BSEN 7671 Table 41.5

[SDH]

[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]Getting a bit of a headache here, hoping someone can break this down for me please.

Are there any instances where table 41.3 of BS7671 do not apply, and table 41.5 will instead apply? [/FONT]
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[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]Example[/FONT]
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[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]32A type C RCBO having a max Zs of 0.58 (after RoT applied).[/FONT]
[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]Measures value of Zs coming in at 1.14.[/FONT]
[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]Ring circuit. [/FONT]
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[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]To me: Doesn't comply and the RCBO should be changed to a type B, having a max Zs after RoT of 1.16.[/FONT]
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[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]What I am told:[/FONT]
[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]A mixture of something about the max Zs being different with an RCBO as oppose a CB (MCB). [/FONT]
[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]Me: Well that doesn't make sense as table 41.3 references RCBO's as well as CB's. [/FONT]
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[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]Because the circuit is protected by an RCD, as long as the Measures Zs is under the value stated in table 41.5 which holds a value of 1667 ohms for a 30mA RCD then it's all fine. [/FONT]
[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]Me: Well that just seems daft. [/FONT]
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[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]As I understand it, if there is an over current on the circuit, the CB will not trip in under 0.4 seconds as the Zs is too high, and the RCD referred to in table 41.5 which is protecting the circuit from an earth fault doesn't react to an over current. [/FONT]
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[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]Or am I missing something obvious here?


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[Simonslimline]

What's the earthing arrangement ?

 

[SDH]

Tn-c-s

 

[Simonslimline]

Ok,

The RCD part of the RCBO is providing additional protection, Although in reality the RCD part will trip under earth fault conditions before the MCB; but as it's a TN system then the Max Zs is 0.58 ohms.

If the earthing arrangement was TT then this would be different. You could then use the 1667 ohms because of "Ra x I delta n <= 50 V.