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Topic Title: 11kV/415V hot site, LV earth electrode resistance
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Created On: 15 February 2011 02:13 PM
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 15 February 2011 02:13 PM
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fshillit

Posts: 18
Joined: 14 August 2002

I'm involved with a project in the UK which has an 11kV/415V distribution transformer supplied from an overhead line. An HV earth electrode has been designed to ensure safe touch and step potentials for HV faults, however as the EPR is above 430V (the site is 'hot'), the HV and LV earths need to be seperated. The earth system will be TN on the LV side.

My question is, what sort of resistance should I be looking at achieving for the LV earth electrode?

The old Electricity Supply Regulations (ESR) 1988 state that the supply neutral conductor of a PME system shall be connected with earth at such points to ensure the resistance to earth is less than 20 ohms. However the Electricity Safety, Quality and Continuity Regulations (2002) which supersedes ESR 1988 does not have this regulation stated.

Thanks,

Francis Shillitoe
 15 February 2011 08:42 PM
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dlane

Posts: 685
Joined: 28 September 2007

The first question I would ask is can the earthing system not be designed so that it becomes a cold site and eliminates the issue to start with.

Second, is this a private network or DNO. If it is a DNO they will have their own standard that will give the maximum resistance for the HV and LV earths.

If it is private then you could still utilise the 20 ohm rule as a guidance even though it is no longer in the regulations. You could get hold of EATS 41-24 that covers the design of substation earthing systems.

Ulimately the earth needs to be low enough so that the correct disconnection times can be achieved and installed outside of the hot zone.

Kind regards

Donald Lane
 16 February 2011 10:46 AM
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fshillit

Posts: 18
Joined: 14 August 2002

Thanks Donald. Point taken about the cold site. Always preferable if practicable. It's a private network. Have got the relevant standards EA TS 41-24, BS7430:1998, etc. There's also a new BS standard in draft, "Draft BS EN 50522:2008 Earthing of power installations exceeding 1kV a.c"

Having done a bit more digging on this, there is some guidance in "ER G12/3 1995 Requirements for the application of protective multiple earthing to low voltage networks":

4.8.2 Substation lv neutral earth electrode resistance shall be sufficiently low for hv protection to clear an interwinding fault and should preferably not exceed 40ohms.

4.8.3 Combined resistance of all lv neutral earth electrodes shall not exceed 20ohms (before the connection of customers' earthing terminals to the neutral).

Francis Shillitoe
 16 February 2011 02:43 PM
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Avatar for timothyboler                                      .
timothyboler

Posts: 227
Joined: 25 July 2008

These are maximum values. It's much better and safer to design the earthing system conservatively. FYI, our project policy at the moment is based on the rating of equipment:

500kVA and less 10 Ohms
500kVA to 1000kVA 5 Ohms
1000KV or greater 3 Ohms

Regards, Tim

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