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Topic Title: DNO Measured Ze too Large for DNO Fuses
Topic Summary: No Earth Fault Protection on Supply Cable for Disconnection Time
Created On: 06 March 2013 12:04 PM
Status: Read Only
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 06 March 2013 12:04 PM
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Posts: 15
Joined: 28 October 2002

I have an installation where we have measured the external earth provided by the DNO and it is coming out at 0.17Ohms.

The DNO have provided a cablehead with fuses at 315A.

This means that there is no earth fault protection on the cable from this cablehead supplying our equipment; i.e max earth allowable for required disconnection time is 0.114Ohm for that size of fuse.

I dont really want to install an RCD unit between the cablehead and our equipment so is there any way round this and how does the DNO get away with this.

 06 March 2013 12:21 PM
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How accurate is your 0.17 figure? If it's a TN-C-S supply it would imply a v.d. of 53.55V at 315A (single phase), which would be outside of UK statutory limits.

Many loop testers are surprisingly inaccurate at low impedances.

- Andy.
 06 March 2013 09:09 PM
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Joined: 17 January 2012

we expect a max of 0.15 for this service size, poss your tester is out.
you can ask us to check.
more likely to be 0.05.

 06 March 2013 09:15 PM
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With that sort of service I'd say the chances of the meter reading correctly are zero.

Given that we are arguing about 0.05 of a ohm, the uncertainty in measurement is likely to be greater than the value of the Ze.

Also, if you manage an earth fault on what will be a short run through the CT chamber and a length of trunking into the mainswitch, please let me leave the room before hand as the bang and flash should be pretty impressive :-)


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