have not been applied correctly.

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#7
In reply to #2

Re: Breaker Curves

07/11/2013 12:39 PM

There used to be some MCB's that where designed for motor protection also. Trip curve was set for motor protection. I think they where by Westinghouse - now by Eaton? called "MPCB" Motor Protection Circuit Breaker.

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#11
In reply to #7

Re: Breaker Curves

07/15/2013 1:59 AM

MCB selection for motors should take into account the method of starting(DOL,star delta-open/closed transition etc) and the duration so that it won't trip. In open transition star-delta starting, during changeover high transient/peak should be considered. Catalogs of manufacturers (eg:Hager) might give example of selecting MCB for motor.

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#8
In reply to #2

Re: Breaker Curves

07/12/2013 1:13 AM

thanks Mr PWSlack

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#3

Re: Breaker Curves

07/10/2013 7:48 PM

You're signature:

In Search Of Intelligent Life, Somewhere, Anywhere.

Don't bother looking at home!

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#6
In reply to #3

Re: Breaker Curves

07/11/2013 3:39 AM

You're Your

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#9

Re: Breaker Curves

07/14/2013 11:32 AM

Do not use MCBs for motor protection. However, you may use an MCB as a back-up switching & protection device in a motor starter feeder with the MCB offering short circuit protection. In such a case, proper co-ordination is required between the MCB, the contactor and the overload relay.

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#10
In reply to #9

Re: Breaker Curves

07/15/2013 1:47 AM

See #7

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#12
In reply to #10

Re: Breaker Curves

07/15/2013 11:23 AM

I beg to differ. MCBs & MCCBs are grossly different products. The overload curve in an MCB is designed with cable protection in mind and the instantaneous protection is a band (i.e.) between 3 to 5 In for B Curve MCB, between 5 to 10 In for C Curve MCB and between 10 to 20In for D Curve MCB.

Whereas in an MPCB, the overload curve is perfectly tuned for motor protection and also the instantaneous release is also very precise.

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#13
In reply to #12

Re: Breaker Curves

07/15/2013 12:12 PM

The point I was trying to make, was that adequate protection for motor applications was available in the familiar "molded case" style of "breaker". If you take apart an MCB and a MPCB you find essentially the same internals, but with different O/L's and trip units installed.

I 100% agree that you have to specifically select the protection device for the application.

Instead of using a standard circuit breaker, and a motor starter with O/L's, you can often use a single device that provides the manual closing, and the motor protection in one assembly, greatly reducing wiring time, panel space, and total cost of the installation.

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#14
In reply to #13

Re: Breaker Curves

07/15/2013 6:13 PM

Agreed! I too am a great fan of MPCB.

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Users who posted comments:

Anonymous Poster (1); eautocad (1); electricalexpert65 (3); GW (3); lyn (1); pnaban (2); PWSlack (2); TonyS (1)

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