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Topic Title: Restricted Earth Fault Protection
Topic Summary: When and Where this is required?
Created On: 12 August 2017 09:00 AM
Status: Read Only
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 12 August 2017 09:00 AM
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Seph83

Posts: 3
Joined: 08 August 2017

I was wondering if theres a British standard that denotes when/where restricted earth fault protection must be used between the LV side of a supply transformer and the LV switchboard / downstream protective device.

I've been told such things as;
- where the cable between the TX / breaker is greater than 15m
- where there is exposure or high risk to damage

Rather than rely on 3rd hand information, I am keen to know if this is covered in a standard anywhere?

many thanks
 12 August 2017 10:16 AM
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Jaymack

Posts: 5357
Joined: 07 April 2004

Originally posted by: Seph83
"where there is exposure or high risk to damage"

Application of REF is used to reduce the effects of damage both to the equipment and system interruption.The term "restricted" comes from the fact that you are only looking at a restricted zone of protection for a unit, not the complete or total zone differential coverage. This restricted zone could include the cabling, but the length of this cable would need to be considered, in relation to the required zone of protection required.

Rather than rely on 3rd hand information, I am keen to know if this is covered in a standard anywhere?

There is probably manufacturers' or electricity supplier's guidance on the application of REF.

Regards
 14 August 2017 09:23 PM
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Nedryerson

Posts: 90
Joined: 12 December 2009

Seph83,

Your advice was good but there is nothing I am aware of which prescribes the length of the tails nor is there a standard.

Basically its down to your own risk assessment which would include the value of the equipment and the transformer KVA. (The bigger the more likely REF would be fitted).
 14 August 2017 09:27 PM
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Nedryerson

Posts: 90
Joined: 12 December 2009

Forgot to mention - you must intertrip the 11kV
 17 August 2017 07:59 AM
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Seph83

Posts: 3
Joined: 08 August 2017

Thanks for the information both.
Nedryerson, out of curiosity, what would you class as a big transformer?
 17 August 2017 08:33 AM
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Nedryerson

Posts: 90
Joined: 12 December 2009

mmmm,

To my mind REF and standby earth fault would be recommended on say, 2MVA and above but it all depends on your assessment of the effects of the fault going undetected.

On really large transformers (generator transformers, Grid substation transformers) differential protection and a host of other devices would be deployed.

Ned
 17 August 2017 12:06 PM
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ToniSM

Posts: 329
Joined: 21 November 2006

I worked for a chemical company with a fairly large 11kV system. REF would be used on transformers of 500kVA or above. As far as I know it was just our requirement.

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