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Topic Title: FUSING FACTORS
Topic Summary: Conductor adjustment
Created On: 15 May 2013 07:02 PM
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 15 May 2013 07:02 PM
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Avatar for Jobbo.
Jobbo

Posts: 912
Joined: 08 July 2010

Evening all

I'm currently working on a project that involves adjusting the conductors to the associated protective device fusing factor (from manufacturer data). This got me thinking about how BS7671 only really recognises BS3036 devices within Regulation 433.1.101 for adjustment.

Looking at the fusing factors I've provided below, a BS 1362 device has similar characteristics. Could someone please explain the reasoning for this? The 1.45 within 433.1.1 (iii) only really relates to circuit breakers.

Is this yet another rule of thumb practice? Are the figures so close to 1 that its not worth considering.

I've shown in the second column the rating factor that would be applied to the conductors for commonly used devices.


Type Fusing factor 1.45 รท I2:I1

BS 88 fuse 1.6 0.906
BS 1361 fuse 1.5 0.967
BS 1362 fuse 1.9 0.763
Type 1/2/3/4 MCB up to 10A 1.5 0.967
Type 1/2/3/4 MCB over 10A 1.35 1.074
Type B/C/D MCB 1.45 1.0
BS 3036 fuse 2.0 0.725


Apologies if this table goes a little weird, as typing this on my iPad.

Regards

Jobbo
 15 May 2013 07:42 PM
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Avatar for OMS.
OMS

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If you ignore the BS 1362, then basically, yes - the thermal component of the protective device for conventional time sits very close to 1.45 and for fuses, thermal acceleration brings the fusing factors for BS 88's much closer to 1.45.

As the fuse is getting hotter, it's closer to it's rupturing point effectively, so the fusing factor appears to get smaller (same concept with ganged circuit breakers mutually heating each other - hence we have a derating factor if the MCB's are at 40C or 50C or 60C etc)

We don't work it out for each case as BS 7671 recognises the very fine tolerances we are talking about (ie so close as to be not worth considering) but does warn about sustained overloads - but only when using the listed CPD's

Of course MCCB's and ACB's are a totally different ball game - and with these, you need to recognise 433.1.1 (iii)

Appendix 4 - section 4 also clarifies

regards

OMS

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Failure is always an option
 15 May 2013 10:45 PM
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Jobbo

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OMS

Thanks for the swift reply.

Just trying to get my head round this, so excuse me if you don't understand my posts (besides it late)

Now I understand that a pvc cable can withstand a 50% overload for an hour, so the 1.45 x Iz factor must have preceded the 60898 devices. So this would suggest to me, that the 1.45 factor was built into the standard when it came about, along with other modern circuit breakers? Therefore giving them a fusing factor of 1.

This now raises a question, when was this 1.45 factor built into the cable standard and does it cover all cable constructions and insulation types.

Regards

Jobbo
 16 May 2013 09:45 AM
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Avatar for OMS.
OMS

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Originally posted by: Jobbo

OMS

Thanks for the swift reply.

Just trying to get my head round this, so excuse me if you don't understand my posts (besides it late) [IMG][/IMG]

No problem, I understand exactly what you are saying - your just a tad too close to the problem - step back and look at what the regs say - and then think why 1.45 - is it because it's a good average approximation for all of the protective devices listed in BS 7671 - thus we have a simple rule.


Now I understand that a pvc cable can withstand a 50% overload for an hour,

Steady - not incorrect, but the concept you want here is "conventional time" - it may be much longer than 1 hour depending on the CPD size.
so the 1.45 x Iz factor must have preceded the 60898 devices.

In general terms, yes - and remember it's based in the era of introduction of PVC (ie thermoplastic) insulation

So this would suggest to me, that the 1.45 factor was built into the standard when it came about, along with other modern circuit breakers? Therefore giving them a fusing factor of 1.

