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Topic Title: Maximum ZS Permitted By BS7671
Topic Summary: What is the correct value
Created On: 07 February 2015 09:00 AM
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 07 February 2015 09:00 AM
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keithredpath

Posts: 543
Joined: 30 March 2002

Is the correct value to be entered for a RCBO Type B 0.57 Ohms or 1333 Ohms (80%) or 1667 Ohms.

What value is entered on the Amtech software?

What value when Amendant 3 kicks in.

Should we be entering 2 values e.g. 0.57/1333 Ohms.

What is the definitive answer?

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keithredpath
 07 February 2015 09:39 AM
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alanblaby

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Where is the 0.57 ohms from?
80% value of 63Amp ocpd?
The correct values are in Tables 41.3.
Adjust as necessary for temperature. Use 80% as a rule of thumb. (The OSG figures are 80% arent they?)
If a TT install, then the RCD is acting as the Fault Protection, so is allowed the higher figures from Table 41.5.
 07 February 2015 09:56 AM
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Dtw21

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Is this not covered in 433. 433.1.1 or more specific 435.1 protection afforded by one device?
 07 February 2015 09:56 AM
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Zs

Posts: 3830
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Keith,

Amtech haven't made any amd 3 changes yet. I should get an alert as my package includes the update when it comes. I'll come back on this when it appears.

Zs

Edit...In passing, one of my gripes about Amtech software is that you enter the details of the protective device and it doesn't show you on screen the required max Zs for your installation. It will go red when you hit the calculate button only if it does not comply. I'm sure it comes out on one of the final reports if you dig hard enough but it isn't an obvious function to view as you work on the design. As such, I expect there are loads of design engineers out there who don't have a clue what the max Zs for protective devices really is. I expect they just attend to the blood bath on hitting the calculate button.

Edited: 07 February 2015 at 10:04 AM by Zs
 07 February 2015 10:34 AM
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OMS

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That's because "Max Zs" is a simple concept for simple installations based on basic Fuse or MCB protection of entirely known characteristics and some big assumptions about the fault current

The requirement, actually, is to ensure that sufficient fault current flows to operate a specific device in a specific time (For ADS)

That fault current is set by voltage, power factor, conductor resistance (and temperature) - depending on how complex the analysis required and the type of protection being used.

To answer the OP, then it depends on what claim is being made on the RCD component - ie is it required for ADS or just for additional protection - ie are you making a demand on the MCB part of the device for earth fault (and short circuit) protection

Regards

OMS

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Let the wind blow you, across a big floor.
 07 February 2015 10:38 AM
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phantom9

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Originally posted by: keithredpath

Is the correct value to be entered for a RCBO Type B 0.57 Ohms or 1333 Ohms (80%) or 1667 Ohms.

What value is entered on the Amtech software?

What value when Amendant 3 kicks in.

Should we be entering 2 values e.g. 0.57/1333 Ohms.

What is the definitive answer?


It is not 1667 ohms!

The 1667 ohms is relevant to a condition that satisfies Ra x I delta n is less than or equal to 50V. The value stated in table 41.5 is the maximum value that satisfies the condition for each rated residual current.

Zs is maximum earth fault loop impedance to operate the fuse element of the disconnection device (overload and short circuit) in the required disconnection time. On TN systems the values in tables 41.2 and 41.3 will be relevant. In TT systems the earth fault loop is satisfied by the condition in 41.5.

In other words two completely different requirements, both of which need careful application.

This is clarified in Note 1 of table 41.5. Note 2 is also stating that earth electrode values should be as low as possible.
 07 February 2015 10:50 AM
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MrP

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The max ZS value of the protective device as per chapter 4 is the max value to be entered. (The circuit will be designed to that max value) not 75%, 80% or whatever %
A comparison is to be made against the max. The max is the max end of
With an RCBO the MCB element is to be used (max ZS) not the RCD element. The MCB element takes precedence over the RCD element you cannot depend on an electronic device to save your life, the circuit should be designed accordingly.
The figures are made up anyway. In other parts of the world they have never heard of ZS nor adopt it. Are those systems less safe.
I would be grateful if anyone could please give my an example of a protective device not operating because the circuit ZS was a tadge higher than that of the value in the book

MrP
 07 February 2015 11:54 AM
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phantom9

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Agreed MrP. I had been entering values of Zs of 80% on the test sheet and qualifying it as 80%. Seen the error of my ways and now use only tabulated value. R-O-T should only be used in the field, not recorded on a test sheet. This is actually what my software does anyway (Tysoft) so since using that its only ever been the max.
 07 February 2015 11:55 AM
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daveparry1

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This is something that's always irritated me a bit about the niceic forms. They very thoughtfully put max Zs figures on a chart at the front of the pad but these are the 80% values, not the figure we need to put in the box!
 07 February 2015 12:11 PM
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phantom9

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Yes NAPIT give us a similar thing with values of Zs and they are all 80%. Useful on site but not when filling out the cert.
 07 February 2015 12:53 PM
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MrP

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Anyone come across a protective device that didn't pop because the Zs was sooooo high be interesting to find out.

