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Topic Title: Quick T-T question
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Created On: 28 June 2012 07:54 PM
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 28 June 2012 07:54 PM
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mitten

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what do i put in the box when it says maximum zs on a T-T ssyem when the Ze is say 40 ohms.
cheers in advance

-------------------------
Jason
 28 June 2012 09:18 PM
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primo

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Assuming that the circuit in question is protected by a 30mA RCD then I believe 1667 ohms is the max Zs.
 28 June 2012 10:26 PM
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Martynduerden

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You put Max Zs for the principal cpd.

Ze whilst not totally irreverent does not influence the max or 80% values for Zs.

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Martyn.

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 28 June 2012 10:34 PM
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daveparry1

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As Primo say's, 1667 ohms if there's a 30m/a rcd present,

Dave.
 28 June 2012 10:46 PM
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Martynduerden

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Originally posted by: daveparry1

As Primo say's, 1667 ohms if there's a 30m/a rcd present,

Dave.


Are you sure?

Situation

TT Older 16th board

Lighting Cct on RCD side 6A BSEN60898 Max Zs 8/6.4

I would suggest that the Max Zs should be 8/6.4 not 1667 as the RCD

Just because the MCB is not going to work for Sc protection does not mean we should ignore it IMO.

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Martyn.

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 28 June 2012 11:06 PM
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primo

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The way I see it, we are looking for a value for earth fault protection for Zs which is 1667 if your primary means of earth fault protection is a 30mA RCD looking to limit touch voltage to 50V. As we are not relying on the MCB to disconnect in the event of an earth fault then surely we DO ignore the max Zs for the MCB as it is unlikely to be achieved in a TT system?
 28 June 2012 11:16 PM
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daveparry1

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Exactly Primo, Zs is to do with earth fault disconnection, nothing to do with s/circuits. Not sure what Martyn is getting at here?

Dave.
 29 June 2012 12:06 AM
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Martynduerden

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What value would you choose in a TN system, The logic here suggests you could do just that...

It complies ...lets use the lower value Vs Oh sh*t better put a higher number in from an upstream device - a bit like relying on a 33kA service fuse for PFC where it exceeds 16kA the 16kA value does not change.

Happy to disagree as no one is ever likely to read the darn thing!

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Martyn.

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 29 June 2012 05:12 AM
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ebee

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I look at it this way.

In a TN system it is the fuse/breaker that primarily gives us the Zs required in order to clear an earth fault.
Any RCD is just giving a secondary (back up) protection and therefore not being relied upon.

In a TT system however we do not have that luxury and it is (almost always) the RCD/Zs combination that is being relied upon.

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 29 June 2012 06:55 AM
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primo

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Originally posted by: Martynduerden


Happy to disagree as no one is ever likely to read the darn thing!


Good point!
 29 June 2012 07:33 AM
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Boyobach

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The value should be 50 volts/Rcd Sensitivity i.e

30 mA = 1666.7ohm
100 mA = 500 ohm
300 mA = 166 ohm
500 mA = 100 ohm

However if it is a special location then the touch voltage may be 25 volts. I'm sure you can do the maths.

Keith
 29 June 2012 07:50 AM
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Legh

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It seems pretty obvious to me that the circuit protection must offer protection, as a design constraint, that complies with table 41.1
For earth faults supplied via a TT system this is not always the case so a RCD is employed. This is not the case where L-N faults are present.

Legh

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 29 June 2012 08:03 AM
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davezawadi

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This box is for the regulations stated maximum figure and is therefore 1667 Ohms for a 30mA RCD. This is exactly the same as other circuits, although you may apply the 80% rule, again to see that you have checked properly! The touch voltage is being misapplied above (below if signed in), the actual touch voltage with an RCD can (is) always be the full supply voltage, the 50V figure is simply used to define the RCD protected maximum EFLI, Sensible electrode resistances or Ze will always be much less than this, but some faults may still give full supply touch voltage, although only for a short period <40ms.

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David
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 29 June 2012 08:21 AM
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Boyobach

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Dave

I merely stated the touch voltage values in order to explain the origin of the TT system Zs values which were previously been stated.

Keith
 29 June 2012 08:35 AM
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OMS

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The touch voltage is being misapplied above (below if signed in), the actual touch voltage with an RCD can (is) always be the full supply voltage, the 50V figure is simply used to define the RCD protected maximum EFLI,


I disagree Dave - the touch voltage will never rise to supply voltage unless we can get rid of all cable impedance.

If you put "Ze" approaching zero the touch voltage will be (asymptotic value) :

Vtouch = Vsupply*(x/x+1)

Where x/x+1 = the ratio between CSA of the phase conductor and teh CPC

So for a 1.0mm2 T&E with a 1.0mm2 CPC , x = 1 and the touch voltage = 115V ie 230 x 0.5

Once you add a value of Ze then the equation expands to

Vtouch = Vsupply * (x/(Ze/R1)+x+1)

ie the denominator increases and thus V touch decreases

The 50V figure essentially ensures enough current is flowing to achieve disconnection in the time required from the touch voltage curve for the relevant actual touch voltage.

regards

OMS

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 29 June 2012 01:47 PM
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Legh

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Surely, the design of an installation should follow the maximum Zs for TN systems and then apply additional protection by the use of RCDs?

Even TT systems offer the choice of either Ia or IAN where ground conditions are unsuitable for a low impedance return path. This, to me, appears as additional protection via the use of electronic monitoring/switching devices...

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 29 June 2012 05:24 PM
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RB1981

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Originally posted by: Legh

Surely, the design of an installation should follow the maximum Zs for TN systems and then apply additional protection by the use of RCDs?


Surely they're being used to provide fault protection rather than additional protection (although they may also be providing this).
 29 June 2012 05:30 PM
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Legh

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Originally posted by: RB1981

Originally posted by: Legh

Surely, the design of an installation should follow the maximum Zs for TN systems and then apply additional protection by the use of RCDs?


Surely they're being used to provide fault protection rather than additional protection (although they may also be providing this).


Surely, you're right !

I have got used to measuring circuit design against 41.1 for TT systems and then apply 41.5.
It makes sense in the long term when there is the likelihood of supplies are being upgraded to pme.

:Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 29 June 2012 06:10 PM
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mitten

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So does everyone agree that the max zsfor each circuit whatever size mcb on a domestic circuit with a 30 ma rcd is 1667?

-------------------------
Jason
 29 June 2012 06:25 PM
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Legh

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Originally posted by: mitten

So does everyone agree that the max zsfor each circuit whatever size mcb on a domestic circuit with a 30 ma rcd is 1667?


Not sure that I do, since 41.5 note 2 gives more guidance on the matter.

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
IET » Wiring and the regulations » Quick T-T question

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