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

[Jim Specs]

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.

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.

I am not interested in playing the regulation game, maybe some other day Lets stick to BS7671 and ignore legalities as it is not relevant to my original post.

Generally agree with what you have stated here. However I would disagree with your conclusion.

I would say of two values, 2.73 ohms is lower than 1667 Ohms and as Table 41.3 mandates a max 2.73 ohm Zs we can not exceed this value as to do so would result in BS7671 non-compliance for overcurrent characteristics of RCBO.

What you are doing is applying Table 41.5 and ignoring Table 41.3. For BS7671 compliance the full standard has to be adhered to in it's entirety.

If you disagree, please elaborate.

 

[Jim Specs]

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.

Well said James, very insightful. Can people write that on test reports instead of recording Zs vales for pass/fail criteria ?

 

[mainline]

I am not interested in playing the regulation game, maybe some other day Lets stick to BS7671 and ignore legalities as it is not relevant to my original post.

Generally agree with what you have stated here. However I would disagree with your conclusion.

I would say of two values, 2.73 ohms is lower than 1667 Ohms and as Table 41.3 mandates a max 2.73 ohm Zs we can not exceed this value as to do so would result in BS7671 non-compliance for overcurrent characteristics of RCBO.

What you are doing is applying Table 41.5 and ignoring Table 41.3. For BS7671 compliance the full standard has to be adhered to in it's entirety.

If you disagree, please elaborate.

If you had an earth fault loop impedance of 1667 ohms, it would need looking at.

If you had an L to N loop impedance of 2.73 it would also need looking at.

 

[Jim Specs]

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

Pick your favorite option from below:

1. I'm using VPN

2. I'm on holidays in Southern Ireland.

3. I'm manufacturing electric skids in Southern Ireland that are going to be installed in the UK.

4. All of the above

 

[Jim Specs]

If you had an earth fault loop impedance of 1667 ohms, it would need looking at.

If you had an L to N loop impedance of 2.73 it would also need looking at.

I note your comment and yet we still have not detailed the Max permissible Zs earth loop impedance for Final Circuit Protected By 30mA Type B 16A RCBO on TN-C-S System.

 

[mainline]

I am not interested in playing the regulation game, maybe some other day Lets stick to BS7671 and ignore legalities as it is not relevant to my original post.

Generally agree with what you have stated here. However I would disagree with your conclusion.

I would say of two values, 2.73 ohms is lower than 1667 Ohms and as Table 41.3 mandates a max 2.73 ohm Zs we can not exceed this value as to do so would result in BS7671 non-compliance for overcurrent characteristics of RCBO.

What you are doing is applying Table 41.5 and ignoring Table 41.3. For BS7671 compliance the full standard has to be adhered to in it's entirety.

If you disagree, please elaborate.

You're correct 2.73ohms is lower than 1667 ohms

You have your answer, 1667 ohms would trip the line to cpc and 2.73 would trip the L to N fault.

 

[mainline]

I note your comment and yet we still have not detailed the Max permissible Zs earth loop impedance for Final Circuit Protected By 30mA Type B 16A RCBO on TN-C-S System.

Max is 1667, but on a tnc-s you would expect it to be much lower

 

[Jim Specs]

You're correct 2.73ohms is lower than 1667 ohms

You have your answer, 1667 ohms would trip the line to cpc and 2.73 would trip the L to N fault.

Thanks, I thought I would have to break out the calculator.

I am going to go out on a limb here and state that 2.73 Ohm Zs detailed on Table 41.3 is only concerned with fault as described in clause 411.3.2.2 between the line conductor and an exposed-conductive-part or a protective conductor in the circuit or equipment. Not a L to N fault.

What say you now ?

 

[mainline]

Thanks, I thought I would have to break out the calculator.

I am going to go out on a limb here and state that 2.73 Ohm Zs detailed on Table

Thanks, I thought I would have to break out the calculator.

I am going to go out on a limb here and state that 2.73 Ohm Zs detailed on Table 41.3 is only concerned with fault as described in clause 411.3.2.2 between the line conductor and an exposed-conductive-part or a protective conductor in the circuit or equipment. Not a L to N fault.

