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Topic Title: Maximum let-through energy of fuse into cable
Topic Summary: Is maximum at current corresponding to a 5 seconds disconnect time
Created On: 10 July 2012 05:57 PM
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 19 October 2012 09:14 AM
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OMS

Posts: 19664
Joined: 23 March 2004

Originally posted by: Spark6

Thanks OMS. I'd love to get all the content of Annex ZA but that might be pushing the friendship :-)

Well I can't send you a copy if that's what you mean, as it comes of our corporate subscription and is only liceneced to the single user.

If you want other values then leyt me know and I'll type them in here - you can collate them in a table.



I'll need to agree to disagree with your statement:

"At 200A the breaker will operate at 0.1 seconds or less as you have reached 10 x In"

Really - if you put 200A through a 20A Type C breaker it will operate in "instantaneous" time ie by convention 0.1 seconds. Equally, the limiting circuit impedance will be exceeded if you can't get 200A flowing

As an example, for a Schneider 20A C60 C Curve breaker, you only need to miss 200A by a fraction and you go can from magnetic to thermal - the difference between less than 0.1 seconds and over 3 seconds clearance time.

If you are reading this from log/log scales stop right there. Once you go over 1.45 x In (so 20 x 1.45 = 29A) then the breaker is already beginning to operate - at that point it will take an hour to open. as you keep increasing that fault current then both the thermal and magnetic parts are pre - loading for the delatching point, and the MCB is becoming increasingly unstable as it gets hotter. The take over point between thermal and magnetic operation will be a region, not a specific point as you have assumed. we don't (usually) have any data for this region as it's unstable.

What we do know is that when we reach the upper limit of the magnetic operation, ie when 200A or mor eis flowing then it will open in 0.1 seconds or less (as I said, somewhere between 0.1 and 0.01 seconds). Increasing fault current beyond 200A won't usually cause faster operation


For the same breaker, 170A (8.5x) could take 5 seconds to disconnect. Yes, you have gone into the thermal region, but if we work on the assumption that no heat will be dissipated from the faulted cable for 5 seconds then you still need to consider it. So from 170A to 199.99A, I'd assume you still need to apply the adiabatic equation to this region regardless of if it is thermal or magnetic. If you slip into the thermal zone and it is still possible to clear the fault in under 5 seconds, this is obviously far worse than clearing a 200.01A fault in under 0.1 seconds.

OK, yes it could be about there and yes, for smaller conductors, you could consifder it to be adiabatic. You know what the maximum duration,t, is for a given conductor size and type (K2S2) - if you think you will have a fault current of 170A flowing then yes, you design for that


Maybe the confusion is that I am talking about C type breakers and it is more common to use B type breakers in the UK? This isn't an issue with B type breakers - just C and D.

Well it may well be an issue, we use plenty of Type c and d breakers - in many applications incorrectly, but there are millions of them out there


I guess my example is bordering on theoretical, but it is possible.

Indeed, you'll recall me mentioning in an earlier post about the designer knowing the failure point of his design - it's up to the designer to determine how credible that fault is - ie will he realy need to look at high impedance faults.

It has just always left me feeling slightly uneasy.

OK - in practice these predicted failure modes don't often happen, but they can in rare cases - I guess it forms part of how "robust" your design needs to be - there will be a diferent aproach to a few socket outlets in someones bedroom to that taken for a safety critical circuit in say the nuclear or petrochm industry

I should point out that the let-through characteristic for the same device suggests some sort of current limitation even around the 5 second region - not sure how that works given that it is obviously not interrupting in the first half cycle anymore, but that should further support that this is not really worth worrying about.

I agree there is a lot of 2.5mm2 cable fed from 20A MCBs and it doesn't seem to be a problem - but I wonder if every now and them a cable gets a little bit toastier than it should be and nobody really notices.....

Almost certainly - but you need to consider also that cable is pretty tough stuff in practice - like circuit breakers it doesn't suddenly change from a cable to a vapourised ball of plasma under fault - it takes time to absorb heat energy - and actually it takes quite a long time to raise conductor temperatures (particularly if some of our assumptions on ambient temp and conductor temp are conservative).

I guess you are discovering that it's not an exact scinece we use at this simple level - we just use relatively wel known state points in the operating cycle and predict (usually with a fair degree of safety margin) what's going to happen along the journy of the fault occuring to the point when the supply has disconnected.

