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High Zs


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34 replies to this topic

#21 SPECIAL LOCATION

SPECIAL LOCATION

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Posted 01 March 2009 - 09:30 PM

Bought my Calcard on wednesday, probably arrive tomorrow so will get to check the tester.


Applaud SmileyApplaud Smiley
You will not be disappointed mcgaw!;)

very handy kept in you meter bag, to pull out while you are one site thinking somethings astray.....
verify meter & leads with known values...

leaves you searching for dodgy cable joints...
OR
confirms you meters gone AWOL so no need to prat about trying to get good R1+R2's

#22 batty

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Posted 01 March 2009 - 09:30 PM

HI guys,
Aside from the way out reading of r1 +r2 on 10mm,
can someone confirm that if you have correct mcb and also protection by an rcd the max zs allowed is 1667 taken from table 41.5 page 50 17th edition.
thanks


Fastman
If you have a high zs I would want to know why. If you have a loose connection it could get worse and cause problems. Would need sorting. Readings of 1667ohms are not acceptable as far as I am concearned.
Batty

#23 fastham

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Posted 01 March 2009 - 09:38 PM

Fastman
If you have a high zs I would want to know why. If you have a loose connection it could get worse and cause problems. Would need sorting. Readings of 1667ohms are not acceptable as far as I am concearned.
Batty


Sorry batty, you misunderstand me.
yes i agree a high zs needs to be clarified, but last year through annual visit of niceic we were told to be putting 1666 on forms as max permitted zs for rcd protected circuit.
I did 17th upgrade course and exam 2 weeks ago and noticed change to 1667, in table 41.5.
I asked tutor if that is the max zs to be recorded and his answer was yes.

Just wanted to know what you guys think.
( i am on this site to continue learning and am grateful for all answers)

#24 SPECIAL LOCATION

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Posted 01 March 2009 - 09:47 PM

HI guys,
Aside from the way out reading of r1 +r2 on 10mm,
can someone confirm that if you have correct mcb and also protection by an rcd the max zs allowed is 1667 taken from table 41.5 page 50 17th edition.
thanks


Don't forget that section 411.5 is referring to TT systems.

In essence what you are saying is correct in as much that a Zs not exceeding 1667ohms will ensure touch voltage cannot exceed 50v with a 30ma RCD / RCBO. BUT... as Batty points out, Applaud Smiley a correctly wired TN-S or TN-C-S would NEVER have a Zs reaching 1667ohm.


It would imply you have dropped a big smelly brown one somewhere on your installation methods & workmanship! Blushing:O

:)



you should be verifying your R1+R2'S match up with your expected cable types & lengths.. so the only high bit would be coming from the External earth loop path, Ze which on a TN-S or TN-C-S it just ain't going to get that high!


TT yes with poor ground conditions... but again Ra's over 200ohm are considered to potentially be unstable.
I don't think I would every be happy with a Zs of 1667ohm in a domestic situation even if TT.


Possibly on a large rambling agricultural installation may be more likely?
but you would need more input off those who do more Agricultural to verify my supposition..
or to see if I need to put the pants on me head again!
:o:^O:^O;)

#25 batty

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Posted 01 March 2009 - 09:50 PM

Sorry batty, you misunderstand me.
yes i agree a high zs needs to be clarified, but last year through annual visit of niceic we were told to be putting 1666 on forms as max permitted zs for rcd protected circuit.
I did 17th upgrade course and exam 2 weeks ago and noticed change to 1667, in table 41.5.
I asked tutor if that is the max zs to be recorded and his answer was yes.

Just wanted to know what you guys think.
( i am on this site to continue learning and am grateful for all answers)


The forms I use you do not have to put max zs so on your forms I would assume it is 1667 but depends on what Rcd is fitted ie if it where a submain to a consumer unit with say a 100mA td rcd it would be different.
Batty

#26 SPECIAL LOCATION

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Posted 01 March 2009 - 10:02 PM

Sorry batty, you misunderstand me.
yes i agree a high zs needs to be clarified, but last year through annual visit of niceic we were told to be putting 1666 on forms as max permitted zs for rcd protected circuit.I did 17th upgrade course and exam 2 weeks ago and noticed change to 1667, in table 41.5.
I asked tutor if that is the max zs to be recorded and his answer was yes.

Just wanted to know what you guys think.
( i am on this site to continue learning and am grateful for all answers)



1667 -vs- 1666 is actually 1666.66667 recurring. (50v / 30ma)

I have NEVER been told to do that??:|:| passed my 17th 11April 2008!

well had my NIC annual inspection 2nd week of Feb...
checked two jobs..
Both full rewires...
Both full RCD/RCBO 17th ed compliant CU's..

