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Calculating Motor Fault Current

02/25/2010 10:43 PM

How to calculate fault current in a motor and to design grounding systeme for the same?

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Guru
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#1

Re: fault current

02/26/2010 3:06 AM

The locked rotor current can be derived by measuring the resistance across the supply terminals and dividing it into the supply voltage.

The full load current will be marked upon it, and the motor overload protection device upstream of it should be set at this figure.

The grounding/earthing system needs to be provided in accordance with applicable national standards, such as British Standard 7671.

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#2

Re: fault current

02/26/2010 4:46 AM

If you don't have any data then you may assume that fault current can be 20 times of FLC of motor. Though MotPro relay will interrupt the supply much before but the earthing system should be capable to handle that amount of current.

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Guru
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#5
In reply to #2

Re: fault current

02/26/2010 6:10 AM

<...fault current can be 20 times of FLC of motor...>

Fault current is determined by the characteristics of the circuit protection device and the cabling. The earthing system needs to be sized to handle it. There is an algorithm in British Standard 7671.

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#6
In reply to #5

Re: fault current

02/26/2010 6:22 AM

I've already mentioned that if you don't have any data then you can assume max fault current= 20 times of FLC.

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Guru
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#9
In reply to #6

Re: fault current

03/01/2010 4:16 AM

Without any data about the circuit protection device, the size, the length and the method of cable installation, that assumption may be invalid, whether repeated or not.

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Guru
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#11
In reply to #6

Re: fault current

01/20/2025 11:32 AM

During a fault, the motor might not be part of the residual circuit, so that assumption is invalid.

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#3

Re: fault current

02/26/2010 5:04 AM

You want the ground fault setting? it is typically 5% (max 10%) of FLA.

If it is Overload- it may be 15-20% above.

After all you do not want equipment to destruct beyond repair in case of fault. Then why do you need relay?

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#4
In reply to #3

Re: fault current

02/26/2010 5:20 AM

I feel the questioner wants to know the design value of earthing system for a motor. He/she may have the protection value already.

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Guru
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#13
In reply to #4

Re: fault current

01/20/2025 11:35 AM

British Standard 7671 can provide this were the installation to be in the UK and designed, installed, tested and maintained by qualified local Electricians. Other countries run different standards.

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Anonymous Poster
#7

Re: Calculating Motor Fault Current

02/27/2010 6:34 AM

question should be explicit for the correct answer. I was reading lot many answer and none may be answering your question. Probably you want to know the fault current contribution from the motor at the time of short circuit. Post the question so as the data may be supplied.

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Anonymous Poster
#8

Re: Calculating Motor Fault Current

02/27/2010 9:41 AM

The maximum motor current will depend on the %age impedence(fault level of system feeding motor) and resistance of motor feeding electrical circuit ie cables etc and will max at motor terminals

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#10

Re: Calculating Motor Fault Current

03/01/2010 1:08 PM

To determine the motor contribution to the first half cycle fault current when the system motor load is known, the following assumptions are generally made:

Induction motors: use 4.0 times the motor full load current (impedence value of 25%)

Synchronous motors: use 5.0 times the motor full load current (impedence value of 20%).

Keep in mind the effect of reactance in an AC system is to cause the intial current to be high then decay towards steady state. The fault current consists of an exponentially decaying DC component superimposed upon a decaying AC current. The rate of decay depends upon the ratio of the reactance to resistance commonly referred to as the X/R ratio. The higher the ratio the longer the high current condition lasts before steady state is reached.

That is essentially how to calculate the motor current addition into a fault currrent calculation. It is not clear to me what you want when you ask for a grounding system for the same.

Could you be asking for what size grounding straps to ground the motor casing? The first part of your question seems to make sense if you are designing something, the follow on makes me wonder what you are doing.

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Guru
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#12
In reply to #10

Re: Calculating Motor Fault Current

01/20/2025 11:33 AM

No-one else seems to know either.

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