Check out a few manufacturers I2t data and compare that with Annex ZA figures - in the vast majority of cases, the manufacturers data will outperform the maximum limits in the standard by a significant margin



Regards

OMS

-------------------------
Failure is always an option
 19 October 2012 01:54 PM
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Spark6

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Hi OMS. No I didn't expect you to send me a copy - I just thought if there wasn't too many combinations you'd be able to share them with us if not too much trouble. No worries if there is too much.

"If you are reading this from log/log scales stop right there. Once you go over 1.45 x In (so 20 x 1.45 = 29A) then the breaker is already beginning to operate - at that point it will take an hour to open. as you keep increasing that fault current then both the thermal and magnetic parts are pre - loading for the delatching point, and the MCB is becoming increasingly unstable as it gets hotter. The take over point between thermal and magnetic operation will be a region, not a specific point as you have assumed. we don't (usually) have any data for this region as it's unstable. "

I'm afraid you have lost me there. I have been using the log/log graph (time/current) characteristic. The characteristic I am referring to shows both upper and lower tolerance. Are you suggesting that the device could operate outside of these limits? If so, that is news to me. The 1.45x you mention has an upper limit of 1 hour disconnection, it could also operate in only 50 seconds at the same current - that gives some idea of the range of these things. I agree there is a grey area between where thermal and magnetic regions have influence but the extent of that region should not exceed the tolerance shown on the time/current curve. Is this not the case?

"Really - if you put 200A through a 20A Type C breaker it will operate in "instantaneous" time ie by convention 0.1 seconds. Equally, the limiting circuit impedance will be exceeded if you can't get 200A flowing"

For the breaker discussed, a 200A current can trip anywhere between 0.01s and 3.8 seconds. A 170A fault between 0.01s and 5 seconds. I agree it is likely it will be more like 0.01s but it "could" be the longer time. I don't see how it can be claimed it will definitely be 0.1 seconds or less. Sorry, I don't think I can post a picture of the curve but a Merlin Gerin C60N Type C breaker is common. I agree the limiting circuit impedance would be exceeded if you can't get 200A flowing, but i'm not talking about a condition you can design for by predicting the minimum fault current. The minimum fault current is determined with a fault of negligible impedance. I'm talking about a high impedance (perhaps arcing fault) that would just "happen" to get you in that 170-200A range, regardless of what the actual calculated min and max fault levels are.

"OK, yes it could be about there and yes, for smaller conductors, you could consifder it to be adiabatic. You know what the maximum duration,t, is for a given conductor size and type (K2S2) - if you think you will have a fault current of 170A flowing then yes, you design for that"
As I said above - the 170A condition is "always" possible - you can't predict the impedance of any fault. Maybe the issue is at what point the process is no longer adiabatic. You can use the adiabatic equation up to 5s, but that doesn't mean the process is adiabatic up to 5s. Unfortunately I have never found a source/standard that mentions the actual adiabatic process duration. Obviously if you start dissapating heat at the 2s mark, the situation improves.

Sorry OMS, I feel like I am stirring the pot, but I don't mean to. I agree it is unlikely and most people I have talked to about this dismiss it straight away. Thanks for the discussion.
 19 October 2012 02:55 PM
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OMS

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Originally posted by: Spark6

Hi OMS. No I didn't expect you to send me a copy - I just thought if there wasn't too many combinations you'd be able to share them with us if not too much trouble. No worries if there is too much.

No trouble at all - If I get a moment this afternoon, I'll post them up for Type B and C curves for Class 3 up to 63A


"If you are reading this from log/log scales stop right there. Once you go over 1.45 x In (so 20 x 1.45 = 29A) then the breaker is already beginning to operate - at that point it will take an hour to open. as you keep increasing that fault current then both the thermal and magnetic parts are pre - loading for the delatching point, and the MCB is becoming increasingly unstable as it gets hotter. The take over point between thermal and magnetic operation will be a region, not a specific point as you have assumed. we don't (usually) have any data for this region as it's unstable. "


I'm afraid you have lost me there. I have been using the log/log graph (time/current) characteristic. The characteristic I am referring to shows both upper and lower tolerance. Are you suggesting that the device could operate outside of these limits? If so, that is news to me. The 1.45x you mention has an upper limit of 1 hour disconnection, it could also operate in only 50 seconds at the same current - that gives some idea of the range of these things. I agree there is a grey area between where thermal and magnetic regions have influence but the extent of that region should not exceed the tolerance shown on the time/current curve. Is this not the case?

