Discuss Earthing on multi core 3 phase system in the Commercial Electrical Advice area at ElectriciansForums.net

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bobc

Hope someone can advise, present cables supplying power to a drive for a 453kW DC motor is 2 x 400mm single core cable per phase. Motor is being upgraded to 630kW, the installer wishes to install an additional 400mm per phase to carry the load for the new motor. There is some debate as to the size of the earth required, is the earth core based on the fact that the phase is 400mm single core albeit doubled up to provide loading capability or is it's size based on the number of 400mm per phase e.g 2 x 400 for 453kW or 3 x 400 for the 630Kw motor. Any assistance would be much appreciated.
 
Per phase? DC? Ignoring that little slip what is the source of the DC? There are several ways of earthing a DC system, each has it’s own requirements.

Sorry but more information is needed.
 
It should be calculated as per adiabatic equation.

S= The CPC conductor

I the value in amperes of the fault current
t time for the protection device to operate at that value ( the sum is squre rooted)

Divided by

K the factor for the resistivity, temperature coefficient and heat capacity of the conductor material.

You may also find that the designer will take other factors into account with something of this size.
 
Saw the DC bit Tony after I posted and read your reply .................yes I missed that but the eyes are not what they were lol.

Yes as it's DC we do need more info on the supply and installation as usual mate spot on.
 
The way I read the op is that these are the supply side of the drive for the dc motor i.e. the ac/dc converter what ever you wish to call it.
The dc motor bit may be a read herring.
However as you both have said, loads more info required.
As well as the drive manuf' instructions needing to be referenced.
e.g. Is there safety earthing and functional earthing required?
Is there a clean earth required.
Is there any separation?
 
Please accept my apologies if I have misled anyone, the present system is as follows - 3 phase supply to an ABB DCS 500 drive, the supply consists of 2 x 400mm cables per phase, DC output from the drive to the motor is 1 x 400mm per pole. The system is being upgraded to a 630kW DC motor, it is suggested to increase the 3 phase supply to the drive to 3 x 400mm per phase and the DC motor supply will be 2 x 400mm per pole, the new drive will be an ABB DCS 800, no issues with safety earths for the drive, the only issue is the size of the earth required between supply and drive and drive to motor. Thanks in anticipation.
 
3 X 400mm seems way over the top to me though, where is the AC supply from?
I’d only use 2 X 500mm per phase for a 1MVA transformer!

Hang on rethink time, is this singles or multicore?

As for 1 X 400mm on each pole of the output to the drive? Some things totally out of kilter here.
 
As per Tony & Malc, plus you should check with the ABB literature.
When I worked for one of their competitors on such kit we had our own earthing requirements for the output of the drive which exceeded those in the national standards.
On these the motor must be earthed back to the drive motor earth terminal.
I would size the earth in accordance with the adiabatic in the absence of any other info as per Malc
 

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