The figures are really made up! We start with a guess, first we used 75% as a comparison then changed 240 to 230 and the comparison was now 80%, now AMD3 have changed the applied voltage again.

Forgive me for being a little cynical as stated there are parts of the world who have never heard of Zs the fire brigade and hospitals must be on permanent standby.

MrP
 07 February 2015 01:10 PM
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keithredpath

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Just to reiterate.

What is the correct value to put on a Condition Report for a BS60898 MCB Type B 32 Amp MCB which is also a BS 61009 RCB

Is it 0.57 Ohms or 1333 Ohms. Should you be entering both figures for the Maximum ZS allowed.

-------------------------
keithredpath
 07 February 2015 01:27 PM
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alanblaby

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1.44ohms, or 1.37ohms for Amd. 3.
 07 February 2015 02:23 PM
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GeoffBlackwell

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Mr P said
The max ZS value of the protective device as per chapter 4 is the max value to be entered. (The circuit will be designed to that max value) not 75%, 80% or whatever %
A comparison is to be made against the max. The max is the max end of

Well there will be as many interpretations of the maximum value as there are assessment bodies ie: absolute maximum; maximum that a loop impedance tester should read; etc.
It is not really important provide a consistent approach is used.


With an RCBO the MCB element is to be used (max ZS) not the RCD element. The MCB element takes precedence over the RCD element you cannot depend on an electronic device to save your life, the circuit should be designed accordingly.

I think you should read 411.4.9 - this indicates that an RCD (which includes RCCBs and RCBOs) can be used to satisfy 411.3.2.2 (ie: meet the required disconnection time) if this cannot be achieved by an overcurrent device.

The figures are made up anyway. In other parts of the world they have never heard of ZS nor adopt it. Are those systems less safe.

There not made up they are based on the research by Biegelmeier and Lee which produced the shock protection curve that many of the IEC (World Wide) based regulations use.

I would be grateful if anyone could please give my an example of a protective device not operating because the circuit ZS was a tadge higher than that of the value in the book

That is not the point - non operation would be void protection but we need much more than simply operation - see 411.5.3 and Table 41.5.

A 30 mA RCD could be caused to operate by say, 23 mA. Now on a 230 volt supply this would imply that the impedance of the earth fault loop path could in the region of 10000 ohms. So an earth electrode with a resistance to earth just below this value might do!

However, the voltage developed across this electrode would be approaching 230 volts.

Now the work of Biegelmeier and Lee indicates that this voltage should not exceed 50 volts (411.5.3) and this results in a maximum figure for a 30 mA device of Ra <= 1667 ohms (Table 41.5)

So the fact that an RCD operates is not, on its own, enough to provide safety - we need much more than that.

MrP


Regards

Geoff Blackwell
 07 February 2015 02:52 PM
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leckie

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So to answer the OP. For the device he has quoted, if the mcb is being used for fault protection then I presume we are all agreeing that the max Zs is 1.44 to be recorded, but if the RCD/RCBO is being relied on because the max Zs exceeds that permitted for the MCB, then we put 1667 ohms for a 30mA device. Although I wouldn't be relying on that figure! I have no idea where the 0.57 ohms figure that Keith mentioned comes from.

I'm not sure about AMD3 figures as my BYB hasn't tuned up yet. Which is frustrating my need to nerd
 07 February 2015 03:02 PM
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daveparry1

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My book has arrived Leckie, I just need to work up some enthusiasm to read it now!
 07 February 2015 03:07 PM
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IsaacThreadbare

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Design to BS7671 and then test to 75% - 80% of the max given. Talking note that it is a 'rule of thumb' and not set in stone so a few whiskers above the % value should not cause you grief (you can make an allowance for actual applied test voltage at the time of test if you want to be pedantic at the time and adjust for temp etc). Or just go for the % of max value of BS7671

Regarding the 1667 Ohm then I would suggest that you forget that for a TN system as it is sometimes used as a cop-out for crap design as in circuits too long etc. If on a TN system you get high values then there is a problem somewhere :-)
 07 February 2015 03:19 PM
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GeoffBlackwell

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Originally posted by: IsaacThreadbare

Regarding the 1667 Ohm then I would suggest that you forget that for a TN system as it is sometimes used as a cop-out for crap design as in circuits too long etc. If on a TN system you get high values then there is a problem somewhere :-)


I disagree - 411.4.9 has been included to allow for situations where getting below the maximum Zs cannot be achieved on a TN. This might be because the DNO can only provide a supply with a Ze not far off the limit.

Regards

Geoff Blackwell
 07 February 2015 04:06 PM
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leckie

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If you have 2.5sq.mm L and N, but only a 1.5sq.mm cpc, you could easy have a situation where the short circuit protection and volt drop are not a problem, but a higher Zs requires the RCD for fault protection.
 07 February 2015 04:14 PM
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John Peckham

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Leckie

"If you have 2.5sq.mm L and N, but only a 1.5sq.mm cpc, you could easy have a situation where the short circuit protection and volt drop are not a problem, but a higher Zs requires the RCD for fault protection".

Sounds like bad design to me!

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
IET » Wiring and the regulations » Maximum ZS Permitted By BS7671

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