Sorry about that - nothing complicated, just that reading a log/log scale is fraught with inacuracy if yoy want to take specific numbers from the graph. The device shouldn't operate outside the limits, no.

The operating time as you've stated is hugely variable depending on which parts of the magnetic and thermal tripping mechanisms are being influenced. The only thing you can say with at least a degree of certainty is:

a - It won't trip at less than 1.12 x In

b - it will trip within an hour at 1.45 x In

c - for any current in between it will trip within an hour - increasing current usually creating faster (thermal) operation

d - It will trip at 0.1 seconds (or less) for currents matching the current multipliers for the MCB type - so Type B breakers will go in 0.1 seconds for currents between 3 and 5 times In, For Type C it will go for currents between 5 and 10 times In

e - There will be a current limiting effect, how much will depend on the MCB energy limitation class and prevailing fualt level (you greater current limiting for higher classifications and less current limiting for increasing fault levels (up the device ultimate breaking capacity)


The tolerance between trailing edge and leading edge won't exceed the range shown

"Really - if you put 200A through a 20A Type C breaker it will operate in "instantaneous" time ie by convention 0.1 seconds. Equally, the limiting circuit impedance will be exceeded if you can't get 200A flowing"


For the breaker discussed, a 200A current can trip anywhere between 0.01s and 3.8 seconds.

Are you sure of that - for a 20A Type C, with a multiplier of 10 x In you are still within the instantaneous operation range - ie 0.1 seconds - if the current is less, it could still go in 0.1 seconds as you are in the range of 5 to 10 x In, ie 100 - 200A, but it may be slower.

A 170A fault between 0.01s and 5 seconds.

Possible, unlikley - and as I said, you are using the breaker outside of it's definite operating points - ie you have installed it in a circuit with sufficiently high impedance to disallow adequate current flow - which needs to be 200A or above. If you suspect only 170A will flow then it's the wrong breaker or the wrong cable - there's the dilemma for the designer - do we consider faults with impedance or assume bolted faults

I agree it is likely it will be more like 0.01s but it "could" be the longer time.

See above

I don't see how it can be claimed it will definitely be 0.1 seconds or less.

I think we are at cross purposes - if you have 200A + flowing it will be 0.1 seconds or less

Sorry, I don't think I can post a picture of the curve but a Merlin Gerin C60N Type C breaker is common. I agree the limiting circuit impedance would be exceeded if you can't get 200A flowing, but i'm not talking about a condition you can design for by predicting the minimum fault current.

It's OK, I can just about conjure up a type C MCB curve

The minimum fault current is determined with a fault of negligible impedance.

Yes, that's our design assumption and is widely used.

I'm talking about a high impedance (perhaps arcing fault) that would just "happen" to get you in that 170-200A range, regardless of what the actual calculated min and max fault levels are.

For sure - it's a choice for the designer if you consider this. BS 7671 doesn't necceessarily require you to account for it - but that's minimum compliance.


"OK, yes it could be about there and yes, for smaller conductors, you could consifder it to be adiabatic. You know what the maximum duration,t, is for a given conductor size and type (K2S2) - if you think you will have a fault current of 170A flowing then yes, you design for that"

As I said above - the 170A condition is "always" possible - you can't predict the impedance of any fault.

I know - the effort in doing so and the impact on the design however can be onerous.

Maybe the issue is at what point the process is no longer adiabatic. You can use the adiabatic equation up to 5s, but that doesn't mean the process is adiabatic up to 5s.


Of course it isn't - perhaps a reference to BS 7454 would be worth your while. What we can say with some certainty is that we can assume the process is adiabatic up to 5 seconds for conductors over 50mm2 for non armoured cables and for significantly less time for smaller conductors



Unfortunately I have never found a source/standard that mentions the actual adiabatic process duration. Obviously if you start dissapating heat at the 2s mark, the situation improves.

It does, hence my earlier coments about the conservative (very) nature of cable ratings


Sorry OMS, I feel like I am stirring the pot, but I don't mean to. I agree it is unlikely and most people I have talked to about this dismiss it straight away. Thanks for the discussion.

