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Topic Title: Rise of Earth Potential Calculation at LV
Topic Summary: Calculating ROEP, Step and Touch Potential
Created On: 28 October 2013 12:21 PM
Status: Post and Reply
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 28 October 2013 12:21 PM
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slimG

Posts: 7
Joined: 11 March 2003

Greeting all,

I have been given an earth fault value of 26.85KA at the LV point of connection (LV busbar) by DNO

Tried calculting the ROEP with 26.85KA / 3 = 8.95KA for single phase earth

Measured earth resistance is 7ohm

From that I calculate ROEP to be V = I R

8950Amp x 7 = 62650V, this ROEP seems way too high for LV supply

Any pointers and also on the side could the calculations for STEP and TOUCH voltage be derived from this.

Thanks for your help.
 28 October 2013 01:13 PM
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hpcompaq

Posts: 78
Joined: 27 September 2007

Hello slimG.

If your DNO is declaring an earth fault value of 26.85kA then this is the single phase to earth value - no need to divide by 3. I don't know the details of your installation but this looks like the three phase symmetrical short circuit current too, and the DNO have assumed a direct zero impedance return path to the Tx star point for the earth fault current hence the same value.

Again I don't know any details of your installation, but if you are introducing 7 ohms into the earth return path then the fault current will be less than 40A. For example, if the Voc of the Tx is 250V then the inherent "resistance" (I know it's not) is 250v divided by 26.85kA giving about 9 mohms. This added to your 7 ohms gives 7.009 ohms and an earth fault current of about 36A.
 28 October 2013 02:57 PM
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slimG

Posts: 7
Joined: 11 March 2003

Thanks for your reply

Basically, there is a 11kv/0.4kv transformer on site in a grp building next to turbine (10metre spacing).

The DNO PCC is at the LV side where the fault parameters apply

What I did not understand is how the Voc affects the fault current. I thought the network fault analysis simulates there been a fault to earth earth on the LV side than 26.85KA can flow to earth. as suppose to the 40amps in the calculated example above .

Also if not, can I use 400V for the calculation instead of 230V single phase to derive the fault current calculated about.

Thanks for your help

Regards
 28 October 2013 03:12 PM
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slimG

Posts: 7
Joined: 11 March 2003


Oh I see I see!

Thanks Thanks
 28 October 2013 03:46 PM
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AJJewsbury

Posts: 11295
Joined: 13 August 2003

Presumably the LV is arranged as a TN system - so the LV earth is acting as a voltage reference only and isn't in the path of L-PE fault current.
- Andy.
 28 October 2013 04:18 PM
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Parsley

Posts: 960
Joined: 04 November 2004

Z= sqrtr2+x2 of the transformer and supply conductors.
If =Uo/Z

The 7 ohms of the earth rod doesn't affect the If that flows into the earthed star point of the transformer.

Regards
 28 October 2013 04:50 PM
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hpcompaq

Posts: 78
Joined: 27 September 2007

Hello.

If the earthing arrangement is as per AJJs suggestion, then the electrode will "effectively" be in parallel with the earth fault return conductor. The current then flowing through the electrode would be insignificant as would the voltage developed across it.
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