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Topic Title: Zs measurements on TT system
Topic Summary: Filling in a PIR
Created On: 07 June 2011 10:35 PM
Status: Read Only
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 07 June 2011 10:35 PM
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Posts: 153
Joined: 09 January 2007


I have recently given up the self employed turmoil to go back on the books due to having what I call an electricians body (bad knees), not enough hours in the day & too much paperwork.

I have an issue with the QS and the way that forms are filled out. On PIRs and EICs he is saying that on a TT system you measure Ra and record it and then on the schedule of tests in the column maximum measured Zs put not applicable.

I was taught that once you have completed all dead tests you measure Zs where safe to do so and record the reading or use the calculated method but there should be no N/A's in this column.

Who is correct?

 07 June 2011 10:40 PM
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In the 'measured Zs' box write the actual measured figure and in the box for maximum Zs allowed by bs7671 write 1667 ohms, assuming a 30 m/a rcd is in use,

 08 June 2011 12:15 AM
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Bit of an academic one really. Your QS is right in that Zs is irrelevant where Ra is known. The key issue is that the required disconnection time is met and the product of Ra and the rcd rating does not exceed 50. On the other hand Zs can replace Ra in the little inequation and so stating it in the appropriate column would seem reasonable if not superfluous.I would recommend R1+R2 values are established for TT systems. Should a TN system replace the TT system then a reasonably accurate refection of Zs can be had.


Lyle Dunn
 08 June 2011 08:37 AM
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John Peckham

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Joined: 23 April 2005

At the top of the schedule of circuit details NICEIC form for Zs it says, "Max Zs permitted by BS7671". For RCDs 411.4.9 and table 41.5 can be applied. So for a 30mA RCd you would write 1667 ohms in the Zs box.

John Peckham

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