single pole rcbo's in a tt system

[unphased]

That is an interesting point about achieving a really low Ra, say 0.8ohms, on a TT. By definition a TT, but where EFLI is concerend as good as a TN.

 

[Sidewinder]

If you check 7671 it says that you only need an rcd on tt if you can't guarantee the efli being low enough.

Think about tn this is still rods etc, just a huge network of them and associated earthed items all in parallel dropping the Ze

 

[park1]

I knew what the RCD is required for, but yes, now IMHO your point is clearer.Yes the RCD is required due to the low current due to the high EFLI in a TT system.

Though, if you stuffed say a dozen rods or a mat in on a steel building you could easily et well below 1 ohm.

And how do we control the value of Rb?

 

[apprentice87]

And...... Interestingly enough..... if, [as Paul says], you:

"stuffed say a dozen rods or a mat in on a steel building you could easily get well below 1 ohm"

That is, got an Ra low enough to permit the use of an OCPD as the means of achieving ADS, then, interestingly enough, you WOULD be able to use the disconnection times normally reserved for use in a TN system.

[see footnote to table 411.1]

john.....

 

[eamonn1983]

All good on the rock apart from the weather step toe. New on here so good to meet you all .

 

[Sidewinder]

And how do we control the value of Rb?

It's late & I'm tired, you'll have to remind me what Rb is?

 

[steptoe]

All good on the rock apart from the weather step toe. New on here so good to meet you all .

good man , do you work for yourself, or a company, Eamonn ?

 

[Steve3948]

It's late & I'm tired, you'll have to remind me what Rb is?

Robbing B*****d

 

[binky]

think he means Ra as in rod resistance, not the Eygptian god

 

[park1]

think he means Ra as in rod resistance, not the Eygptian god

No, I mean Rb.

 

[Evans Electric]

think he means Ra as in rod resistance, not the Eygptian god

I think that every time I see Ra TBH

 

[steptoe]

wasnt Ra somebodies god too though? :|

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personally I think $park is trying to be a bit clever and has gotten Rd a little bit wrong,

 

[park1]

wasnt Ra somebodies god too though? :| Yes yours

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personally I think $park is trying to be a bit clever and has gotten Rd a little bit wrong,

Don't be silly

 

[steptoe]

well, I for one would like Rb explained to me,

I'll admit, Im stumped,

Ive never heard of it, or do I even know how to measure it,

:C

 

[Jono Pashley]

Rb = Remaining beer, measurement usually taken at the fridge. Guinness

 

[park1]

well, I for one would like Rb explained to me,I'll admit, Im stumped,

Ive never heard of it, or do I even know how to measure it,

:C

Subtract Ra

 

[steptoe]

from what?

Ra is a definitive measurement, there is nothing to subtract it from,

I think someone has been yanking your chain and making stuff up $park. seriously, youve been had.!

 

[park1]

from what?Ra is a definitive measurement, there is nothing to subtract it from,

I think someone has been yanking your chain and making stuff up $park. seriously, youve been had.!

lmfao, that made me chuckle.

Subtract Ra from Zs to approximate Rb

 

[Jono Pashley]

Hmmm....either ive had one too many or something seems a might fishy

 

[steptoe]

lmfao, that made me chuckle.Subtract Ra from Zs to approximate Rb

why cant you just explain to me what Rb is?

Ive never heard of it,

and you cannot, CANNOT, subtract a resistance from an impedance, they are 2 different things, :shakehead