Find Zs in the new OSG

[Richsand]

Hi all

I understand that With a 30mA RCD the max Zs permissable becomes 1667ohms but where is this stated in the new (green) onsite guilde or has this been amended?

Thanks

 

[Nicky Tesla]

its a calculation to do with the 50v max touch voltage ie: 50v/0.03=1666.6666

 

[SPECIAL LOCATION]

Hi allI understand that With a 30mA RCD the max Zs permissible becomes 1667ohms but where is this stated in the new (green) onsite guide or has this been amended?

Thanks

Hello Richsand.....

welcome to the forumbulator...

I know what you are trying to say... But your terminology is actually wrong....

1/

Max Zs does NOT become 1667ohms with a 30ma RCD.

The max Zs for circuits protected by a 30ma RCD on a TN system must still comply with the values given in Appendix B ..

page 113 onwards...

2/

The OSG only considers TT systems with 30ma RCD protection due to high Ra's unable to meet Zs values....

See 3.5.3 page 25.

With 30ma RCD protection

BS7671 quotes the 1667ohms figure in table 41.5 page 57 of the regs... {& what Nicky T refers to.}

But the On Site Guide recommends "the must not exceed 200ohms" figure as per the guidance note 3.

as Ra's above 200 ohms can be unstable..

See 7.2.6. page 70..

3/

this info is pretty much the same as it was in the red book so nothing has been amended or changed AFAIK.

hope that helps.. Guinness

P.S..

You need to be looking in the index for stuff about 'disconnection times' NOT 'max Zs'

 

[apprentice87]

Hi Special Location!!

Hmm, not sure i understand...I have not got the book in front of me so i am just going by memory here mind!!!!!!

You say that;

"Max Zs does NOT become 1667 with a 30ma RCD.

The max Zs for circuits protected by a 30ma RCD on a TN system must still comply with the values given in Appendix B .."

Without looking at appendix B, i have no idea what it says, but having a guess here, I presume it gives the values of Zs to achieve the appropriate disconnection times in the normal way, I.E. A TN system with ADS catered for by means of an OCPD.

But how does this square with the reg that states that you can use an RCD instead of an OCPD to achieve disconnection times in a TN system [obviously you still have to make provision for overcurrent protection too] which would otherwise be impossible, presumably because of high Zs??

john..

 

[SPECIAL LOCATION]

Hi Special Location!!Hmm, not sure i understand...I have not got the book in front of me so i am just going by memory here mind!!!!!!

You say that;

"Max Zs does NOT become 1667 with a 30ma RCD.

The max Zs for circuits protected by a 30ma RCD on a TN system must still comply with the values given in Appendix B .."

Without looking at appendix B, i have no idea what it says, but having a guess here, I presume it gives the values of Zs to achieve the appropriate disconnection times in the normal way, I.E. A TN system with ADS catered for by means of an OCPD.

But how does this square with the reg that states that you can use an RCD instead of an OCPD to achieve disconnection times in a TN system [obviously you still have to make provision for overcurrent protection too] which would otherwise be impossible, presumably because of high Zs??

john..

Re read the OP's posts...

To the layman / student it reads that every circuit with a 30ma RCD has max Zs = 1667ohms (irrespective of earthing types)

Which is WRONG!!!!

I have attempted to clarify as best as I can...

and TBH I cannot be arsed to type out any quotes form the OSG..

pages & paragraphs & appendix's are all listed for others to read If They Want To...

GuinnessGuinness

 

[apprentice87]

Hi SL,

Yes... as you quite rightly say......

P.S..

You need to be looking in the index for stuff about 'disconnection times' NOT 'max Zs'

john...

 

[Nicky Tesla]

so what do you put in the max Zs column? 200ohm ?