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Topic Title: Max Zs & TT Installation
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Created On: 27 January 2009 10:16 PM
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 02 October 2009 07:12 PM
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Avatar for Testit.
Testit

Posts: 2962
Joined: 06 August 2007

Splitting hairs?...

1667 relates to the 50V touch voltage when applied to 30ma RCDs.

The idea of finding 1667ohms acceptable is absurd... IMO

Ra should be the rod resistance or stipulate which method you are using.. but I wont carry on and just agree to disagree as I'm signing off for a cuppa......

-------------------------
Online Services - http://propertydevelopment.org.uk

Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.
 02 October 2009 07:21 PM
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OMS

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1667 relates to the 50V touch voltage when applied to 30ma RCDs.


Indirectly yes - the equation does not show that touch voltage is limited to 50V - it simply ensures sufficient current is flowing to trip the device in 40ms, generally a figure of 2 x 1 delta N.

Ra should be the rod resistance or stipulate which method you are using


Read the definition of Ra again (411.5.3) - it is the combined resistance of the electrode and the erthing condutor to the point where it connects to the exposed conductive parts

Regards

OMS

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Failure is always an option
 02 October 2009 07:51 PM
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John Peckham

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At the top of the maximum Zs box on my forms it says, " Maximum Zs permitted by BS761".

My stock saying on these occasions is, "if all else fails follow the instructions".

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John Peckham

http://www.astutetechnicalservices.co.uk/
 02 October 2009 09:37 PM
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Jaymack

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Originally posted by: daveparry1

As i'm niceic and use their certs i'm thinking maybe put down max Zs allowed as 100 ohms, although I know this is an nic recommendation and not BS7671


It comes from BS 7430 which suggests that a reading exceeding 100ohms may be unstable.

Regards
 04 October 2009 12:34 AM
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Avatar for Testit.
Testit

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That only took ten attempts to log in

Read the definition of Ra again (411.5.3) - it is the combined resistance of the electrode and the erthing condutor to the point where it connects to the exposed conductive parts


Indeed who could disagree?

Such exactness.... Bravo and all that.... I usually write as I'm thinking and dont check for correctness always, who'd have guessed?.... But specificities aside... the overall point remains.. namely the reading of 1667ohms being an acceptable max permissible Zs as referred to on schedules of certs is maybe misleading. Just as an observation table 41.5 is clearly related to Zs to ensure operation of RCD in accordance with 411.5.3 which relates predominantly to ADS and 50V touch voltage..

I would say its clear enough in relation to the regulations about the use of 1667ohms in conjunction with ADS, volt drop, PSCC, 200ohms guidance and other regulation notes, and indeed high values of earth resistance may be utilised in specially designed situations, but generally there appears to be a lot of confusion from people practicing about what values in practice are acceptable...

Personally I would amend the regulation to incorporate the 200ohms figure to account for practicalities of earthing characteristics, and make a note relating to higher earth resistances being a judgement call by the designer not to exceed 1667ohms to comply with 411.5.3.. as opposed to the note referring to the 200ohms figure.... but that's just me of course..

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Online Services - http://propertydevelopment.org.uk

Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.
 05 October 2009 10:01 AM
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Avatar for OMS.
OMS

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But specificities aside... the overall point remains.. namely the reading of 1667ohms being an acceptable max permissible Zs as referred to on schedules of certs is maybe misleading


Again, ther max permissible Zs value is that used by Table 41.5 (It is used in the absence of Ra). For a 30mA RCD, the value of 1667 ohms ensures adequate residual current flows to operate the device in 40ms - essentially the time is derived from the IEC touch voltage curve for an assumed asymptotic touch voltage. If you assume the human body is about 1000 ohms and add it to 1667 you should then see a fault current of about 90mA - more than enough to fire the RCD.

The electrode resistance will be recorded seperately on the EIC/PIR for comparison and a check on "stability". (542.2.2)

I quite agree that any circuit presenting values approaching 1667 should be checked to determine the electrode resistance initially and if that is acceptable the circuit length will need to be questioned regarding short circuit protection and volt drop.

Regards

OMS

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Failure is always an option
 06 October 2009 09:42 PM
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CarlCosby

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Originally posted by: Alexg

While your here, and while I'm feeling stupid.



whats the SCC of a standard BS 1361 fuse?


Depends on type of fuse, 1361 type 1 are 16.5 kA and type 2a / 2b are 33 kA. I could be wrong as I'm going on memory, I'm sure I will be stood corrected if I am wrong though.

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Carl
 06 October 2009 09:46 PM
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CarlCosby

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Originally posted by: OMS

1667 relates to the 50V touch voltage when applied to 30ma RCDs.




Indirectly yes - the equation does not show that touch voltage is limited to 50V - it simply ensures sufficient current is flowing to trip the device in 40ms, generally a figure of 2 x 1 delta N.



Ra should be the rod resistance or stipulate which method you are using




Read the definition of Ra again (411.5.3) - it is the combined resistance of the electrode and the erthing condutor to the point where it connects to the exposed conductive parts



Regards



OMS


Hi OMS, where is this 2 x I delta n coming from?

I'm lost in all this but learning

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Regards
Carl
 07 October 2009 09:26 AM
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AJJewsbury

Posts: 11462
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Hi OMS, where is this 2 x I delta n coming from?

Ia for an RCD to disconnect within 0.2s (TT disconnection time at 230V) - table 3A of appendix 3.

- Andy.
IET » Wiring and the regulations » Max Zs & TT Installation

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