Difference Between Over Current and Overload Relays

An overload is a higher-than normal current flowing in the normal, intended path. For example, due to a mechanical overload on a motor.

A Short-circuit current is a higher-than-normal current flowing through an un-intended path.

Both are called overcurrents.

i would set the relay at name plate FLA of the motor, even if actual load current is lower.

thanks a million, Is ovl relay = Fla Then over current setting is 110 % of full load current . Is this correct .

In IEC countries, the trip threshold of an overload relay is between 105 and 120% of the set current which should be the FLA. So, setting a relay at the nameplate FLA protects the motor correctly. If you set the relay at 110%, the motor will not be protected.

In USA, there is a 115% service factor, which means that the motor can safely take some overload. Setting of the relay may be different in USA, i hope some experts will enlighten us.

This thread also help:

http://cr4.globalspec.com/thread/60385/Over-Load-and-Over-Current (especially see the comment # 2)

- MS

As you can tell by reading the other excellent comments in this thread it all depends on what type of overcurrent protection you are using.

My Electrical code requires the following:

Time Delay fuses= not in excess of 175% of your FLA

Non-Time Delay fuses=not in excess of 300% of your FLA

Circuit breakers=not in excess of 250% of your FLA

When it comes to overload protection my code states that you would set the overload to 125% of your FLA if the service factor is 1.15 or greater, but if it's less the 1.15 then you would set them at 115% of your FLA.

These are just the common settings that could be used, there are many other options out there for protection. But I would suggest you look at the electrical code in your area and get the help of a qualified person if you are still not sure.

how to calculate service factor for motors.

All are valuable comments indeed!

Service factors are usually given by the manufacturer, maybe from tests; I don't think "calculated" is an exact term. Usually, one just looks at the motor nameplate.

GAs to EE65 and msamad.

i have been trying to get people to understand that motors can burn out even without overload, due to clogged airways (especially in textile industries). Not many takers you will be surprised. Actually, since most engineers today want to do their work in front of clean computers rather than dirty their hands on actual products, we can only expect and sigh at this degeneration.

And msamad, the good info about US codes....very valuable, thanks.