EICR code - type C breakers with undersized CPC

[Welchyboy1]

Mainly on commercial lighting with twin and earth 1.5mm or 1mm on 6A and 10A type C breakers

Im sure there was a table in bs7671 requiring larger CPC sizes for type C breakers than supplied with twin and earth, hence why people use fb200 or singles etc

I guess it must be restrictEd by let through current, how would you determine it on an existing circuit

What’s your opinion on maybe a code for this on a Type C MCBs
or what about on a Type C RCBO?

thank you

 

[westward10]

New one on me provided the cpcs can carry the expected fault current which they almost certainly can I see no reason to Code. If in doubt calculate the minimum size required but I very much doubt they would be inadequate.

 

[Pretty Mouth]

Do you mean table B7 on P132? I think it's sort of a one-size-fits-all guide. As @westward10 says, better to calculate, using manufacturer's data if you can

 

[Welchyboy1]

Do you mean table B7 on P132? I think it's sort of a one-size-fits-all guide. As @westward10 says, better to calculate, using manufacturer's data if you can

Yes that’s the table I saw before thank you

but then that surely directly contradicts table 7.1(i) in which it states many variants of compliant final circuits on type C devices with < 1.5mm CPC,s

only limited by ZS etc

am I missing something?

 

[Pretty Mouth]

Yes that’s the table I saw before thank you

but then that surely directly contradicts table 7.1(i) in which it states many variants of compliant final circuits on type C devices with < 1.5mm CPC,s

only limited by ZS etc

am I missing something?

The 2 tables are concerned with different things, at different ends of a circuit:

Table 7.1 gives us a maximum length of a circuit. Voltage drop and max Zs are the main limiting factors, and we're concerned with how these affect the far end of the circuit - if they're good there, then they're good throughout the circuit.

Table B7 gives a minimum conductor size for BS60898 breakers only, based on the adiabatic equation. Note that for BS60898 breakers, the let through energy tends to increase with fault current. Higher let through energy = thicker conductor required. Fault current is highest at the origin of the circuit, so if the conductor is thick enough to take the fault current there, it's good throughout the rest of the downstream circuit.

So, for example, a very low loop impedance at the origin might permit a very long circuit length in terms of VD and Zs, but you might find you have to increase the conductor size anyway to meet adiabatic requirements.

 

[timhoward]

@Pretty Mouth thank you, your response is much clearer than the draft I started last night and gave up on!
I'd got to an understanding that unless you are next door to the substation most of the time the <=3Ka column would apply.
In turn for type B breakers you would generally always be exceeding these values anyway. The type C column (at first glance) only really creates issues with 1.0 sq mm lighting circuits I think.
I hadn't noticed it was only related to BS60898 breakers too.

This would appear to finally provide a reason for 1.5mm for lighting circuits in limited circumstances!

 

[Welchyboy1]

The 2 tables are concerned with different things, at different ends of a circuit:

Table 7.1 gives us a maximum length of a circuit. Voltage drop and max Zs are the main limiting factors, and we're concerned with how these affect the far end of the circuit - if they're good there, then they're good throughout the circuit.

Table B7 gives a minimum conductor size for BS60898 breakers only, based on the adiabatic equation. Note that for BS60898 breakers, the let through energy tends to increase with fault current. Higher let through energy = thicker conductor required. Fault current is highest at the origin of the circuit, so if the conductor is thick enough to take the fault current there, it's good throughout the rest of the downstream circuit.

So, for example, a very low loop impedance at the origin might permit a very long circuit length in terms of VD and Zs, but you might find you have to increase the conductor size anyway to meet adiabatic requirements.

Please bear with me here

So as I mainly use ready calculated circuits from the final circuits section on the OSG

That details that a 1.00/1.00mm t&E circuit is a compliant circuit to install on a 6 or 10 amp type C circuit breaker provided the ZS and VD are within limits but the the other table indicates if the fault current at origin is under 3KA (which would cover most situations) the CPC must actually be a min of 1.5mm and the final circuit charts in this respect are not compliant?

So generally a 1mm cpc cannot be used on type c mcbs?

I kinda assumed the de-rated Max zs values for type C mcbs were to account for this

 

[pc1966]

This would appear to finally provide a reason for 1.5mm for lighting circuits in limited circumstances!

Exacpt for 1.5mm T&E having a 1mm CPC...

 

[pc1966]

So generally a 1mm cpc cannot be used on type c mcbs?

