TT system - maximum earth fault loop impedence?

[ainsdale_gas]

I've recently completed a part p defined scope course but all of our testing was on TNS or TNCS systems, in our house we have a TT system and in the OSG I can't find much information on the maximum earth fault loop impedence, just says that it is likely to be higher than their TN tables? I might ring the IEE helpline tomorrow.
What do you use for your maximum figures on TT systems?

 

[Octopus]

The maximum "allowed" is 200 ohms, however you should endeavour to get below 100 ohms!

Fingers crossed you've got RCD's fitted.

 

[BOROROBBO]

as above 200 ohms is maximum

 

[telectrix]

with TT, as said prevoiusly, max. is 200ohms, but the lower the better. however, with TT, you need 30mA RCD /s or RCBOs on all ccts. max. Zs is entered at 1667ohms. as you will be relying on the rcd for fault protection.

 

[ainsdale_gas]

yes all the circuits are protected by a 30mA RCD, it is an RCCB? We had a full re-wire by a NICEIC registered installer 10 years ago.
Can you clarify on the max zs being entered as 1667 ohms? I'm not sure what you mean here, if the max is still 200? Thanks for the quick response guys, made up.
What about rcd trip out times? are these different with a TT system? I read something in the on site guide about this but got a bit lost. Don't worry I done these tests tens of times in college and it is sinking in, I look forward to getting more familiar with the tests when I get a part p mft in a few weeks.

 

[Deleted member 9648]

The 200 ohms figure is actually Ra not Zs.....in effect your Ze should be max 200 ohms and Zs on final circuits may be higher...ie Ra+ R1+R2.
On a TT ignore the max Zs figures in 7671 for overcurrent protective devices and use the max Zs for RCD's,(as it is the RCd which provides earth fault protection on a TT).....given in table 41.5.
The 200 ohms figure is to allow for soil conditions deteriorating and Ra increasing.....in practice a much higher Ra will still meet the requirements.....the 200 ohms is a rule of thumb recommendation,not a regulation.The 1667 figure is the maximum Zs where earth fault protection is by a 30ma rcd (table 41.5....and note2)

The disconnection time on a TT is 0.2s for final circuits not exceeding 32a,and 5s for distribution circuits and finals exceeding 32a .(table 41.1.....411.3.2.4)

 

[telectrix]

sorry, wasn't quite clear in my last post. max. Ze should be under 200. it's the max. Zs ( for each circuit ) that is 1667.

 

[ezzzekiel]

confused? you will be, welcome to our world lol.....

resistance between rod and main earth (sometimes called Ze or Ra) should be less than 200 ohms whereas all final circuits should have a max Zs figure (loop) of less than 1667 ohms.

pretty much what they said grrrrrrrrrrrr

 

[telectrix]

as i don't understand binary , am i a 1-5 person or a 6-10?

 

[Deleted member 26818]

Yes or no, depends.

 

[telectrix]

on whether i'm having an OFF day?

 

[mattmoo1974]

You managed to confuse me and i know all this stuff

 

[ainsdale_gas]

That is great thanks, I'm clear now that the max Ra for a TT system is 200ohms and for Zs 1667 ohms. Are they both likely to be much higher or just the Ra?
Which page is the table on for RCD disconnection times for TT systems? I'm struggling to find it sorry.

 

[topquark]

Yes or no, depends.

Quantum computing strikes again

 

[Deleted member 26818]

That is great thanks, I'm clear now that the max Ra for a TT system is 200ohms and for Zs 1667 ohms. Are they both likely to be much higher or just the Ra?
Which page is the table on for RCD disconnection times for TT systems? I'm struggling to find it sorry.

Disconnection time for TT installations are page 46 Table 41.1 in BS7671.

 

[muzza1]

Great lads, very informative. Thanks.