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Topic Title: Taking into account source impedance
Topic Summary: Is it important?
Created On: 21 December 2012 08:48 AM
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 21 December 2012 08:48 AM
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NickEbsworth

Posts: 6
Joined: 19 December 2011

I have a 100A fuse at the distribution board feeding a length of 16mm2 cable to a piece of plant, drawing 64A.

When I size the cable, the aim is to ensure that the cable resistance is low enough to ensure that a short between L and N at the far end of the cable will allow enough current to flow to blow the 100A fuse within the required time (in this case 5seconds)

However, the source impedance from the supply transformer into the property will affect the current that can flow and therefore will affect the length of the cable that I can fit between the fuse and the plant.

As far as I can tell, the source impedance is never taken into account when calculating the length of the cable between the fuse and equipment....

Am I missing something?
 21 December 2012 08:54 AM
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Avatar for OMS.
OMS

Posts: 19689
Joined: 23 March 2004

Yup, you are missing a fundamental point. the fault current has to have a circuit or loop to flow around the source impedance forms part of that loop. So you must include the source impedance otherwise the calculation is pointless.

the source impedance may be small, and the cabling up to the point of (in your example) the 100a fuse may be almost all rectance with very low resistance - but it is efectively an impedance and you need to account for it.

how you account for it is driven by how accurate you ned to be in your short circuit assessment

regards

OMS

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 21 December 2012 09:09 AM
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AJJewsbury

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As far as I can tell, the source impedance is never taken into account when calculating the length of the cable between the fuse and equipment....

In practice it's usually covered by line-earth fault loop impedance calculations (Zs) as the external L-earth impedance is usually either the same as L-N (TN-C-S) or higher (TN-S or TT) and within the installation the c.p.c. is usually no larger than the live conductors - hence if calculated Zs is OK, L-N loop impedance should be too.

- Andy.
 21 December 2012 09:35 AM
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NickEbsworth

Posts: 6
Joined: 19 December 2011

Using the ELI reading is a good idea when measuring a TN-C-S system but MAY give me a high reading when measuring a TN-S or (worst case) a TT system.

As I am dealing with relatively high currents (64A) the source impedance must be low (certainly below 0.24 Ohms) or it becomes theoretically impossible to have 'any' length of cable between the 100A fuse and the plant!

One thought that I had was to measure the PSCC at the distrib board and calculate the source impedance from that - thoughts?
 21 December 2012 09:44 AM
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Avatar for OMS.
OMS

Posts: 19689
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One thought that I had was to measure the PSCC at the distrib board and calculate the source impedance from that - thoughts?


If you can measure PSCC at the DB you don't need source impedance

I think what you mean though is you can measure impedance at the DB and the instrument estimates PSCC from that - with potentially a significant margin of error.

Either way, if you have information at the DB, and you know the cable size and length, you can determine far end fault current - from there you can observe disconnection time and from that determine if the condutor is adequately protected from short circuit faults by calculation.

Depending on how accurate you need to be you may also need to account for temperature effects on resistance and for reactance in the system.

Regards

OMS

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Failure is always an option
 21 December 2012 09:45 AM
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NickEbsworth

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Oh - and my ELI us usually far too high to blow the fuse in the required time for a L-E fault, so a 300mA Selective type A RCD is used to protect against this fault.
 21 December 2012 10:18 AM
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NickEbsworth

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Joined: 19 December 2011

OMS - yes, I kind of got that the wrong way round - as you rightly state the instrument measures source impedance and then calculates PSCC. However, as the instrument only indicates PSCC, we have to do the reverse calculation to get the source impedance (even if it is only an approximate figure)

Thanks for all the input to this issue so far - it helps.
 21 December 2012 10:33 AM
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OMS

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Well you can use your earth loop tester to measure the line - neutral loop if you want - you'll gert an answer in Ohms. the tster is simply doing the sums for you when you select PSCC

Regards

OMS

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 21 December 2012 09:47 PM
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peteTLM

Posts: 3166
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Nick,

the outcome and end impedance is perfectly predictable by calculation.
EG you have a BS1361 fuse in this case 100A, requires a max Zs of 0.35 ohms. This is fixed.
Lets say you have a supply of Ze 0.11. This is also fixed.
You have 0.24 ohms to use up on your cable which you can vary in size up and down by calculation to achieve the end result, rather than a hit and hope.

Is your machine single or 3 phase. Whats the cable type? as 16mm might not do it for your 100A cpd.

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 21 December 2012 11:11 PM
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alancapon

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Originally posted by: peteTLM
. . . Lets say you have a supply of Ze 0.11. This is also fixed. . .

I am goint to disagree here. If you have your own HV/LV transformer, then it will be fixed. If you take a supply from the DNO at LV, then it may vary, depending on the configuration of the network - either up or down, as the network is reconfigured to take account of faults, maintenance or supply reinforcement.

Regards,

Alan.
 21 December 2012 11:14 PM
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peteTLM

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Originally posted by: alancapon

Originally posted by: peteTLM

. . . Lets say you have a supply of Ze 0.11. This is also fixed. . .


I am goint to disagree here. If you have your own HV/LV transformer, then it will be fixed. If you take a supply from the DNO at LV, then it may vary, depending on the configuration of the network - either up or down, as the network is reconfigured to take account of faults, maintenance or supply reinforcement.



Regards,



Alan.


Haha, ok, bad choice of wording. Variable, but out of your control........hows that? Fixed as in you have to lump it!

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
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