I kinda assumed the de-rated Max zs values for type C mcbs were to account for this

The lower Zs values are to meet disconnection times, to ensure you always hit the magnetic trip point (5-10 In, instead of 3-5 for B-curve).

The OSG quotes the generic I2t limits for MCBs that all have to meet, where as many manufacturers have significantly less I2t. For example, here are some Hager values:

https://download.hager.com/hagergroup.co.uk/files_download/Technical_Information/13%20Appendix_3.pdf

Using k=115 for PVC copper wire then the 1mm I2t limit is about 13.2k A2s so for their 6A C-curve it is met to a PFC of 6kA and for their 10A C-curve to a PFC of over 3kA (same really for B-curve). In most properties, and certainly any moderate distance down some 1mm cable, you would be very unlikely to exceed 3kA.

 

[timhoward]

Exacpt for 1.5mm T&E having a 1mm CPC...

I was thinking singles when I wrote that as had just finished feeding some!

 

[Welchyboy1]

The lower Zs values are to meet disconnection times, to ensure you always hit the magnetic trip point (5-10 In, instead of 3-5 for B-curve).

The OSG quotes the generic I2t limits for MCBs that all have to meet, where as many manufacturers have significantly less I2t. For example, here are some Hager values:

https://download.hager.com/hagergroup.co.uk/files_download/Technical_Information/13%20Appendix_3.pdf

Using k=115 for PVC copper wire then the 1mm I2t limit is about 13.2k A2s so for their 6A C-curve it is met to a PFC of 6kA and for their 10A C-curve to a PFC of over 3kA (same really for B-curve). In most properties, and certainly any moderate distance down some 1mm cable, you would be very unlikely to exceed 3kA.

So forgive me for being a ---- but on this chart, does this say - applies to all circuits up to and including 16A for PFC of 3KA UNDER or does it mean OVER 3KA

<3KA means any installations Under or equal to 3KA yes? so effectively in any situation a minimum CSA of at least 1.5mm is required on a type C breaker according to this table

So how is it compliant to have a 1mm/1mm 10A Type C, final circuit from the Final circuit charts on Page 66 Table 7.1(i) OSG?

I understand what you are saying that each table are measurements from either end of the circuit but from table B7, the circuit wouldnt be compliant to begin with

Im confused

 

[westward10]

If you are worried calculate it as per BS7671 don't worry about the OSG.

 

[pc1966]

So forgive me for being a ---- but on this chart, does this say - applies to all circuits up to and including 16A for PFC of 3KA UNDER or does it mean OVER 3KA

It is saying under (up to and including, strictly speaking).

<3KA means any installations Under or equal to 3KA yes? so effectively in any situation a minimum CSA of at least 1.5mm is required on a type C breaker according to this table

What the OSG table is saying is that all MCBs complying with "energy limiting class 3" specifications meet Table B7.

So in the general case if you have a C-curve MCB of 16A or less then the CSA should be at least 1.5mm for PFC values under 3kA.

So how is it compliant to have a 1mm/1mm 10A Type C, final circuit from the Final circuit charts on Page 66 Table 7.1(i) OSG?

What I was saying is the manufacturer's specification could well be better (i.e. less let-through energy). Hence my example where the Hager 10A C-curve is meeting the adiabatic for 1mm at 3kA PFC.

I understand what you are saying that each table are measurements from either end of the circuit but from table B7, the circuit wouldnt be compliant to begin with

Yes, I would agree that it is very bad practice to assume a fault is at the end of a cable since damage could occur anywhere.

Im confused

With any luck less so now! The reality is the OSG would normally be your pass/fail test, but here when it is a fail on the generic MCB you could then look at the manufacturer's data to see if it is actually OK.

RCBO are not necessarily any better. While the RCD side of them will trip on 15-30mA so they appear to protect better, when it comes to a hard fault it is going to be the magnetic trip coil of the MCB side that determines how quickly the contacts separate, and from that what sort of a I2t gets past it. The mag trip can go in a few milliseconds, where as the RCD side of things takes typically 10-30ms so is not "energy limiting" in the sense of trying to go before peak current, as a whole 50Hz cycle could be passed before it decides to trip.

But you sometimes find RCBO that are as fast if not faster than some MCBs, it really does come down to design details and so it is back to scouring the data sheets to see what they deliver.

 

[Welchyboy1]

It is saying under (up to and including, strictly speaking).

What the OSG table is saying is that all MCBs complying with "energy limiting class 3" specifications meet Table B7.

