Refurbished Thermal Overload Relay?

[Herbrata]

Is it possible that Thermal Overload Relay not tripped above the set point after determined time achieve?


Like:
Set 50A, Class 10.
The current condition is already above 50A - let say 60A, for more than 20 seconds, and the Relay still not tripped.


What part of the Thermal Overload Relay is broken?
Is that kind of fault common?


Thanks for your attention.
Warm regards,
Herbrata

 

[petersonra]

It depends on the style of relay. Sometimes the setting on the dial really is the setting. Sometimes there is a service factor built in to the setting so it may be that a 50A setting is really 62.5 A.

Also if it is a real thermal element, it is possible it is being cooled somehow such as by some kind of air flow in the cabinet where it is located.

 

[jim dungar]

Is it possible that Thermal Overload Relay not tripped above the set point after determined time achieve?


Like:
Set 50A, Class 10.
The current condition is already above 50A - let say 60A, for more than 20 seconds, and the Relay still not tripped.

Class 10 means that the device can carry 600% current for 10 sec.

You are asking about a 120% overload. I would not be surprised to find it hold more than 3 minutes. At this point, there are way too many variables to do more than guess.

 

[kwired]

Class 10 means that the device can carry 600% current for 10 sec.

You are asking about a 120% overload. I would not be surprised to find it hold more than 3 minutes. At this point, there are way too many variables to do more than guess.

And as mentioned if motor FLA were 50A and you set the overload to 50 - the actual trip setting probably is 1.25% of 50 Device may hold indefinitely if motor is loaded to 120% of rating.

 

[Jraef]

Here is an OL realy trip curve for a typical IEC device.
(Click for larger)
So if you extrapolate 120% of the setting on the Class 10 curve, the average trip time is around 6 minutes.

Here is another one from Franklin Pumps, which adds their MOTOR'S thermal damage curve.

This is showing you WHY you would not use a Class 20 OL relay on their pump, because you can see how the motor thermal damage curve is BELOW the Class 20 protection curve. In your case if you have a Class 10 relay and a typical IEC motor, you would be fine.

Side note:
MOST IEC overload relays have ALREADY taken into account the 120% pick-up point in the motor curve, so the instructions will tell you to ALWAYS set it at the motor nameplate FLA. If you add the 120% BEFORE making the setting, you are going to burn out the motor. RFTM...