All my EIC certs had the max Z's values from table 41.3 written in them.....
no comments were made about either?:|


I still tend to er on the side of caution... what if your RCD don't work?
be nice to know the MCB has best possible chance of doing the business with a live-earth fault?

#27 fastham

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Posted 01 March 2009 - 10:06 PM

Thanks batty and special location.

I am well aware of checking and verifying figures on circuits, i am a test engineer for an niceic approved contractor.
It was more of a paperwork question rather than practice.
I asked the question of the tutor as i am aware 411.5 is for tt supply and this was the reason for my question. why were the nic quoting it on paperwork.
I dont like enetering figures just cause i am told to do so by the nic. i wanted to know where they got it from and why.

Thanks again

#28 fastham

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Posted 01 March 2009 - 10:13 PM

1667 -vs- 1666 is actually 1666.66667 recurring. (50v / 30ma)

I have NEVER been told to do that??:|:| passed my 17th 11April 2008!

well had my NIC annual inspection 2nd week of Feb...
checked two jobs..
Both full rewires...
Both full RCD/RCBO 17th ed compliant CU's..

All my EIC certs had the max Z's values from table 41.3 written in them.....
no comments were made about either?:|


I still tend to er on the side of caution... what if your RCD don't work?
be nice to know the MCB has best possible chance of doing the business with a live-earth fault?


Interesting,
to clarify it was not 17th course that pointed it out, i asked them the question.
It was in our last nic visit march 2008.
As we our due our next visit at the end of this month i will again be asking the question again.

Sorry if this seems pointless to you guys, but i always ask questions.
Once again thank you for your answers

#29 SPECIAL LOCATION

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Posted 01 March 2009 - 10:18 PM

Interesting,
to clarify it was not 17th course that pointed it out, i asked them the question.
It was in our last nic visit march 2008.
As we our due our next visit at the end of this month i will again be asking the question again.

Sorry if this seems pointless to you guys, but i always ask questions.
Once again thank you for your answers




erm???

NOPE!

most defiantly not pointless.....
may take a couple of posts to get to the bottom of what angle you are approaching from...

but all good banter & debate IMHO!
Guiness Drink:YGuiness Drink:Y


(children's warning.... DON'T mix your drinks!:D:^O:^O:^O)

#30 batty

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Posted 01 March 2009 - 10:22 PM

Interesting,
to clarify it was not 17th course that pointed it out, i asked them the question.
It was in our last nic visit march 2008.
As we our due our next visit at the end of this month i will again be asking the question again.

Sorry if this seems pointless to you guys, but i always ask questions.
Once again thank you for your answers


I would think 200 ohms would be max myself but I probably am wrong.

#31 fastham

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Posted 01 March 2009 - 10:38 PM

I would think 200 ohms would be max myself but I probably am wrong.


Hi batty
I understand the theory of what the nic are saying :- Ra x I∆n ≤50v
or 50v / .03 = 1666.6Ω
I just dont get why we were asked to put it on forms and obviously others (special location) have not.


Anyway enough questions time for a Guiness Drink

Thanks

#32 batty

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Posted 01 March 2009 - 10:50 PM

Hi batty
I understand the theory of what the nic are saying :- Ra x I∆n ≤50v
or 50v / .03 = 1666.6Ω
I just dont get why we were asked to put it on forms and obviously others (special location) have not.


Anyway enough questions time for a Guiness Drink

Thanks


If tt system on 30mA rcd then 1667 ohms is max but as an rcd could be unstable over 200ohms I would think that should be max value. For tns and pme you would use other tables in red book.
Batty

#33 NozSpark

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Posted 01 March 2009 - 10:57 PM

The thing is in this instance you cannot use the fact that the circuit is RCD protected to get away from the fact that the measured Zs is high.... for a 15m run you should know that it should be no where near the values that you have got and that there is something wrong.

#34 batty

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Posted 01 March 2009 - 10:59 PM

The thing is in this instance you cannot use the fact that the circuit is RCD protected to get away from the fact that the measured Zs is high.... for a 15m run you should know that it should be no where near the values that you have got and that there is something wrong.


Thats what I said Noz.
Batty

#35 NozSpark

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Posted 01 March 2009 - 11:02 PM

Even if the reading that you got was just under the max permissible, it should still be sounding alarm bells for a run that short




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