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

 

[Jim Specs]

1667 ohms, table 41.5.

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

Agreed, Zs is earth loop impedance as stated in my original post.

Point noted on Zs not concerning faults between live conductors. As per my original post, at best, Zs can only infer the impedance of line/neutral for overcurrent short circuit scenario, perhaps did not need to be mentioned in my original post.

Yes, 1667 Ohms is the value detailed in Table 41.5 for 30mA RCD. However an RCD does not provide overcurrent protection. For the overcurrent overcurrent characteristics of RCBO the max permissible Zs value is detailed in Table 41.3 which is 1.37 Ohms for Type B 16A RCBO as per my original post.

Why are you ignoring the overcurrent characteristics of RCBO earth loop impedance Zs values detailed in Table 41.3 ?

 

[Pretty Mouth]

Why are you ignoring the overcurrent characteristics of RCBO earth loop impedance Zs values detailed in Table 41.3 ?

What regulation makes you think they should be considered?

 

[Jim Specs]

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:

You go on to also say in regards this:


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.

Yes, fair catch Tom, I should have stated 2.37 Ohms not 1.37 Ohms for table 41.3.

You state mentioning TN-C-S is pointless. I find it surprising that you would make such a clearly false statement. There are going to be quite a few people reading that bemused. The earthing system is indeed important when determining max Zs for a circuit. BS7671 Section 411 specifies different Zs criteria for different earthing systems TN, TT, IT. Also worth noting that for a TN-C system BS7671 clause 411.4.5 does not allow an RCD to be used for fault protection. So knowing this, I specifically specified a TN-C-S system in my original post. It is critical for a person to know the earthing system in order to provide a concise response to my original post. Otherwise people will provide responses for multiple sicarios and start talking about IT systems, etc. You appear to have a gap in your knowledge when you state mentioning TN-C-S is pointless.

Yes I am asking what is the Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO for TN-C-S System, without providing any further information in regards to design current, volt drop, etc, etc. This seems to be causing some confusion for you. You think you do not have enough information to answer the question, but in fact you have all the information that is required. This is because BS7671 provides clearly defined max permissible Zs for the specific circuit detailed in my original post.

You keep trying to design a circuit. Put yourself in the mind of the installation contractor instead. The installation contractor will install as per the design issued by the consultant. The installation contractor is not concerned with design current, etc, etc. However once the installation contractor completes the install they will have to test the installation as per BS7671 section 643. The installation contractor may populate GENERIC SCHEDULE OF TEST RESULTS in appendix 6. On this, they will have to record an actual value for Zs and compare it to the max permissible Zs to determine if it has passed or failed testing. The installation contractor will use the max Zs tables provided in BS7671 Chapter 4 to determine pass/fail. So you should now understand that BS7671 provides clear max Zs values for the circuit detailed in my original post and you do not require additional information and you do not need to make further assumptions about the circuit.

You state won't be wasting anymore time on this but I suspect that is false and you will indeed spend more time in responding

 

[Jim Specs]

What regulation makes you think they should be considered?

I believe the earth loop impedance Zs values for overcurrent characteristics of RCBOs detailed in Table 41.3 should be adhered to so that we do not have BS7671 non-compliance.

I realize I'm asking the exact same question again but only because you did not answer the first time. Why are you ignoring RCBO earth loop impedance Zs values detailed in Table 41.3 ?

 

[Jim Specs]

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.

Appreciate the input. Yes, fair catch , I should have stated 2.37 Ohms not 1.37 Ohms for table 41.3.

I take your point in regards 80% as per APPENDIX 3. However I intentionally stated Zs in my original instead of Zs(m). BS7671 does not state that a circuit is non-compliant if the 80% equation is not met. There are other options available as detailed in BS7671, therefore the 80% equation is not the ultimate decider when determining max permissible earth loop impedance

I am trying to keep answers concise by narrowing down on a very specific Zs scenario detailed in my original post and not go into Zs(m) considerations detailed in APPENDIX 3.

 

[James]

I do believe that this conversation is useless.
overcurrent protection is just that. ensuring the circuit disconnects if the current goes over the rated current.

If you have such a high impedance between live and N that the protective device can't trip, then you do not have a high enough current flow to worry about.

the rcd portion of the device in question will deal with any fault to earth in a timely manor.

for anyone not understanding this, a little refresher training would be useful.

 

[Pretty Mouth]

I believe the earth loop impedance Zs values for overcurrent characteristics of RCBOs detailed in Table 41.3 should be adhered to so that we do not have BS7671 non-compliance.

Which regulation do you believe requires this? It helps us to help you if you answer this. If you don't know, just say so.

Why are you ignoring RCBO earth loop impedance Zs values detailed in Table 41.3 ?

See BS7671 definitions: "fault protection".
411.4.5 permits either an RCD or overcurrent device for fault protection.
An RCBO is both an RCD and overcurrent device.
We are looking for a maximum permitted Zs for the B16 30mA RCBO.
See dictionary definition :"maximum"
Table 41.3 offers a maximum Zs for the overcurrent part of a B16 RCBO of 2.73 ohms.
Table 41.5 offers a maximum Zs for the RCD part of a B16 30mA RCBO of 1667 ohms.
Of the two values, 1667 is higher, and is permitted by 411.4.5. Therefore 1667 is the maximum permitted Zs for a B16 30mA RCBO to BS61009.