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Topic Title: Double Ipf for TPN?
Topic Summary: What is entered on the Installation cert and the test schedule?
Created On: 10 January 2013 11:26 AM
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 10 January 2013 11:26 AM
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IanLloyd

Posts: 65
Joined: 17 April 2002

Paragraph 7 of 2.7.16 in GN3 (p 54) says that where the voltage between lines cannot be measured directly with the loop test instrument then, as a rule of thumb for 3 phase supplies, the maximum balanced prospective short circuit level will be approximately twice the single-phase value.

Page 95 in Appendix 5 of GN3 shows a completed "model" installation certificate indicating that the supply is 400/230V with an Ipf of 0.77kA and a Ze of 0.3 Ohms - clearly the Ipf is derived from the single-phase loop impedance and has not been doubled even though it is a 3 phase supply. Page 99 in Appendix 5 of GN3 shows a completed "model" from 4 for a three phase installation. In the box for Ipf, it is clearly based on the single-phase loop impedance and has not been doubled.

This seems to be a contradiction and I am wondering what value should be entered into these documents?
 10 January 2013 09:46 PM
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leckie

Posts: 2014
Joined: 21 November 2008

Well I measure between phases and enter that value.

On single phase I measure between line and earth and line and neutral and enter the highest value.

Hope this doesn't lead to another debate about filling in boxes.
 23 January 2013 08:15 AM
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IanLloyd

Posts: 65
Joined: 17 April 2002

Hmmm...still not clear what we should do here. (Looking at P92 of GN3, I see the model form there has 9.0kA for Ipf with a Ze of 0.28 - for that value of Ze, shouldn't the Ipf be 230/0.28 = 0.86kA? Typo, I guess).

Paragraph 7 of 2.7.16 does state "...maximum balanced prospective short circuit level..." - so, is this just a "bolted short" between phases or 3 phase to earth?
 23 January 2013 10:21 AM
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AJJewsbury

Posts: 11768
Joined: 13 August 2003

The values don't necessarily have to be double the Uo/Ze value - that's an erring-on-the-side-of-safety estimate. Real values (or values measured L-L with an appropriate instrument) might well be a bit lower than double (line impedance might well be a lot higher than earth) - although I find it hard to imagine a situation where they wouldn't be noticeably higher than the single-phase L-E fault current.

Typo, I guess

Seems likely. Unfortunately the new example forms were added rather late in the process and so missed out on much of the reviews that the rest of the document got. If I were you I'd contact the IET with your observations and see if they'll issue an errata sheet for it (like they usually do: http://electrical.theiet.org/w...ions/updates/index.cfm ).

- Andy.
 23 January 2013 12:59 PM
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IanLloyd

Posts: 65
Joined: 17 April 2002

Thanks Andy,

Yes, I realise that doubling the single phase Ipf is providing a margin of error - it's just trying to establish what should be entered into the EIC and test schedule.

I've dropped a line to the IET regarding the apparent error.
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