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Atul Chahure
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Necessity of OLR

09/08/2014 7:40 AM

While designing a DOL starter, we use MCB, contactor and OLR. OLR provides overload protection. MCB provides both overload and short-circuit protection. Then why to use OLR? Can we design a DOL starter with just MCB and contactor?
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pnaban
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#1

Re: Necessity of OLR

09/08/2014 7:59 AM

OL protection cannot be adjusted in MCB as in OCR.OL setting depends on motor current which varies from motor to motor on voltage,speed,kW etc

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JohnDG
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#2

Re: Necessity of OLR

09/08/2014 8:30 AM

The MCB is to protect the wiring, not the motor.

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JohnDG
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#10
In reply to #2

Re: Necessity of OLR

09/09/2014 6:34 AM

OK. 'fess up. Why was that "off topic?" ( it had 2GAs this morning). You can post anon if you like, but I'd like to know.

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kvsridhar
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#11
In reply to #10

Re: Necessity of OLR

09/09/2014 7:03 AM

i noticed that too earlier. Reinforces my opinion that a GA should not be negated by an OT. i hope the CR4 Admin will take note. That comment by you was GA. i have tried to restore justice.

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JohnDG
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#12
In reply to #11

Re: Necessity of OLR

09/09/2014 7:12 AM

Thanks. Not worried about actually getting a GA, more curious as to why someone thought it was OT.

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kvsridhar
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#13
In reply to #12

Re: Necessity of OLR

09/09/2014 7:32 AM

Some frustrated troll going through male menopause i bet. i have seen several in action here. No right-thinking person would vote something OT if the author has not done so, wouldn't you think?

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#14
In reply to #2

Re: Necessity of OLR

09/13/2014 12:58 AM

How the MCB knows that it protects the wiring only ??
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PWSlack
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#3

Re: Necessity of OLR

09/08/2014 10:14 AM

...and the overload relay is there to protect the motor, not the wiring.

The topic is called "co-ordination", and British Standard 7671 gived full protocols.

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PWSlack
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#4
In reply to #3

Re: Necessity of OLR

09/08/2014 10:59 AM

*gives*

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SolarEagle
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#6
In reply to #4

Re: Necessity of OLR

09/08/2014 2:01 PM
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kvsridhar
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#5

Re: Necessity of OLR

09/08/2014 11:27 AM

If you study the catalogues of OLRs from good manufacturers, you will see that they have many features like (a) Ambient compensation (b) Single-phasing protection (c) auto- or manual-resettability etc. Above all, they give much closer protection to the motor than any MCB or MCCB can. MCB & MCCBs are primarily meant as short-circuit protection, not overload.

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Paulusgnome
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#7

Re: Necessity of OLR

09/08/2014 4:41 PM

If you don't like thermal overload relays, you could always use one of these motor protection circuit breakers.

This is a GE example, but all of the other major manufacturers have equivalent products.

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V. Natarajan
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#8

Re: Necessity of OLR

09/08/2014 11:36 PM

Overload relay is a thermal protection which normally comes with adjustable settings ranging from 75% to 120% of FLC.If we some time want willfully to overload the motor it is possible with OLR.Most motors are manufactured with 10% over load.MCB is primarily for short circuit protection for all downstream of MCB.MCB comes with fixed short cicuit ratings like 10 KA.20KA etc.MCB does not meant for OL.And OLR is thermal and MCB is magnetic
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Jimh77
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#9

Re: Necessity of OLR

09/09/2014 12:47 AM

NO.

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