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Topic Title: zs or ze
Topic Summary: ze on tt system
Created On: 10 July 2014 12:55 AM
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 10 July 2014 12:55 AM
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gchar

Posts: 27
Joined: 30 May 2014

Muxium ze on tt system is/are there any give value
 10 July 2014 06:32 AM
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Jaymack

Posts: 4754
Joined: 07 April 2004

Originally posted by: gchar
Muxium ze on tt system is/are there any give value

Rather than formicating the site with basic q's in tortuous English, a search will answer all. Well someone has to say it!

Regards
 10 July 2014 09:27 AM
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AJJewsbury

Posts: 11692
Joined: 13 August 2003

Maximum Ze for TT on a public network is usually 21 Ohms (compare with 0.35 for TN-C-S and 0.8 for TN-S). HOWEVER Ze doesn't include the resistance of the soil around the customer's electrode (as that's part of the customer's installation, not the external supply). Zs at the origin (including the customer's electrode resistance) needs to be low enough both for RCDs to operate under earth fault conductions and to ensure that exposed conductive parts can't exceed 50V above true earth due to earth leakage. The last bit is usually the most onerous and can be calculated from 50V/Idn of the RCD - so for example 1667 Ohms for a 30mA device and 100 Ohms for a 500mA device. Table 41.5 in the regs gives you a full rundown. Add to that there's a recommendation that a customer's electrode shouldn't exceed 200 Ohms on the basis that if it's that poor then it'll not have good enough contact with the soil to remain adequate during very dry or freezing conditions.

(Strictly speaking is a small difference between Zs and Ra and some parts of the regs limit Ra rather than Zs, but I'll try not to confuse things further with that now).

- Andy.
 10 July 2014 09:38 AM
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John Peckham

Posts: 7577
Joined: 23 April 2005

Andy

The 21 ohms is the usual maximum declared value of the suppliers electrode not the actual value and does not include the consumers earth electrode resistance or the resistance of the soil in between so it is not Ze. That assumes you can get the DNO to understand your question and get them to declare anything even the day of the week!

If you measure the electrode (I hate the word steak unless it comes on a plate) resistance with an earth resistance tester then you are measuring Ra if you include the resistance of the earthing conductor. If you use a loop impedance tester on the earth electrode then you are measuring Ze. If you measure the resistance/impedance with the bonding connected then you are measuring Zs, I could debate the later point.

Call me a pedant if you will but I have been called a lot worse without leaving home.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 10 July 2014 09:50 AM
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slittle

Posts: 3585
Joined: 22 November 2007

I had the pleasure of a TT system on this years ECA inspection.

Took a bit of explaining that Ze / RA depending how you want it measured was under 5 ohms !

The joys of old electrodes, nearby substations and very conductive Essex Clay

Stu
 10 July 2014 11:32 AM
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AJJewsbury

Posts: 11692
Joined: 13 August 2003

Call me a pedant if you will

moi?

The 21 ohms is the usual maximum declared value of the suppliers electrode

Or 20 Ohms for their electrode (as per BS 7430) plus 1 Ohm for the rest of the conductive path? (source, lines etc).

and does not include the consumers earth electrode resistance or the resistance of the soil in between

Agreed (I think I said that).

so it is not Ze

Depends on your definition of Ze. Part 2 says 'external to the installation' - isn't the customer's electrode part of the customer's installation?

- Andy.
 10 July 2014 11:58 AM
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John Peckham

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Yes the consumers earth electrode is part of the consumer's installation but using an earth resistance tester you measure Ra not Ze.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 10 July 2014 11:58 AM
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OMS

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Joined: 23 March 2004

Depends on your definition of Ze. Part 2 says 'external to the installation' - isn't the customer's electrode part of the customer's installation?


I agree Andy - Ze is limited to the suppliers system as you stated - which includes not just the suppliers electrode but the rest of the junk to make it a conductive path - collectively not exceeding 21 ohms with Rb not exceeding 20 ohms

Ra is the resistance of the consumers electrode and earthing conductors connecting to exposed conductive part or parts

Regards

OMS

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