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mcgaw81

High Zs

Question

I wired a shower last week and went back to connect it up and test today, however on testing the Zs of the circuit i get a reading of 0.93ohms, and the MCB maximum is 0.92ohms (50amp Type B) in regs, on the back of my NICEIC test books its 80% which allows 0.73ohms.

More detail, what I have is a Sub Main supplied in 16mm Twin and Earth from a 60amp switch fuse. I have install new 4 way enclosure with 63amp RCD 50amp MCB (shower) 16amp (1x Double skt).

At the top of the RCD i get 0.22ohms, socket is 0.44ohms and the high reading for the shower. Everything else tests ok, my R1+R2 was 0.46ohms. Now i know i could have done my Zs by calculation and would have had a satisfactory result but why is my R1+R2 ok but high Zs.

What are my options of reducing this, different Type MCB, maybe type 1? What other options do I have?

Thanks in advance.

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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

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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

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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)

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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.

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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

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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

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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.

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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

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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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