So in the general case if you have a C-curve MCB of 16A or less then the CSA should be at least 1.5mm for PFC values under 3kA.

What I was saying is the manufacturer's specification could well be better (i.e. less let-through energy). Hence my example where the Hager 10A C-curve is meeting the adiabatic for 1mm at 3kA PFC.

Yes, I would agree that it is very bad practice to assume a fault is at the end of a cable since damage could occur anywhere.

With any luck less so now! The reality is the OSG would normally be your pass/fail test, but here when it is a fail on the generic MCB you could then look at the manufacturer's data to see if it is actually OK.

RCBO are not necessarily any better. While the RCD side of them will trip on 15-30mA so they appear to protect better, when it comes to a hard fault it is going to be the magnetic trip coil of the MCB side that determines how quickly the contacts separate, and from that what sort of a I2t gets past it. The mag trip can go in a few milliseconds, where as the RCD side of things takes typically 10-30ms so is not "energy limiting" in the sense of trying to go before peak current, as a whole 50Hz cycle could be passed before it decides to trip.

But you sometimes find RCBO that are as fast if not faster than some MCBs, it really does come down to design details and so it is back to scouring the data sheets to see what they deliver.

Thank you everyone for your very detailed answers, it makes it much clearer

I did actually ask the NAPIT technical guy about this today and it was all news to him, he had never seen the table B7 before and told me if it complies on the final circuits page it should be fine

I wonder how many general electricians are even aware of this?

Last question, Does anyone have any data for MEM memshield 2, C type 6a &10a mcb's and whether they too would be compliant on a 1mm CPC?

So im assuming if its not compliant, on an EICR a C3 code would be due maybe even a C2 which could have significant ramifications if its not possible to swap it over to a B type MCB

 

[Pretty Mouth]

Last question, Does anyone have any data for MEM memshield 2, C type 6a &10a mcb's and whether they too would be compliant on a 1mm CPC?

 

[westward10]

Thank you everyone for your very detailed answers, it makes it much clearer

I did actually ask the NAPIT technical guy about this today and it was all news to him, he had never seen the table B7 before and told me if it complies on the final circuits page it should be fine

I wonder how many general electricians are even aware of this?

Last question, Does anyone have any data for MEM memshield 2, C type 6a &10a mcb's and whether they too would be compliant on a 1mm CPC?

So im assuming if its not compliant, on an EICR a C3 code would be due maybe even a C2 which could have significant ramifications if its not possible to swap it over to a B type MCB

Can you reference this "non compliance" in BS7671.

 

[pc1966]

Thank you everyone for your very detailed answers, it makes it much clearer

I did actually ask the NAPIT technical guy about this today and it was all news to him, he had never seen the table B7 before and told me if it complies on the final circuits page it should be fine

I wonder how many general electricians are even aware of this?

I suspect very few...

Last question, Does anyone have any data for MEM memshield 2, C type 6a &10a mcb's and whether they too would be compliant on a 1mm CPC?

So im assuming if its not compliant, on an EICR a C3 code would be due maybe even a C2 which could have significant ramifications if its not possible to swap it over to a B type MCB

The data for the Eaton Memshield MCBs lacks detailed info on the I2t let-through, which is disappointing.

However, given the OSG lists 1.5/1.0 circuits as compliant in Table 7.1 for 6/10A MCBs of both B & C curve I doubt it is a significant risk in practice. Has the DB in question got a PFC near or above 3kA?

I suspect if it is down in the 1-2kA region then really nothing to worry about.

 

[Welchyboy1]

Can you reference this "non compliance" in BS7671.

Not as such really, without delving into technical data for individual brands of mcb devices it seems

 

[Welchyboy1]

I suspect very few...

The data for the Eaton Memshield MCBs lacks detailed info on the I2t let-through, which is disappointing.

However, given the OSG lists 1.5/1.0 circuits as compliant in Table 7.1 for 6/10A MCBs of both B & C curve I doubt it is a significant risk in practice. Has the DB in question got a PFC near or above 3kA?

I suspect if it is down in the 1-2kA region then really nothing to worry about.

Ok bit of a storm in a teacup then

 

[pc1966]

Just to add that this BEMA guide has the maximum let-through specifications on pages 16 & 17:

BEAMA Guide to Low Voltage Circuit Breaker Standards june 2015 Corrigendum 2020

www.beama.org.uk

And yes, I should be finding more interesting things to do on a Saturday morning!