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Discuss 1mm cable on a C curve 6amp mcb ? in the Electrical Forum area at ElectriciansForums.net

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So I was chatting with a friend tonight (also an electrician) and we were talking about disconnection times of C curve MCBs (as you do) and he was convinced he had a distant memory that 1mm t&e cable couldn't effectively be protected by a C curve 6amp mcb but couldn't give me a good reason. I've never heard of this and couldn't think of a reason why this would be the case, as long as the max measured Zs of the circuit is within the max permitted Zs of the breaker, and the cable has sufficient CCC its all good right?...
 
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Julie.

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So I was chatting with a friend tonight (also an electrician) and we were talking about disconnection times of C curve MCBs (as you do) and he was convinced he had a distant memory that 1mm t&e cable couldn't effectively be protected by a C curve 6amp mcb but couldn't give me a good reason. I've never heard of this and couldn't think of a reason why this would be the case, as long as the max measured Zs of the circuit is within the max permitted Zs of the breaker, and the cable has sufficient CCC its all good right?...
It can't.

The minimum size for type c would be 1.5mm^2

If the fault level is over 3kA then the minimum would be 2.5mm^2

So a normal t&e 1.5/1.0/1.5mm^2 would not be suitable either as the cpc is less than 1.5mm^2.

You can determine this during normal cable sizing calculations when you do the Adiabatic calculation for fault current, or you can just reference it in table B7 in the osg.
 

pc1966

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Hager's 6A C MCB would be OK for 1.5mm CPC size at 10kA fault current, and 1mm for 3kA fault current, but that is manufacturer-specific design.
 

Ian1981

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For a type C 6 amp mcb upto fault levels of 6Ka , a 1mm cpc is fine as the let through through energy is 13kA2S giving a minimum cpc size of 0.99mm2
 
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So at what point on the circuit do I determine the fault current to be used in my Adiabatic equation? The end where max measured Zs is taken and the fault current is less, or at the installation origin where min Zs is calculated and the fault current is the highest?
 

Baddegg

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For a type C 6 amp mcb upto fault levels of 6Ka , a 1mm cpc is fine as the let through through energy is 13kA2S giving a minimum cpc size of 0.99mm2
would anyone go with it being that tight though? Not Necessarily directed at you Ian just interested in views 😁
 

Ian1981

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would anyone go with it being that tight though?
Domestically your never going to see a fault current of that magnitude so yes, even commercially it’s rare.
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So at what point on the circuit do I determine the fault current to be used in my Adiabatic equation? The end where max measured Zs is taken and the fault current is less, or at the installation origin where min Zs is calculated and the fault current is the highest?
I’ve never needed to use the adiabatic for circuit breakers, often the disconnection time exceeds that of the curves given in appendix 3 so we refer to the let through energy of the device. As long as this is equal to or less than the energy withstand of the conductors then it’s fine.
 
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Domestically your never going to see a fault current of that magnitude so yes, even commercially it’s rare.
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I’ve never needed to use the adiabatic for circuit breakers, often the disconnection time exceeds that of the curves given in appendix 3 so we refer to the let through energy of the device. As long as this is equal to or less than the energy withstand of the conductors then it’s fine.
Thanks Ian, I think I'm getting it now... I know the energy let through data is obtained from the manufacturer of the device but is there a generic table for the energy withstand of the conductors?
 

Ian1981

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Thanks Ian, I think I'm getting it now... I know the energy let through data is obtained from the manufacturer of the device but is there a generic table for the energy withstand of the conductors?
There’s a generic table for BSEN 60898 devices like what table B7 in the on site guide uses but these give pessimistic values so if possible manufacturers data should be used as these allow smaller sizes like this from Hager and MK.
919BB294-2761-46CE-BED9-D0B7FFF1BE5A.jpeg
 
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There’s a generic table for BSEN 60898 devices like what table B7 in the on site guide uses but these give pessimistic values so if possible manufacturers data should be used as these allow smaller sizes like this from Hager and MK.
View attachment 59017
Which book is that from if you don't mind me asking?
 

pc1966

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The IET Electricians design guide
It is well worth getting a copy of that book as it has a few good worked examples of the sort of calculations you might need to do. Though if you are looking at a typical setup using standard circuits then the OSG tables have most of the answers.
 
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Thanks Ian, Ive ordered the book.
Out of interest is there a page on there for Crabtree Type C?

Presumably if this breaker were a 61009 RCBO we wouldn't be having this conversation because it would trip at 30mA earth fault current anyway, therefore energy withstand of cpc vs. energy let through of device isn't a consideration... am I right?
 

Julie.

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Thanks Ian, Ive ordered the book.
Out of interest is there a page on there for Crabtree Type C?

Presumably if this breaker were a 61009 RCBO we wouldn't be having this conversation because it would trip at 30mA earth fault current anyway, therefore energy withstand of cpc vs. energy let through of device isn't a consideration... am I right?
NO!!

The trip time of a rcd or rcbo - rcd portion is likely to be slower than the instantaneous trip of the mcb or rcbo-mcb portion therefore would be a larger let through, perhaps the same at best.

So if you have say 6kA fault level, a
6A hager Mcb may trip on instantaneous with 13kA2s let through - so trips in 0.00036 seconds that's 0.36 ms .

When you test an rcd or rcbo do you get a trip time this quickly?

the rcd portion is slower than the instantaneous/magnetic part of a mcb or mcb aspect of a rcbo even at the higher currents.

