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Topic Title: Zs calc for a 110V sockets
Topic Summary: Full example
Created On: 21 September 2015 02:14 PM
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 21 September 2015 02:14 PM
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sarandis10

Posts: 56
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
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AJJewsbury

Posts: 16117
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
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AJJewsbury

Posts: 16117
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
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gkenyon

Posts: 4982
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 TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 21 September 2015 03:12 PM
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sarandis10

Posts: 56
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
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sarandis10

Posts: 56
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
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AJJewsbury

Posts: 16117
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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
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sarandis10

Posts: 56
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
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GeoffBlackwell

Posts: 3800
Joined: 18 January 2003

Try this

Regards

Geoff Blackwell
 21 September 2015 05:18 PM
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AJJewsbury

Posts: 16117
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
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mapj1

Posts: 9716
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
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lyledunn

Posts: 1112
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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
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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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