Max Zs for TT.

dlt27 · Jan 31, 2015

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.

baldsparkies · Jan 31, 2015

dlt27 said:
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 · Feb 1, 2015

baldsparkies said:
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 · Feb 1, 2015

D Skelton said:
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 · Feb 1, 2015

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

Engineer54 · Feb 1, 2015

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....