Instantaneous tripping time of motor overload

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m sleem

m sleem

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For the motor load, the breaker is 250% of FLA & the motor overload is 115% of FLA as well as the disconnect.
My question is what is the appropriate trip time that is taken by the overload to protect the cable and the disconnect?
 
m sleem said:
But how the overload permits the starting current without tripping?
Click to expand...
The typical overload in US equipment is a Class 20 device, which means it can hold 20x current for 6 secs. Should be 6X for 20secs
The typical European overload is a Class 10, which can handle 10X for 6 sec. In the US, this is called a fast trip overload.Should be 6X for 10secs
There is also a slow trip Class 30, which can handle 30X for 6 seconds.Should be 6X for 30secs

These trip points are part of the criteria used when determining the 'inverse time' curve of the overload relay.

As long as the starting current is below these values and is gone before 6 seconds, the overload will not trip.Should be 6X
For current other than these points, you need to look at the trip curve for the overload relay.
 
Last edited:
jim dungar said:
The typical overload in US equipment is a Class 20 device, which means it can hold 20X current for 6 secs.
The typical European overload is a Class 10, which can handle 10X for 6 sec. In the US, this is called a fast trip overload.
There is also a slow trip Class 30, which can handle 30X for 6 seconds.

These trip points are part of the criteria used when determining the 'inverse time' curve of the overload relay.

As long as the starting current is below these values and is gone before 6 seconds, the overload will not trip.
For current other than these points, you need to look at the trip curve for the overload relay.
Click to expand...
Jim,
I always thought it was the other way around....that is a Class 20 will trip in 20 seconds at 6x current.
 
don_resqcapt19 said:
Jim,
I always thought it was the other way around....that is a Class 20 will trip in 20 seconds at 6x current.
Click to expand...

"NEMA-rated overload relays are classified by how quickly they must trip at 600% of rated tripping current. Since the ultimate tripping current is generally 125% of motor full-load current per NEC Sec. 43032(a)(1), this means that a Class 20 overload relay must trip within 20 sec at 750% of full-load current, from a cold start (overload relay at ambient temperature). Similarly, a NEMA Class 10 overload relay must trip in 10 sec, ...."

But if it is a mathematically exact inverse time curve, that would mean that it would also trip in 6 seconds at 20x current. :)
 
jim dungar said:
The typical overload in US equipment is a Class 20 device, which means it can hold 20x current for 6 secs. Should be 6X for 20secs
The typical European overload is a Class 10, which can handle 10X for 6 sec. In the US, this is called a fast trip overload.Should be 6X for 10secs
There is also a slow trip Class 30, which can handle 30X for 6 seconds.Should be 6X for 30secs

These trip points are part of the criteria used when determining the 'inverse time' curve of the overload relay.

As long as the starting current is below these values and is gone before 6 seconds, the overload will not trip.Should be 6X
For current other than these points, you need to look at the trip curve for the overload relay.
Click to expand...
Using class 10 means 6X in 10secs, and breaker size is 2.5X, so what is the trip time of breaker to be coordinated?
 
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