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 Topic Title: Zs calc for a 110V sockets Topic Summary: Full example Created On: 21 September 2015 02:14 PM Status: Read Only Linear : Threading : Single : Branch
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 21 September 2015 02:14 PM sarandis10 Posts: 51 Joined: 21 November 2011 I am trying to calculate the Zs of a 110V sockets which are fed from a 230/110V trans so I have: VA :6000 Trans impedance : 4.9 % Primary imp Zp:0.31455 (taken from the Amtech ) Secondary Voltage :110V Primary Voltage :230V R1+R2, for a 6mm cable with no cpc : 3.08mOhm/m from the table F.1 on the on-site guide. L:10m Zsec= 0.31455*((110/230)^2+(4.9/100)) * ((110^2/6000)+(0.00308*10) = 0.178 Ohms So I compare this figure with the table 41.6 and find out if the mcb I am choosing to use can disconnect on time. Are these calcs right? 21 September 2015 02:31 PM AJJewsbury Posts: 15838 Joined: 13 August 2003 R1+R2, for a 6mm cable with no cpc : 3.08mOhm/m from the table F.1 Are you really sure you have no c.p.c.? - Andy. 21 September 2015 02:44 PM AJJewsbury Posts: 15838 Joined: 13 August 2003 from the table F.1 on the on-site guide. Also check what conductor temperature that table is for and what temperature your conductors will likely be in service (worst case). - Andy. 21 September 2015 03:11 PM gkenyon Posts: 4932 Joined: 06 May 2002 If it's Earth Fault, is the Voltage to Earth 110 V, or is it a Centre-Tapped system, to the voltage to earth is actually 55 V (as would be usual for the yellow sockets used on the RLVS in the UK)? ------------------------- EUR ING Graham Kenyon CEng MIET TechIOSHG Kenyon Technology LtdWeb-Site: www.gkenyontech.com 21 September 2015 03:12 PM sarandis10 Posts: 51 Joined: 21 November 2011 well, you are right i do have one but i just used this as an example. i quite liked the number thats why. ha! what i am after is someone to confirm that the cals are correct. thanks for your reply! 21 September 2015 03:14 PM sarandis10 Posts: 51 Joined: 21 November 2011 it is 55V-0V-55V , so it is centre -tapped. will this affect my cacls? 21 September 2015 03:30 PM AJJewsbury Posts: 15838 Joined: 13 August 2003 will this affect my cacls? Yup.Think about what's involved in the earth fault loop on the secondary side. - Andy. 21 September 2015 04:41 PM sarandis10 Posts: 51 Joined: 21 November 2011 well in that case i will change the formula from 110V to 55V and from 6000VA to 3000VA. i think thats the only change that needs to be done! what u recon? 21 September 2015 05:06 PM GeoffBlackwell Posts: 3793 Joined: 18 January 2003 Try this Regards Geoff Blackwell 21 September 2015 05:18 PM AJJewsbury Posts: 15838 Joined: 13 August 2003 Conductor temperatures? If you're using 20 degree tables you might need to increase your R1+R2 values by around 20% (factor of 0.004 per degree C) - unless you're comparing the result with the 80% ROT tables from GN3 or the OSG instead of the BS 7671 ones and need to have the primary loop impedance adjusted too. - Andy. 21 September 2015 10:58 PM mapj1 Posts: 9366 Joined: 22 July 2004 Be aware that while the above is true, it is not that common to worry about the Zs of a 55-0-55 supply to the same extent as a 230v one, as there are very few fault conditions where the earthed human is exposed to a voltage greater than 50 or so, and considering a general L-E short at the appliance, then the exposed voltage is more like half of 55V if the live core and the earth are the same cross-section, and that is neglecting the transformer voltage droop, so there is almost never the same urgency of disconnection for safety of life. And of course an "all wires" phase to phase to earth fault gives even less of a touch voltage. It is normally voltage drop and inrush that determine maximum useful cable lengths. ------------------------- regards Mike 22 September 2015 06:27 PM lyledunn Posts: 1107 Joined: 13 August 2003 It is only but right that Geoff gets a round of applause for his comprehensive explanation of the issue. We are very fortunate on this forum to have so many erudite chaps who are willing to take time out to educate others. ------------------------- Regards,Lyle Dunn 22 September 2015 10:53 PM Inrush Posts: 741 Joined: 19 December 2007 I am trying to calculate the Zs of a 110V sockets which are fed from a 230/110V trans so I have: VA :6000 Trans impedance : 4.9 % Primary imp Zp:0.31455 (taken from the Amtech ) Secondary Voltage :110V Primary Voltage :230V R1+R2, for a 6mm cable with no cpc : 3.08mOhm/m from the table F.1 on the on-site guide. L:10m Zsec= 0.31455*((110/230)^2+(4.9/100)) * ((110^2/6000)+(0.00308*10) = 0.178 Ohms So I compare this figure with the table 41.6 and find out if the mcb I am choosing to use can disconnect on time. Are these calcs right? As others have said, if you are calculating for earth fault the voltage and VA figures should be halved. Your calculation will err on the side of caution, however it's not strictly accurate to just add the TX impedance to the reflected primary impedance. To be more accurate you really need to know the primary side reactive and resistive components (possibly more resistive), then reflect these individually and add to the transformer impedance - for such a small TX I'd assume this to be 100% reactive. You can then calculate the overall system impedance. Unfortunately I think Amtech only gives overall impedance, however I could be wrong on this. Thanks
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