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Topic Title: Earth fault loop impedance
Topic Summary: Resistor in line on the earth of a circuit
Created On: 06 December 2012 07:06 PM
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 06 December 2012 07:06 PM
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Neil73

Posts: 2
Joined: 06 December 2012

Can anyone help me and explain it to me
Whilst doing an EFLI on a lighting circuit with a 10 amp Crabtree MCB and found it to be 56 ohms. I started to look into the problem as I am looking to add lights to the circuit and another circuit was tripping the RCD when testing on EFLI.
I have a 10mm earth which goes to a junction box which has a 25watt 470r resistor in it and then the earth splits and goes to 3 lighting circuits. My query is what is the resistor there for and can someone explain what it does. I havent come accross this before.
Thanks for your help
 06 December 2012 07:24 PM
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John Peckham

Posts: 7459
Joined: 23 April 2005

Its not a student test rig is it? I have resistors in my test rigs I use at college carefully hidden from the students. The only other place you might find that is on an IT installation such as in school laboratories but I doubt you have one of those as the resistor is usually of a higher value.

Probably the most likely explanation is the usual carp installation carried out by a f**kwitt, I see a lot of his work.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 06 December 2012 07:27 PM
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daveparry1

Posts: 6205
Joined: 04 July 2007

It might be someones attempt at stopping an rcd tripping, unlikely to work though at only 470 ohms!

Dave.
 06 December 2012 07:57 PM
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slittle

Posts: 3510
Joined: 22 November 2007

Originally posted by: John Peckham




Probably the most likely explanation is the usual carp installation carried out by a f**kwitt, I see a lot of his work.



No need to call my work carp John
 06 December 2012 09:07 PM
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antric2

Posts: 1055
Joined: 20 October 2006

Evening,
Has the customer got any dimmable LEDS in the circuit?
Regards
Antric
 06 December 2012 09:23 PM
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Neil73

Posts: 2
Joined: 06 December 2012

Hi
No dimmable LEDs its at a train depot if that helps
Thanks
Neil
There is certainly something fishy about this job
 06 December 2012 09:45 PM
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OMS

Posts: 19668
Joined: 23 March 2004

Have you got electrified lines nearby - typical railway standards used to require a resistor to limit D.C leakage in the CPC - usually in conjunction with a DC immune RCD - but I'm guessing without more information - this was where there was a potenial for contact with traction voltages

Regards

OMS

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Failure is always an option
 07 December 2012 04:51 PM
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Delbot321

Posts: 77
Joined: 06 November 2012

They often put resistors in the earth path to stop high DC currents using the domestic electrical system earth as a return path - this is typical in train care sheds and the likes. As an example: you have a power tool/lead light (usually 110v but not always) and you want to use it on a train or near the running rails. The potential of the running rail (and any carriage) will be a few volts different to the earth on the power tools - this will cause a current through the domestic earthing system of tools leads and conductors. This will easily be 30+ amps which is a lot more than it was designed to carry. There is no fault on the system it's just what happens with 750vDC systems. To overcome this they usually put a 470 ohm resistor in the earth conductor (usually in the socket outlet) to stop a DC current flowing. The circuit MUST have RCD protection in order to provide fault protection against electric shock - as we know the impedance can be as high as 1666 ohms for a 30mA RCD so you can see a 470 ohm resistor wouldn't be a problem.

I have seen very similar systems in British Telecom telephone exchanges on some equipment that needs the earthing separate from other domestic systems but again their is RCD protection in place too.

Hope this helps in your situation - if someone has put it in then there may be a very good reason for it - equally they could be putting it there as a totally inappropriate solution to another problem.
 07 December 2012 05:21 PM
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Inrush

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Joined: 19 December 2007

As stated by OMS and Delbot321 this resistor is most likely installed to limit d.c traction current leakage, as stated in Network Rail standard: NR/SP/ELP/27242

There are also restrictions on the use of neutral connections in d.c traction areas, so you need a copy of the NR standard.
 11 December 2012 05:47 PM
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AJJewsbury

Posts: 11462
Joined: 13 August 2003

as we know the impedance can be as high as 1666 ohms for a 30mA RCD

Technically, up to 7666 Ohms would be OK on a TN system (0.4s disconnection, no requirement to limit to 50V in non-trip conditions).
- Andy.
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