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Topic Title: Zs for main cutout
Topic Summary: TN system - loop impedance at head - main fuse - will it blow?
Created On: 27 April 2013 03:46 PM
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 27 April 2013 03:46 PM
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anton

Posts: 13
Joined: 09 September 2003

Hi I have been looking at the Z values for installs and tns is 0.8 ohm which is a bit higher than the 0.4 - 0.5 ohm (at 100%) that 7671 gives for 100 amp cut-out fuses for 5 second blow? I have checked many a Ze/s at the head and come-up with values over these (but still within book). Am I missing something? - will these fuses blow at their rated current? Does anyone know how these work? Thanks in advance.

Edited: 27 April 2013 at 04:00 PM by anton
 27 April 2013 04:06 PM
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slittle

Posts: 3585
Joined: 22 November 2007

They won't blow at their rated current. Look at the time/current charts.

Bear in mind too that BS7671 doesn't apply before the meter output terminals


Stu
 28 April 2013 02:25 PM
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ennisteve

Posts: 10
Joined: 10 December 2012

Hi

The cut out fuses are there to provide current limitation to the supply authority cable.
 28 April 2013 03:48 PM
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OMS

Posts: 19851
Joined: 23 March 2004

Originally posted by: ennisteve

Hi

The cut out fuses are there to provide current limitation to the supply authority cable.


I doubt it - they are usually there to provide basic short circuit protection for that part of the clients installation prior to his first point of protection - it's why DNO's have a cable length limit on tails before the consumer has to provide protection.

Current limitation (or overload protection) is usually back at the distribution fuse racks and will be pretty coarse indeed - it's quite probable that DNO's would run cabling and transformers to more than 130% of rated capacity for reasonable durations.

regards

OMS

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