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Topic Title: First Faults on IT (Earth Free) 650v Railway Feeders
Topic Summary: First Faults on IT (Earth Free) 650v Railway Feeders
Created On: 26 November 2010 09:05 PM
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 26 November 2010 09:05 PM
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JWallace85

Posts: 2
Joined: 26 November 2010

Hi

Can someone please explain why small fault currents flow when there is a single earth fault on an IT circuit. I undertsand it is linked with capacitive coupling. Can anyone please explain whay this happens?

Thank you!

James
 07 December 2010 11:39 AM
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tomcryer

Posts: 2
Joined: 14 September 2003

By earth free, are you thinking of something like the London Underground which has both positive and negative conductor rails? In that situation, the conductor rails are referenced to earth by bias resistors at each substation. Hence in the event of a single pole earth fault, the only path for any fault current is via those bias resistors.
 07 December 2010 05:14 PM
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ArthurHall

Posts: 735
Joined: 25 July 2008

I have no experiance on railway supplies but I have worked with 3 phase IT supplies at 600V fed from a delta winding. If one phase shorts to earth current flows through the capacitance of the insulation on the other phases, the bigger the capacitance i.e. the longer the cables then the higher the currents.
The usual reason for IT is not so that the conductors are safe to touch but so that the system can stand one fault and still operate.
All the systems I have worked on have had earth monitoring systems which alarm if they detect an earth fault. The monitoring systems can be a lamp from each phase to earth, if a phase shorts to earth then the lamp goes out or a relay which monitors voltage between an artificial star point and earth.
 10 December 2010 09:18 AM
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tomcryer

Posts: 2
Joined: 14 September 2003

I'm maybe missing the point here then - what's an IT feeder / supply?
 13 December 2010 04:45 PM
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broadgage

Posts: 1264
Joined: 07 August 2007

Originally posted by: tomcryer

I'm maybe missing the point here then - what's an IT feeder / supply?


One without any intentional connection between any current carrying conductor and earth. No earthed neutral, center tap, star point or equivalent.

Prohibited for public supplies but sometimes used for special applications.
May continue in service in the event of a single earth fault, unlike a system with an earthed neutral which would be dissconnected by the opening of fuses or circuit breakers.
 10 May 2011 02:11 PM
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rallen

Posts: 5
Joined: 28 January 2002

Originally posted by: broadgage

Originally posted by: tomcryer



I'm maybe missing the point here then - what's an IT feeder / supply?




One without any intentional connection between any current carrying conductor and earth. No earthed neutral, center tap, star point or equivalent.



Prohibited for public supplies but sometimes used for special applications.

May continue in service in the event of a single earth fault, unlike a system with an earthed neutral which would be dissconnected by the opening of fuses or circuit breakers.


In this context, it's used for railway signalling power supplies in order to improve reliability. Regulation 411.6.3.1 of BS 7671 requires that insulation monitoring is provided to detect a first earth fault and this is standard practice for new installations on Network Rail.

In response to the original question, I would agree that any residual fault current following a first earth fault will be the result of capacitive coupling. What is your concern - is it that the magnitude of this current could cause an electric shock to someone touching a part of the system beived to be at earth potential following an earth fault?

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rallen
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