That's why you can't just use a rcd, proper overcurrent protection is still needed (phase-neutral faults not withstanding)


So say the rcd/rcbo had a trip time of 1.2ms (much less than typical test times due to 6kA current) then the let through would be 6kA x 6kA x 0.0012 = 43.2kA2s way more than the let through of the mcb section on magnetic/instantaneous trip!
 
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NO!!

The trip time of a rcd or rcbo - rcd portion is likely to be slower than the instantaneous trip of the mcb or rcbo-mcb portion therefore would be a larger let through, perhaps the same at best.

So if you have say 6kA fault level, a
6A hager Mcb may trip on instantaneous with 13kA2s let through - so trips in 0.00036 seconds that's 0.36 ms .

When you test an rcd or rcbo do you get a trip time this quickly?

the rcd portion is slower than the instantaneous/magnetic part of a mcb or mcb aspect of a rcbo even at the higher currents.

That's why you can't just use a rcd, proper overcurrent protection is still needed (phase-neutral faults not withstanding)


So say the rcd/rcbo had a trip time of 1.2ms (much less than typical test times due to 6kA current) then the let through would be 6kA x 6kA x 0.0012 = 43.2kA2s way more than the let through of the mcb section on magnetic/instantaneous trip!
Thanks Julie, Yes that does make sense now.
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It can't.

The minimum size for type c would be 1.5mm^2

If the fault level is over 3kA then the minimum would be 2.5mm^2

So a normal t&e 1.5/1.0/1.5mm^2 would not be suitable either as the cpc is less than 1.5mm^2.

You can determine this during normal cable sizing calculations when you do the Adiabatic calculation for fault current, or you can just reference it in table B7 in the osg.
Julie do you agree that table B7 errs on the side of caution and actually when manufacturer specific data is looked at (As per Ians posts) that a 1mm CPC is likely to be OK on C6 MCB?
 

Julie.

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Thanks Julie, Yes that does make sense now.
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Julie do you agree that table B7 errs on the side of caution and actually when manufacturer specific data is looked at (As per Ians posts) that a 1mm CPC is likely to be OK on C6 MCB?
Don't know really, I always work to the standard type figures as per b7 or similar, however if it's close, or not suitable, I then check with manufacturer's data, which is usually OK then, but in that case I need to specify a particular manufacturer.

Then there is the usual issue, someone checks up with normal data and proudly declares that the design is wrong!
 

Ian1981

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Problem with using the generic values is that you will find that if using t&e and type C ocpd’s are specced then you’d need to install 2.5 mm for say lighting circuits which is in most cases, ridiculous.
Manufacturers data usually allows a 1mm cpc and the fault current is rarely so high as to require a larger CSA, however it’s possible in some situations that it may need to be increased.

Using manufacturers data for say maximum earth fault loop impedance values may also allow a larger number than the generic values given in bs7671 appendix 3, tho I’ve always just stuck to them really.
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Don't know really, I always work to the standard type figures as per b7 or similar, however if it's close, or not suitable, I then check with manufacturer's data, which is usually OK then, but in that case I need to specify a particular manufacturer.

Then there is the usual issue, someone checks up with normal data and proudly declares that the design is wrong!
Off the topic a bit but can I ask you as a designer, when working on a design new say with its own TX on site or say an existing supply for a refurbishment like a school, what do you figure do you use for a starting Ze?
Do your design allow room for adjustment of this value?
Reason I ask is that I’m tired of getting a design and finding that their Ze value and thus their maximum Zs values for protecting distribution circuits is way off and often above the maximum value allowed for say Mccb’s.
cheers:)
 
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Julie.

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The issue really is that when you are doing the initial design, it can be months or even years away from the actual installation, and you could have little data.

I always try to size cables and equipment for the maximum fault level that's likely to occur, and the protection such that everything still works if the fault level is much lower, but this is difficult as you don't want to over-specify as this really puts the cost up.

Even at the later stages you often still won't have genuine figures from the dno, they usually just supply stock figures.

So unfortunately you are in a position of ensuring a cost effective design, which is suitable for fault levels you don't know!

With site transformers there is often another issue, and that is you have to assume a primary fault level and a size/voltage impedance of the transformer, it has happened to me in the past where I have specified a transformer size, only for the installer to use something larger (and lower impedance) as they often take it as a minimum. (As this is normal for kit sizing, - we can use 10kA mcbs in place of 6kA, 10 way board rather than 8 way, a 3kVA 230/110V transformer is ok to use instead of 2kVA etc)
 

Ian1981

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The issue really is that when you are doing the initial design, it can be months or even years away from the actual installation, and you could have little data.

I always try to size cables and equipment for the maximum fault level that's likely to occur, and the protection such that everything still works if the fault level is much lower, but this is difficult as you don't want to over-specify as this really puts the cost up.

Even at the later stages you often still won't have genuine figures from the dno, they usually just supply stock figures.

So unfortunately you are in a position of ensuring a cost effective design, which is suitable for fault levels you don't know!

With site transformers there is often another issue, and that is you have to assume a primary fault level and a size/voltage impedance of the transformer, it has happened to me in the past where I have specified a transformer size, only for the installer to use something larger (and lower impedance) as they often take it as a minimum. (As this is normal for kit sizing, - we can use 10kA mcbs in place of 6kA, 10 way board rather than 8 way, a 3kVA 230/110V transformer is ok to use instead of 2kVA etc)
Many thanks Julie.
 
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Thankyou all. Its been very enlightening, appreciate your help as always.
 

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