TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

[Jim Specs]

Further to my last: What is the non-compliance?

Non-compliance would be exceeding max permissible Zs value permitted by BS7671. I have detailed permissible Zs value in my original post for Final Circuit Protected By 30mA Type B 16A RCBO on TN-C-S System.

As per my original post, I have read a few articles stating that under BS7671, max permissible Zs earth loop impedance for above circuit is 1667 Ohms.

I believe this to be BS7671 non-compliance, which stems people reading Table 41.5 and ignoring overcurrent characteristics of RCBO detailed in Table 41.3.

My post is quite simple but people are somewhat hesitant to detail BS7671 Max permissible Zs for above circuit.

 

[Tom256]

Jim there is no non-compliance and I am not hesitant to detail the answer. What you want is me to confirm a misunderstanding

The Zs values given in table 41.3 that you refer to enable the disconnection times to be met for earth fault as per 411.3.2.1 and 411.3.2.2, 411.3.2.3, 411.3.2.4. set against the time / current curves for the relevant devices in Appendix 3.
[Sometimes higher values of Zs can be applied when device manufacturers show their devices operate within required times at higher values of Zs. The values in 41.3 are somewhat generic and individual device manufacturers data may vary].

However, If this requirement is met by RCD (always backed up by MCB - note under high fault values the mcb should break first as already explained why earlier in the thread) then refer to 411.4.204 which takes you to chapter 43.
There is no let through energy values for RCD's . There is no time / current curves for RCD's in Appendix 3.

The Zs values do not apply to line to neutral calculations. Other factors need to be considered for example on the circuit in question is overload protection required or is it overload and fault protection that is required. Voltdrop requirements are they met, Adiabatic confirmation - See regulation 434.5.2 where the time requirements for fault current disconnection are set out.
You then calculate device and cables etc accordingly. Even then confirmation of cable size withstand under earth fault can be confirmed by calcualtion or selection.
This is not all about a figure in table 41.3

There is a lot more which I am not going to go into - unless you pay me - but be assured when all the elements are correctly considered there is no non-compliance by using an RCD set against a Zs value higher than shown in table 41.3.

 

[Jim Specs]

Jim there is no non-compliance and I am not hesitant to detail the answer. What you want is me to confirm a misunderstanding

The Zs values given in table 41.3 that you refer to enable the disconnection times to be met for earth fault as per 411.3.2.1 and 411.3.2.2, 411.3.2.3, 411.3.2.4. set against the time / current curves for the relevant devices in Appendix 3.
[Sometimes higher values of Zs can be applied when device manufacturers show their devices operate within required times at higher values of Zs. The values in 41.3 are somewhat generic and individual device manufacturers data may vary].

However, If this requirement is met by RCD (always backed up by MCB - note under high fault values the mcb should break first as already explained why earlier in the thread) then refer to 411.4.204 which takes you to chapter 43.
There is no let through energy values for RCD's . There is no time / current curves for RCD's in Appendix 3.

The Zs values do not apply to line to neutral calculations. Other factors need to be considered for example on the circuit in question is overload protection required or is it overload and fault protection that is required. Voltdrop requirements are they met, Adiabatic confirmation - See regulation 434.5.2 where the time requirements for fault current disconnection are set out.
You then calculate device and cables etc accordingly. Even then confirmation of cable size withstand under earth fault can be confirmed by calcualtion or selection.
This is not all about a figure in table 41.3

There is a lot more which I am not going to go into - unless you pay me - but be assured when all the elements are correctly considered there is no non-compliance by using an RCD set against a Zs value higher than shown in table 41.3.

Agreed, Zs values in Table 41.3 enable disconnection times to be met as detailed on the top of the table. As per my original post it states 1.37 Ohms for Type B 16A RCBO.


Agreed, RCD always backed up by MCB (or fuse). However I am keen for people to stay focused on the specific scenario in my original post which is Final Circuit Protected By RCBO on TN-C-S System. Not separate RCD and MCB.


Agreed, Zs values do not apply to line to neutral calculations. As per my original post, at best, Zs can only infer the impedance of line/neutral for overcurrent short circuit scenario. There are many factors that go into a design, however I am keen for people to stay focused on the specific subject matter of my original post which relates specifically to Max Permissible Zs for a specific circuit.


Agreed, BS7671 Table 41.3 is based off generic time/current curves. Individual protective device manufactures may have different time/current curves which can be considered but do not have to be considered. As per BS7671 411.4.202 the Table 41.3 can be used instead.


No more hesitation, give a direct answer. As per BS7671 what is the Max permissible Zs for Final Circuit Protected By 30mA Type B 16A RCBO on a TN-C-S System ?

