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by GScrimES - no comments
So Mr. C calls and starts of with:- “Why this rubbish overload does not trip when the current is higher then the motor current?”.[all]
Circuit breaker tripping curve



At first I am a bit confused…why it does not trip?

So I ask:- Ok help me out here…

What is the overload set at?

Mr. C: 132A

And what current is the motor full load current rating?

Mr. C : “132Amp”

And what current was the motor physically pulling?

Mr. C: “139Amp”.

And for how long was the motor pulling this 139Amp?

Mr. C: “What does time have to do with it?” he asks? He even mumbled something to the tune of a swear word in the background.

This was my explanation:

I ask: Should an overload trip at the setpoint it is set at?

“Yes” he says. “It must trip because that is what the overload is set for”.

For quite a few moments I just sat there thinking:-

So the motor is designed to run at 100%FLC but you want it to trip? Why?

And I can feel the tension building between us.

Mr. C: “Lets say it does not trip at FLC – I told you it was pulling more the overload setting and that is why I called you. This is a rubbish product”.

I thought for a moment what would be the best approach to explain this.

Ok, so if your machine went through a temporary glitch and it pulled 120% of the overload trip point setting for 10 seconds – which will not damage the motor or your machine – will you be a happy chappy if it tripped, stopping all production…I don’t think so?

But if it continuously pulled 120% over current which – as the latent heat build up in the motor – will ONLY damage the motor over time, we do want the overload to eventually trip.

But if your machine pulled 500% of the O/L setting for 10 second would you want it to trip – Most definitely I would think.

So, that is the way the overload is designed – the higher the load current the quicker it trips and at less than 115% for NEC and 105% for IEC trip setting it will not trip at all…this is referred to as the  Proportional Inverse Tripping curve.

Mr. C: “Ohhhh…I did not know”. And a soft voice says: “Thank you, bye”.

So the bottom-line is this:

A overload will not trip at the set rating but as the overload current increases it will progressively trip for a shorter trip time.

For a more technical approach:

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