Max Zs for TT.

[dlt27]

Don't know if I'm being a proverbial div, but could someone tell me the max Zs for 80A 1361( distribution cct )on a TT system with disconnection time of 1.0 sec. Thanks.

 

[telectrix]

0.354 by my calc. where's your BS7671>?

 

[telectrix]

and why 1.0sec.?

 

[D Skelton]

and why 1.0sec.?

Oops. Having a blonde moment are we?

 

[telectrix]

oops. just reread it's TT.

 

[D Skelton]

oops. just reread it's TT.

Someone once told me to RTFQ. It's stood me in good stead ever since

 

[telectrix]

Oops. Having a blonde moment are we?

is my post otherwise correct? 650A being the required A?

 

[telectrix]

Someone once told me to RTFQ. It's stood me in good stead ever since

haha. just becouse you've just composed a nice essay in that other thread, don't let it go to your head. :49:

 

[D Skelton]

is my post otherwise correct? 650A being the required A?

Couldn't tell you, I'm sat in a pub without my BGB.

 

[telectrix]

or i'll set monty on yer. :smilielol5::smilielol5:

 

[D Skelton]

haha. just becouse you've just composed a nice essay in that other thread, don't let it go to your head. :49:

Naa, I don't let essays go to my head, only my perfectly formed buttocks

 

[telectrix]

Couldn't tell you, I'm sat in a pub without my BGB.

lucky you. i'm stuck at home. going to a do tonight though.

 

[Deleted member 9648]

Don't know if I'm being a proverbial div, but could someone tell me the max Zs for 80A 1361( distribution cct )on a TT system with disconnection time of 1.0 sec. Thanks.

scrub that....blonde moment!

 

[GMES]

Couldn't tell you, I'm sat in a pub without my BYB.

Changed that for you mate, I thought better of you Damian!!

 

[D Skelton]

Changed that for you mate, I thought better of you Damian!!

I ain't buying it until I need to mate. The current one is chock a bloc full of errors. No thanks IET, I'll wait until they're corrected and then I'll invest.

 

[baldsparkies]

Don't know if I'm being a proverbial div, but could someone tell me the max Zs for 80A 1361( distribution cct )on a TT system with disconnection time of 1.0 sec. Thanks.

0.40 Ohms
So wouldn't be achievable on a TT system, meaning disconnection times would need the use of an RCD so!!
Looking at 1 second disconnection times for distribution circuits, and 0.2 seconds for all others.
With a 100ma Rcd employed a max value of 500 ohms.
All that said, stability issues are considered relevant with anything over 200 ohms by BS 7671
Or as the NICEIC would advise (They make there own rules up) 100 ohms.
Final circuits are often quoted 1667 ohms when disconnection times are afforded by a 30ma rcd but thats generic and I personally prefer the tabulated values.
That's my take, hope it helps. :icon12:

 

[HappyHippyDad]

0.40 Ohms
So wouldn't be achievable on a TT system, meaning disconnection times would need the use of an RCD so!!
Looking at 1 second disconnection times for distribution circuits, and 0.2 seconds for all others.
With a 100ma Rcd employed a max value of 500 ohms.
All that said, stability issues are considered relevant with anything over 200 ohms by BS 7671
Or as the NICEIC would advise (They make there own rules up) 100 ohms.
Final circuits are often quoted 1667 ohms when disconnection times are afforded by a 30ma rcd but thats generic and I personally prefer the tabulated values.
That's my take, hope it helps. :icon12:

It would at Trev's home!

 

[Knobhead]

I ain't buying it until I need to mate. The current one is chock a bloc full of errors. No thanks IET, I'll wait until they're corrected and then I'll invest.

So you’re waiting for the 18[SUP]th[/SUP] which will be full of mistakes.

What you don’t seem to realise they have a professional reputation to uphold.

 

[telectrix]

buy a fake copy from amazon, errors may have been corrected.

 

[Engineer54]

If you have a stable sub 1 ohm TT system, i wouldn't be still using the 1 sec and 0.2 sec values, i'd be quite happy to use TN disconnecting times.... And getting a TT system below 1 ohm is achievable if you go about things in the right way. The last time i checked my own houses TT system in Cyprus last year, was 0.88 and has been consistently dropping for the last few years. I wouldn't call the ground type particularly good either....