 

[cireland]

Theoretically it is 1667 ohms, however even on a TT system, anything over 200 ohms should be considered unstable.
The max Ze on a TN-C-S is 0.35 is it not?
What kind of circuit are you running?

 

[Tom256]

TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

As per my original post, I have read a few articles stating that under BS7671, max permissible Zs earth loop impedance for above circuit is 1667 Ohms.

I believe this to be BS7671 non-compliance, which stems people reading Table 41.5 and ignoring overcurrent characteristics of RCBO detailed in Table 41.3.

However if the line/neutral impedance is excessively high, for example 1667 Ohms, then it will take a longer time before it will disconnect. I would think this could lead to serious issues.

You talk of non-compliance and 'could lead to serious issues' without setting out what this would be when asked, I post , you agree then you copy / paste the same phrases and it starts again.

In all of this you have not set out any details of a final 'circuit'. You mention only a 16 A type B 61009 30mA protective device the figure 1667 ohms and 1.37 ohms... which is not the figure for a type b, 1.37 is the type C which is an easy mistake to make when you are not used to using the tables (I was not going to mention it but...)

Mentioning TN-C-S is pointless and I cannot see the relevance yet would love to hear why TN-C-S. You then throw in table - table 41.5 and whilst I am at it lets also include table 53.1.

For it all, all you are asking in effect is what is the Zs figure from Table 41.5 for a 30mA RCD and we all here know that figure is 1667 ohms.
And the rcd as used for Additional Protection can easily meet that requirement.

So lets talk about rcd for fault protection, considering also that at high levels of fault current the MCB may well trip before the rcd. And lets include an actual circuit outline from your figures.

TN-C-S you say, ok, Ze lets go with 0.35 ohms. Your R1 + R2 on this is therefore 1,666.65 ohms. You select a B16 30mA 61009 no idea why. We have no design current and I cannot imagine what cable we are using.

That done we require the minimum and maximum earth fault currents for the circuit using Cmin and Cmax which is going to be exciting.
Because of the RCD element of the 61009 used for ADS we calculate the cpc for worst case and somehow have to explain how you have an R1 value considerably >1500 ohms and all kinds of volt drop issues.

In your opening post you state:

A 1667 ohm Zs would seem to be non-compliant in regards to overcurrent characteristics of RCBO detailed in Table 41.3

You go on to also say in regards this:

I believe this to be BS7671 non-compliance, which stems people reading Table 41.5 and ignoring overcurrent characteristics of RCBO detailed in Table 41.3.

It has been explained Jim several times now, nothing is being ignored the tables are being correctly applied. I do not know what you are looking to achieve, it does not appear to be a wish to understand, as such I won't be wasting anymore time on this.

 

[newfutile]

Agreed, Zs values in Table 41.3 enable disconnection times to be met as detailed on the top of the table. As per my original post it states 1.37 Ohms for Type B 16A RCBO.


Agreed, RCD always backed up by MCB (or fuse). However I am keen for people to stay focused on the specific scenario in my original post which is Final Circuit Protected By RCBO on TN-C-S System. Not separate RCD and MCB.


Agreed, Zs values do not apply to line to neutral calculations. As per my original post, at best, Zs can only infer the impedance of line/neutral for overcurrent short circuit scenario. There are many factors that go into a design, however I am keen for people to stay focused on the specific subject matter of my original post which relates specifically to Max Permissible Zs for a specific circuit.


Agreed, BS7671 Table 41.3 is based off generic time/current curves. Individual protective device manufactures may have different time/current curves which can be considered but do not have to be considered. As per BS7671 411.4.202 the Table 41.3 can be used instead.


No more hesitation, give a direct answer. As per BS7671 what is the Max permissible Zs for Final Circuit Protected By 30mA Type B 16A RCBO on a TN-C-S System ?

A 16 amp type B would be closer to 2.18 at 80% values.
but at a push you could go up to 200 ohms .
unless there are conditions out of your control i would always design to the lower figure.

 

[westward10]

You are in Southern Ireland how come you are referring to BS7671?

 

[Pretty Mouth]

As per BS7671 what is the Max permissible Zs for Final Circuit Protected By 30mA Type B 16A RCBO on a TN-C-S System ?

1667 ohms, table 41.5.
Zs = "earth fault loop impedance", as defined by BS7671.
Zs does not concern faults between live conductors.

See also:
definitions, "earth fault loop impedance"
the title to chapter 41
regulation 411.3.2.1
the titles to tables 41.2 - 41.5