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# MCB Characteristic Curve & Overload / Shortcircuit Protection

09/10/2014 5:11 AM

Please let me know how to read Overload / Short circuit ranges from a MCB's Tripping Characteristing Curve. Assume that the MCB is a 10kA, C curve as per IEC 60898

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#1

### Re: MCB Characteristic Curve & Overload / Shortcircuit Protection

09/10/2014 5:16 AM

Declined. An MCB is not suited for use as a motor overload device. Contact the manufacturers directly to obtain characteristic curves for the device in question.

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#2
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### Re: MCB Characteristic Curve & Overload / Shortcircuit Protection

09/10/2014 5:28 AM

I have a C type characteristic curve. When I is equal to 5In to 10In vertical asymptote. to time zero (in the graph time is in Vertical axis & current rating in horizontal axis).

I want to know 5In - 10In is overload range or short circuit range?

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#3
In reply to #2

### Re: MCB Characteristic Curve & Overload / Shortcircuit Protection

09/10/2014 5:36 AM

The graph is being read correctly though not interpreted correctly: one axis is the current, the other the trip time and to differentiate between <...overload range or short circuit range...> would be meaningless.

For the correct protection of a motor an overload device is needed in addition to a wiring protection device. The topic is called "co-ordination" and British Standard 7671 gives full details.

If in doubt, consult a qualified Electrical Engineer.

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#4
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### Re: MCB Characteristic Curve & Overload / Shortcircuit Protection

09/10/2014 5:56 AM

Curve is attached.

Please define the areas of Overload protection & Short Circuit Protection.

Anonymous Poster #1
#5
In reply to #4

### Re: MCB Characteristic Curve & Overload / Shortcircuit Protection

09/10/2014 6:03 AM

Why?

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#7
In reply to #2

### Re: MCB Characteristic Curve & Overload / Shortcircuit Protection

09/10/2014 11:59 PM

As i mentioned in the PM, 5x cannot be called an overload. It is a 'low' short-circuit.

One definition you can think about : "Overload" is a higher-than-normal current through the normal, INTENDED path. "Short Circuit" is a higher-than-normal current through an UNINTENDED path.

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#6

### Re: MCB Characteristic Curve & Overload / Shortcircuit Protection

09/10/2014 10:06 AM

Postpone the interview until you've had time to get the answers to all the interview questions.

Otherwise, it won't go well.

Good Answer (Score 2)
Anonymous Poster #2
#8

### Re: MCB Characteristic Curve & Overload / Shortcircuit Protection

10/10/2014 12:24 AM

Hi!

The tripping curve of a MCB is quite important because it decides the protective functionalities such as overload or short circuit protection. It is described as a characteristic curve with a couple of parts. Each part has a different function to protect electrical equipment. For example, the overload protection curve is always laid at the bottom of the co-ordinate while the fault protection ones are set at above areas of the co-ordinate. Therefore, the protective devices will operate correctly.

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