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Topic Title: PEFC and Ze on a TT system
Topic Summary: TT system
Created On: 18 December 2009 05:45 PM
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 18 December 2009 05:45 PM
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nige296

Posts: 72
Joined: 04 December 2009

Just testing the Ze at origin on a TT system which was 256 ohms. The PSC is 1.1KA, 0.22 ohms. The PEFC is 1A. I was expecting the PEFC to be a lot higher and Ze a lot lower. The main earthing cable is unprotected coming straight from the floor. The system has a Residual Current Operated Circuit Breaker 100ma trip. Please advice on next course of action (in the process of rewiring the house) no experience on TT systems!

Edited: 18 December 2009 at 06:11 PM by nige296
 18 December 2009 06:18 PM
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mikejumper

Posts: 1701
Joined: 14 December 2006

You record the higher reading as the PFC, either P-N or P-E.

In TT systems the P-E loop impedance is likely to be always lower than the P-N loop impedance.

Your Ze is a tad high at 256 Ohms, the on site guide recommends not greater than 200 Ohms
 18 December 2009 08:04 PM
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luggsey

Posts: 261
Joined: 20 January 2008

50v touch voltage... 50v/0.1mA=500ohm
If the spike is 256 under worst conditions the RCD will be ok to protect for fault as far as the CU. But you should really fit a plastic enclosure 100mA(s) type or same spec as CU main switch with the rest of the installation split between two 30mA RCD and possibly smoke detectors on it's own RCBO.
That's what I would do!
 18 December 2009 08:30 PM
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mikejumper

Posts: 1701
Joined: 14 December 2006

Originally posted by: luggsey
But you should really fit a plastic enclosure 100mA(s) type or same spec as CU main switch with the rest of the installation split between two 30mA RCD and possibly smoke detectors on it's own RCBO.
)

If a dual 30mA RCD CU is fitted then why would you need a 100mA RCD upstream, all circuits are 30mA protected and overall isolation is via the DP switch in the CU.
 18 December 2009 10:29 PM
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luggsey

Posts: 261
Joined: 20 January 2008

A lot of older TT installs have a seperate RCD, so the tails are fault protected.
The mains board 'should' have reinforced insulation on the busbars if you fit a main switch only as there is no fault protection to them, you are also limited to a class2 plastic CU this way.
Easier to fit an RCD(s) main switch IMO.
 19 December 2009 09:21 PM
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StudyMate

Posts: 3
Joined: 29 November 2009

I'd be very tempted to check out the condition of the elcetrode outside. would have thought you should be lower than 256 ohms with a decent electrode. Theoretical max for a 100mA RCD is 500 ohms but regs suggests 200 max.

I'd have a look at whats on the other end of the earthing conductor!

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 20 December 2009 03:48 AM
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ebee

Posts: 5664
Joined: 02 December 2004

"In TT systems the P-E loop impedance is likely to be always lower than the P-N loop impedance"

???

Gritish Bas

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 20 December 2009 02:21 PM
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mikejumper

Posts: 1701
Joined: 14 December 2006

Originally posted by: ebee
"In TT systems the P-E loop impedance is likely to be always lower than the P-N loop impedance"
???
Gritish Bas


Oops, well spotted ebee, other way round of course.
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