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Topic Title: RCBO Zs
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Created On: 04 June 2008 04:33 PM
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 04 June 2008 04:33 PM
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Avatar for EddyCurrents.
EddyCurrents

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iv been doing some passed papers for my 17th today and one of questions was this;

A 6A BS EN 61009 RCBO with a maximum value of earth fault loop impedance of 1.92ohms is type?
a. A
b. B
c. C
d. D

But then it got me thinking that a 61009 is a type of RCD which would likely be rated at 30mA, which would then give a max Zs of 1667, which is what I would put down on my PIR test sheet.
But I was then redirected to page 49 in the new regs showing a table of max Zs for 60898's and 61009's
But how can these be the same given the formula on the previous page
Zs x In = Uo and In when considering an RCD is the rated residual operating current.

Any opinions? I seem to be finding loads of odd questions like this
 04 June 2008 04:49 PM
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rocknroll

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You need to work it out from your OSG page 51, Table 7.2B

Type B is 5In
Type C is 10In
Type D is 20In

So
Uo / 6 x In
Type B
230 / 6 x 5 = 7.6 ohms

oops it not that one lets try Type D

Uo / 6 x In
Type D
230 / 6 x 20 = 1.916666667 ohms

yippee got it Have a smoke bro

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 04 June 2008 at 04:57 PM by rocknroll
 04 June 2008 04:56 PM
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EddyCurrents

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yes that is perfectly fine for an mcb 60898, but not an rcbo surely? as it is a type of rcd
I think it is rather strange having both on the same table.
 04 June 2008 05:04 PM
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rocknroll

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The primary circuit protection device is the MCB, the RCD portion is supplemental to this;

On your PIR you should be entering the Zs value, if you had software this will do this automatically, you design your circuits to the Zs value to ensure that your touch voltage in the event of fault is less than 50V , Yes OMS I havent finished with that other thread yet and will post some more thoughts for you all.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 04 June 2008 05:08 PM
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ebee

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Yep I see what you mean.
A type D Breaker would need 1.92 ohms max to disconnect.
A 30mA RCD would need 1667 ohms to disconnect.
So if you think as a Type D RCBO as both units cascaded then why does 1.92 apply and not 1667?

This question has been asked a few times but I can`t recall the best answer.

Personally I`d prefer to rely on an MCB but not an RCD and just consider an RCD as a back up that might work.
But in a TT system then you`ve not much choice because a breaker will almost always not disconnect

-------------------------
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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 04 June 2008 05:11 PM
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OMS

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Yes OMS I havent finished with that other thread yet and will post some more thoughts for you all.



Ahhh - pearls before swine RnR

You need to live fast and die young on this forum

OMS

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Failure is always an option
 04 June 2008 05:13 PM
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EddyCurrents

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0.03 x 1667 = 50v Done
The fault current causing automatic disconnection of supply is 30ma not 20x6A

The type D current characteristics come into effect for overcurrent, not, fault current, which is the purpose of taking a Zs reading
 04 June 2008 05:19 PM
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rocknroll

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Lets put it this way, say you were an unexperienced electrician just got their 2391 went to a PIR and found a ring main wired in 4 core bell-wire , with a 32A RCBO, you did a loop impedance test and it was 1660 ohms would you pass it as ok

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 04 June 2008 05:50 PM
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ebee

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4 core bell wire.
Now there`s a luxury it`s usually 2 cores.

Three distinct types of protection.
1/ Earth fault.
2/ Overload
3/ short circuit.

Maybe different devices or the same device.

If an MCB is protecting against 2/ & 3/ then may an RCD protect against 1/ in a TN system?

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 04 June 2008 05:51 PM
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dg66

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How about table 41.3 no need to calculate.Also you cant take the OSG into an exam(unless things have changed)

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Dave(not Cockburn)
 04 June 2008 06:10 PM
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EddyCurrents

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How about table 41.5 which is Max Zs to ensure RCD operation for non-delayed RCDs to 61008 & 61009 Yes thats 61009 again how can it have 2 tables with different values?

The bell wire......Now strange as it may seem, the bell wire under an earth fault would be OK because it would be capable of takin more than 5 times the RCD In being 150mA. But it would obviously fail on other matters such as current carrying capacity volt drop and so on, but Zs NO!
 04 June 2008 06:28 PM
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OMS

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Is this an appropriate time to mention short circuit currents and the inability of an RCD to detect a P_N fault ?

OMS

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Failure is always an option
 04 June 2008 06:56 PM
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bobdenton

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Originally posted by: EddyCurrents

iv been doing some passed papers for my 17th today and one of questions was this;



A 6A BS EN 61009 RCBO with a maximum value of earth fault loop impedance of 1.92ohms is type?

a. A

b. B

c. C

d. D


Its an open book exam. You're being asked if you know how to use the look up tables in the regs. You should know there's a table of values of Zs for RCBOs. You're expected to be able to find it and look at the values for 6A RCBOs, find the value 1.92 Ohms and look at the rubric to see the type of RCBO it refers to.

You're reading more into the question than is actually there, that's why it seems odd.
 04 June 2008 07:00 PM
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ebee

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Originally posted by: OMS

Is this an appropriate time to mention short circuit currents and the inability of an RCD to detect a P_N fault ?



OMS


No

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 04 June 2008 07:18 PM
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OMS

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Ok then - I won't

Off to get my coat

OMS

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Failure is always an option
 04 June 2008 07:19 PM
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EddyCurrents

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A P-N fault will be how the D type part of the RCBO deals with the fault according to the relevant graph. But this has nothing to do with Zs at all. Only a fault current to earth relates to Zs and this will be taken care of by the RCD side of the RCBO.

EG if an RCBO was to be used on a TT system with an electrode resistance of 50 ohms for example, the RCBO would have no chance of complying with the max Zs if you where to use the MCB rating, but if you where to use the correct rating of 1667 ohms it would. Now im NOT reading too much into it,as someone suggested , and making things more complicated than they are , this is the way it is in reality, and not on some exam sheet.
 04 June 2008 07:27 PM
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OMS

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OK - so back to your question why does the limiting values of Zs on your RCBO relate to the "MCB" bit and not to the "RCD" bit then - cos 1666 certainly aint one of the possible answers is it.

I'll give you a clue - indirect shock protection, reliable, votage independant as opposed to supplementary protection, unreliable, voltage dependant

Regards

OMS

-------------------------
Failure is always an option

Edited: 04 June 2008 at 07:30 PM by OMS
 04 June 2008 07:44 PM
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bobdenton

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Now im NOT reading too much into it,as someone suggested , and making things more complicated than they are , this is the way it is in reality, and not on some exam sheet.


Sorry, just giving you the benefit of some hard learned examination technique. If you're preparing for an exam you're more likely to do well in the exam if you prepare for the exam instead of preparing for reality.
 04 June 2008 08:15 PM
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normcall

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Isn't that cheating?

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Norman
 04 June 2008 09:12 PM
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EddyCurrents

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agreed bobdenton, just give me a list of the questions i should answer incorrectly to get it right.

Anyway i have just delved a bit further and found something at the beginning on page 5, it says "IT has been clarified that where an RCBO is referred to in theses Tables, only the overcurrent characteristic of the device is being considered." and a new table 41.5 gives max values of Zs for RCDs to 61008 and 61009.
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