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Topic Title: Calculation of short circuit current
Topic Summary: **Calling all number crunchers**
Created On: 30 June 2013 09:37 PM
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 30 June 2013 09:37 PM
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alanjosephbrown

Posts: 20
Joined: 01 September 2012

Good evening folks,

To set the scene; I have a containerised service module with a very simple electrical installation which consists of 1 control panel which supplies/switches a twin 36W Flu luminaire and supplies a star/delta starter panel. The star delta panel in turn supplies and controls a 3 phase 11KW motor. Easy so far.

The unit is to be sent to a client's site so they are asking all the normal questions - load, voltage, frequency etc. BUT a new one for me, the client is requesting a figure for max short circuit amps so that they can select a suitable protective device to supply the equipment.

The only experience i have had of this kind of thing Is doing a PSCC for a fixed installation and praying the figure is less than that of the already installed MCBs. I have always thought the short circuit current was a relationship between cable sizes and proximity to the local transformer.

It seems the client requires some kind of calculation to do with the equipment alone. I

Can anybody shed some light on this or perhaps point me in the direction of some relevant literature? I really don't know where to start with this one.

Many thanks in advance

Regards

Alan
 30 June 2013 09:49 PM
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dlane

Posts: 690
Joined: 28 September 2007

Hi Alan,

I would think tht they are asking for the withstand capability of your equipment. I assume that you have a distribution board in the container that the incoming supply is fed into . The breakers in that board will have a breaking capacity which is a figure they could use to ensure that their supply will be compliant with your container installation.

The only other figure you could supply them is the earth loop impedance of the installation within the container that they could utilise for a PSCC calculation.

Ultimately the actual current that would flow in a fault is going to be based upon the impedance of their supply, which you would not know, along with your installation ELI.

Kind regards

Donald Lane
 01 July 2013 10:33 PM
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alanjosephbrown

Posts: 20
Joined: 01 September 2012

Hi Donald thank you for your response, after some digging, i found the reason for the equipment details request was this from DNV regs 2.7-2 (4.5.3);
"Modules shall have their in-take circuits and components rated for defined values of nominal voltage,
frequency, current, short circuit level and type of system, e.g. IT, TN-S, etc. This data shall be clearly marked
at the connection point."
So by your reckoning Donald do you think the way forward would be to survey all the protective devices and find the one with the lowest KA rating and use this as the value for the whole assembly?

I can see what you are saying about taking an EFLI reading, but of course this would change from installation to installation due to different Ze on-site.

Best Regards

Alan
 01 July 2013 10:44 PM
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Zuiko

Posts: 521
Joined: 14 September 2010

I can see what you are saying about taking an EFLI reading, but of course this would change from installation to installation due to different Ze on-site.



There would be a minimum impedence though, and therefore maximum short-time short circuit current.
DNOs rarely put in 11kV / 400V transformers rated more than 1MVA with more than 5% impedence. This is to limit the short circuit current on their network. Normally it's about 26 kA (max). This will be a fault right on the LV busbars at the substation. Downstream at your motor, you are not going to an impedence anywhere near that low.


As has been suggested you could always measure the total impedence back to the transformer from where you are going to plug the thing in!


If the supply is PME, Ze should be below 0.35, but is often somewhat higher; separate N/E Ze should be below 0.8.
 02 July 2013 04:49 PM
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dlane

Posts: 690
Joined: 28 September 2007

If you are utililising DNV regs then I assume that this is for an offshore installation?

Calculations for offshore fault levels are carried out to a different standard than the onshore calcs and I am not that familiar with them.

If they are asking for a nominal value for short circuit level then to give them the maximum short circuit withstand would be one option.

The second option is if you know the earth loop impedance of your supply where you test the cabins then you could take readings of the installation in the module and subtract it from the value you have for your test supply to obtain an earth loop impedance just for your module. It may change when the module is installed offshore but that would be due to their supply and outside your scope.

Kind regards

Donald
 02 July 2013 06:51 PM
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Avatar for OMS.
OMS

Posts: 19463
Joined: 23 March 2004

"Modules shall have their in-take circuits and components rated for defined values of nominal voltage, frequency, current, short circuit level and type of system, e.g. IT, TN-S, etc. This data shall be clearly marked at the connection point."


So just state what you've designed to (or tested to) - ie 400/230V 50Hz, Full load amps, XxKA for 1 second, three seconds or whatever as RMS and peak values.

Regards

OMS

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Failure is always an option
 04 July 2013 02:45 PM
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Avatar for timothyboler                                      .
timothyboler

Posts: 229
Joined: 25 July 2008

I'm not entirely clear from your description - does the starter panel contain 11kV busbars or is it just a stand-alone control panel? Most classification societies including DNV require that switchgear and controlgear over 1000V is fully type-tested. If it's just the LV panels we're interested in then you can based the design of a type-tested panel or calculate the fault withstand strength - it's not just the MCBs it's the physical withstand strength of the enclosure as well. If the panel is rated at less than 10kA you don't need to perform any short-circuit withstand tests per IEC 60439-1. You'll find some info on specifying SC ratings in that standard.

Regards, Tim

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