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Topic Title: Overload protection device
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Created On: 25 May 2010 04:55 PM
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 25 May 2010 04:55 PM
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Joined: 25 July 2008

I have a 3ph induction motor (I rated=15.5A) connected to the 400V busbar through a MCB(I n=20A) and 2.5mm2 Cu/PVC cable.(I z=25A). it satisfy Iz>In>Ib equation.
Now I am going to fix aoverload protection device close to the motor.The selected overload relay is (16-24)A range. what is the set value in this O/L relay? How can i calculate the set value? is there any thumb rule to set O/L relay?
Is there any advantage to have a seperate O/L relay rather than using MCB's overload function
 25 May 2010 08:47 PM
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The MCB is in place for short circuit protection of the cabling and starter, the motor overload is there to protect the motor if too much load is put onto it or it develops mechanical faults etc.

The IEC standards specify thermal overloads for no operation at 1.05 times and tripping at 1.2 times so the overload is usually set to the motor FLC or sometimes its running current. However you wil usually find a lot of people will set them to 1.1 to 1.2 times motor FLC.

Kind regards

Donald Lane
 01 June 2010 09:04 PM
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The disadvantage of using a MCB to protect a motor circuit is it can trip due to the starting motor current which is about 6.5 times the motor full load current under direct-on-line start.

Since the motor starting current will only exist for 20s - 30s, the advantage of using an overload relay is it has a time delay in its overload detection function. It will turn a blind eye to the starting current when the motor starts but after a set time (30s), its overload sensing function will kick in and protect the motor cable against overloads.

As Donald said, you could set the overload setting at 10% or not more than 20% of the motor FL current value. However, it is best for you to consult the motor manufacturer's manual in order to protect the motor winding from insulation failure caused by overloads.

Once you sorted the OL device, you will need to install a short circuit protection device at the beginning of the motor cable (the MCC panel). The device should provide protection against short circuit currents (3-ph, 2-ph, 1ph-earth etc) and often it will also provide shock protection function too. Its size is depending on:
1. what it needs to protect against,
2. the starting method of the motor,
3. the type of protection device you intend to use (MCBs or fuses),
4. the protection device's immunity to the starting current of the motor.



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