What say you now ?

is only concerned with fault as described in clause 411.3.2.2 between the line conductor and an exposed-conductive-part or a protective conductor in the circuit or equipment. Not a L to N fault.

What say you now ?

Oh, sorry, I thought you weren't interested in playing the regulation game.

411.4.204 Where an RCD is used to satisfy the requirements of Regulation 411.3.2.2, 411.3.2.3 or 411.3.2.4
the maximum values of earth fault loop impedance in Table 41.5 may be applied.

 

[Jim Specs]

Oh, sorry, I thought you weren't interested in playing the regulation game.

411.4.204 Where an RCD is used to satisfy the requirements of Regulation 411.3.2.2, 411.3.2.3 or 411.3.2.4
the maximum values of earth fault loop impedance in Table 41.5 may be applied.

BS7671 is not a regulation, it is a Technical Standard. There is a distinct difference.

BS7671 clause 411.4.5 states for fault protection the following may be used

(i) An overcurrent protective device
(ii) An RCD.

As an RCBO integrates and RCD and MCB, it would seem a bit unclear to the the installation contractor testing if the RCD was selected for fault protection.

When reading 411.4.204 how does the installation contractor testing the install know if the RCD integrated into the RCBO has been installed for fault protection and not additional protection ?

 

[mainline]

BS7671 is not a regulation, it is a Technical Standard. There is a distinct difference.

Under the Health & Safety at Work Act section 17, you could be prosecuted for non-compliance with the BS 7671 regulations. It is widely accepted that compliance with the regulations and associated requirements will also ensure you comply with the 1989 Electricity at Work Regulations.

BS7671 clause 411.4.5 states for fault protection the following may be used

(i) An overcurrent protective device
(ii) An RCD.

As an RCBO integrates and RCD and MCB, it would seem a bit unclear to the the installation contractor testing if the RCD was selected for fault protection.

When reading 411.4.204 how does the installation contractor testing the install know if the RCD integrated into the RCBO has been installed for fault protection and not additional protection ?

Because a competent electrician would be able to make sense of his readings and understand the requirements of the installation.

My suggestion is to befriend one and ask if they will educate you.

 

[Jim Specs]

Because a competent electrician would be able to make sense of his readings and understand the requirements of the installation.

My suggestion is to befriend one and ask if they will educate you.

It's not quite that simple unfortunately.

For large installations it is common for a consultant to produce a design and the electrician will then install to that design. The design instructs the electrician to install Final Circuit Protected By 30mA Type B 16A RCBO on a TN-C-S System.

It should be clear to you now, that it is not always going to be obvious to the electrician if the RCD integrated into the RCBO has been installed for fault protection or has been installed for additional protection.

Will the electrician educate me on their magical guessing skills in this scenario ?

 

[mainline]

It's not quite that simple unfortunately.

I'm afraid it is quite simple.

For large installations it is common for a consultant to produce a design and the electrician will then install to that design. The design instructs the electrician to install Final Circuit Protected By 30mA Type B 16A RCBO on a TN-C-S System.

Gosh, you learn something new every day.

It should be clear to you now, that it is not always going to be obvious to the electrician if the RCD integrated into the RCBO has been installed for fault protection or has been installed for additional protection.

I think to an electrician it would be, or maybe I need another 40yrs or so to understand.

Will the electrician educate me on their magical guessing skills in this scenario ?

He may just ignore you.

 

[Jim Specs]

I'm afraid it is quite simple.

Gosh, you learn something new every day.

I think to an electrician it would be, or maybe I need another 40yrs or so to understand.

He may just ignore you.

Repeating your false statements won't achieve much.

It's not quite that simple unfortunately.

For large installations it is common for a consultant to produce a design and the electrician will then install to that design. The design instructs the electrician to install Final Circuit Protected By 30mA Type B 16A RCBO on a TN-C-S System.

It should be clear to you now, that it is not always going to be obvious to the electrician if the RCD integrated into the RCBO has been installed for fault protection or has been installed for additional protection.