Not at all mate - it's a discussion forum, we have a few that run to a 1000 posts and there's a fair bit of disagreement - mostly good natured.

just remember if people dismiss it straight away, it usually means they don't know the answer - all of your points are valid - it's taken a few posts to get to the crux of it but i suspect the both of us and a few browers are learning something, do you think



Regards

OMS

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Failure is always an option
 19 October 2012 03:24 PM
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Spark6

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Thanks OMS

"No trouble at all - If I get a moment this afternoon, I'll post them up for Type B and C curves for Class 3 up to 63A"

That would be great if it isn't too much to ask.

"For the breaker discussed, a 200A current can trip anywhere between 0.01s and 3.8 seconds.

Are you sure of that - for a 20A Type C, with a multiplier of 10 x In you are still within the instantaneous operation range - ie 0.1 seconds - if the current is less, it could still go in 0.1 seconds as you are in the range of 5 to 10 x In, ie 100 - 200A, but it may be slower."

Yes, very sure. 10 x In is the edge of the tolerance range, where the ceiling of the region is 3.8s and the floor is 0.01s. Ask me about 10.000000000000001 x In and I can guarantee you something alot closer to the sub 0.1 s figure :-)

"Not at all mate - it's a discussion forum, we have a few that run to a 1000 posts and there's a fair bit of disagreement - mostly good natured.
just remember if people dismiss it straight away, it usually means they don't know the answer - all of your points are valid - it's taken a few posts to get to the crux of it but i suspect the both of us and a few browers are learning something, do you think"

For sure! It is better not to shine the light on some of these accepted practices. The results can be .... inconveinient :-)

I'm going to open up a new topic based on how to these let-through characteristics should be interpreted if the breaker is cascaded with an upstream breaker or backup with a fuse. I think I have hijacked this thread enough. Keep an eye out for it, I'd be keen to see what you think. Might do it tomorrow.
 19 October 2012 04:09 PM
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OMS

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Yes, very sure. 10 x In is the edge of the tolerance range, where the ceiling of the region is 3.8s and the floor is 0.01s. Ask me about 10.000000000000001 x In and I can guarantee you something alot closer to the sub 0.1 s figure :-)


OK - you have to be looking at specific data here, or reading a time current curve - BS EN 60898-1 would require that breaker to operate at 100mS (0.1 seconds) or less for 10 x In

The test defined is:

9.10.2.3 For circuit-breakers of the C-type
A current equal to 5 In is passed through all poles, starting from cold.
The opening time shall be not less than 0,1 s and not more than:
- 15 s for rated currents up to and including 32 A,
- 30 s for rated currents above 32 A. 
A current equal to 10 In is then passed through all poles, again starting from cold.
The circuit-breaker shall trip in a time less than 0,1 s.


My emphasis

So either we are not actually discusssing the same thing or your MCB isn't compliant. I suspect the former because I'm not quite sure of how you are describing the ceiling and floor of the (assumed) 5x and 10x In region.

Max let through energy for type B (Class 3) MCB's

SSC <16A 20, 25, 32A 40A 50, 63A
3.0kA 15000 18000 21600 28000
4.5kA 25000 32000 38400 48000
6.0kA 35000 45000 54000 65000
10kA 70000 90000 108000 135000

Max let through energy for type C (Class 3) MCB's

SSC <16A 20, 25, 32A 40A 50, 63A
3.0kA 17000 20000 24000 30000
4.5kA 28000 37000 45000 55000
6.0kA 40000 52000 63000 75000
10kA 80000 100000 120000 145000

regards

OMS

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Failure is always an option
 19 October 2012 11:04 PM
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Spark6

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Thanks for the data OMS. Does the annex have type D data too?

Yes, I am looking at specific data (Merlin Gerin C60N Type C). I see where our confusion lies now.

Yes, you are right, at 200A (10 x In) it will clear in under 0.1 seconds, but at 199.999A it might not. Again this is a semantic argument, but I am just demonstrating the point that for any value below 200A it is not guaranteed. So at 199.999A it is will obviously still break in that time but it doesn't need to as per the standard. The ceiling I am talking about is where the region is bounded by the upper thermal portion of the curve. The standard eludes to the ceiling (at 5xIn) by saying a device must not be slower than 15 seconds, which is exactly the ceiling value at 5 x In for the Merlin Gerin breaker I am talking about.
If you look in the wiring regs figure 3.5 shows a 60898 type c breaker characteristic. This is the worst case characteristic - it shows the magnetic region at 10 x In. For 200A you will get less than 0.1s. but for 199.99 you *may* take 5 seconds to clear. The 5 second "knee" in the plot is the ceiling I am talking about. This graph doesn't show the best case, where it is possible to disconnect in under 0.1 seconds for a 199.99A fault. But yes, I completely agree, 200A is guaranteed under 0.1 seconds, as per the standard.
 22 October 2012 03:00 PM
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OMS

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Originally posted by: Spark6

Thanks for the data OMS. Does the annex have type D data too?

It doesn't, the annex (and the marking) is only applicable for Type B and Type C up a rating of 63A


Yes, I am looking at specific data (Merlin Gerin C60N Type C). I see where our confusion lies now.

Me too


Yes, you are right, at 200A (10 x In) it will clear in under 0.1 seconds, but at 199.999A it might not. Again this is a semantic argument, but I am just demonstrating the point that for any value below 200A it is not guaranteed. So at 199.999A it is will obviously still break in that time but it doesn't need to as per the standard. The ceiling I am talking about is where the region is bounded by the upper thermal portion of the curve. The standard eludes to the ceiling (at 5xIn) by saying a device must not be slower than 15 seconds, which is exactly the ceiling value at 5 x In for the Merlin Gerin breaker I am talking about.

Ok - I guess we are back to your high impedance fault and the depressed fault current consequently.


If you look in the wiring regs figure 3.5 shows a 60898 type c breaker characteristic. This is the worst case characteristic - it shows the magnetic region at 10 x In. For 200A you will get less than 0.1s. but for 199.99 you *may* take 5 seconds to clear. The 5 second "knee" in the plot is the ceiling I am talking about. This graph doesn't show the best case, where it is possible to disconnect in under 0.1 seconds for a 199.99A fault. But yes, I completely agree, 200A is guaranteed under 0.1 seconds, as per the standard.

For sure - I guess this is all about undertsanding the circuit breaker, what the standard allows and what the impact of that is on design.

Given a variable starting point such as conductor temperature and the effect on resistance and a simplistic approach to how hot a conductor can get under fault and the further effect on resistance, then I guess we would have to say that any design that barely achieves 200A fault current at design, is going to be suspect, unless you have also considered the impact of a much longer fault and the lack of current limiting effect of faults longer than 0.1 seconds and increased the cable sizes accordingly.

More usual would be to keep the fault level higher by design and if a high(er) impedance fault is supected, inctreasing the cable size accordingly.

A quick check would be to use the limiting Ze and apply that to the P-N loop - if it's close then there's a problem,




Regards

OMS

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Failure is always an option
 06 September 2013 02:53 AM
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Spark6

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I know this is old, but a quick question about the let through energies quoted in annex ZA of 60898.

Does a device comply to the lower current rating let through energies as well as the figure nominated at the devices rated capacity.

E.g. Does a 20A, Type C breaker rated at 10kA have a let-through profile where of 17kA2s@3kA, 28kA2s@4.5kA, 40kA2s@6kA and 80kAs@10kA ? Does it comply with the lower current ratings too, or for a 10kA device, does it only comply with the last let-through energy (80kA2s).

Any help would be appreciated.
 06 September 2013 06:38 PM
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weirdbeard

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Hi spark6, as far as I can say, 10kA would be the maximum rating for the device so i would say for any figure lower than this the device is considered compliant.
 09 September 2013 11:11 AM
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AJJewsbury

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Hi spark6, as far as I can say, 10kA would be the maximum rating for the device so i would say for any figure lower than this the device is considered compliant.

I think spark6 is asking the question the other way around. According to annex ZA a higher rated device (e.g. 10kA) has a larger energy let-through than a lower rated (e.g. 3kA) device - without regard to of the actual level of the fault current.

So say you had two 16A C type MCBs - a 3kA rated one and a 10kA rated one. A 3kA rated device would, according to annex ZA have a let-though of 18,000 A2s, but a rated 10kA version 84,000 A2s. If installed at a point where the the PFC was say 3kA would the 10kA rated device behave like a 3kA device and only let 18,000A2s through, or is it still allowed to let up to 84,000A2s through?

I think the short answer is that BS 60898 doesn't say (as least not that I've been able to see), but the IET in their tables (in GN3, OSG etc) tabulate the same figures based on prospective fault current, not the device's rating - so (unless they've cocked up) they seem to have grounds for presuming that a 10kA device will behave no worse than a 3kA device for a 3kA fault current.

- Andy.
IET » Wiring and the regulations » Maximum let-through energy of fuse into cable

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