As I said, it's an average - I can't vouch for it, you'll need someone older and who was there - but I believe it derives from one of those smoke filled rooms where the principal engineer of a large UK contracting and engineering firm scribbled a few calcs on the back of a fag packet and worked his way back from the thermal withstand and limiting temperatures of "modern" PVC cable - it's where the unique UK "5 second disconnection time" came from


This now raises a question, when was this 1.45 factor built into the cable standard and does it cover all cable constructions and insulation types.

I think we need to go back to early versions - certainly prior to 15th edition - and no, it doesn't cover all cable types and constructions because it doesn't cover all CPD types - there is a relationship between the two that's inalienable - basically, what goes through the CPD has to be soaked up by the cable based on the cable losses delivering energy to the load. To avoid evaluating every cable in every circumstance we use a simple set of empirical rules - that don't often fail, but are not foolproof - hence the warning in BS 7671.

There is (or can/should be) much more thought required in the mating of CPD's and cables that doesn't get a mention in BS 7671 - except the 1.45 requirement (and the link to rewirables). For example it doesn't discuss CPD derating for ambient temperature etc, alterations to volumetric heat capacity (ie fudging K factors) etc etc

So, if you really are looking to evaluate an existing system based on real cables and real CPD data it's not as simple a s just deriving a few factors based on 1.45/I2 - you first need to adjust I2 for it's actual temperature (basically my earlier comment why a BS 88 fuse exhibiting an upper bound og 1.6 x In will actually be closer to 1.45 x In due to the increasing temperature of the fuse assembly as it runs into an increasing overload situation - both internally within the fuse body and externally as the enclosure in which it's mounted also warm up - think of it like preloading the device - it's already moving up the pre arcing point of the curve - so it has less to go when it finally opens.

You'll remember we had a dialogue about this regarding a sample question you set when you were all off on your designers course - your example had at least 5 fully loaded circuits all with a design current approaching 20A - i was demonstrating to you why selecting a CPD of 20A and a cable sized to suit based on a group of 5 wasn't the correct answer - just the one that C&G gave - because they use a simple method - I was challenging why a design course didn't give you the background information to this issue - so at least there is an awareness of it - and why I showed you that actually a 25A device was required for that 19.?? A load.

Any help ?

regards

OMS



Regards

Jobbo


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Failure is always an option
 16 May 2013 11:05 AM
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Avatar for Legh.
Legh

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Another excellent thread....

You'll remember we had a dialogue about this regarding a sample question you set when you were all off on your designers course - your example had at least 5 fully loaded circuits all with a design current approaching 20A - i was demonstrating to you why selecting a CPD of 20A and a cable sized to suit based on a group of 5 wasn't the correct answer - just the one that C&G gave - because they use a simple method - I was challenging why a design course didn't give you the background information to this issue - so at least there is an awareness of it - and why I showed you that actually a 25A device was required for that 19.?? A load.


Now, as I would estimate the problem. If I had a 20A CPD covering a circuit load of 19+A I would expect the CPD to run warmer than average. Therefore its ambient working temperature would be higher than normal bringing it, as you say, closer to its pre-arching point and so reducing its fusing factor.
Maybe this is useful for certain circuits where close overload protection might be required. Not withstanding the extra thermal strain and reduced life placed on the protective device . Other circuits such as motors might not appreciate this level of close protection and would rely on its own overload protection within its control gear.
So then I might utilize the warming effect to advantage. Not sure for a general purpose design that all 5 circuits would benefit though

I assume I'm getting 'warm' in this assumption?

Legh

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 16 May 2013 11:43 AM
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OMS

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On fire, Legh !!

Although to be correct, at 19.XX Amps, the MCB won't be troubled by the cable per se - more that the In of 20A isn't - it's the pre loading by temperature impacting the fusing factor that effectively reduces In below 20A - and probably (in this case) below Ib


Regards

OMS

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Failure is always an option
 17 May 2013 04:38 PM
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Jobbo

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OMS

Sorry for the delay is replying, I've had a really manic week

The information you have provided is very helpful, so thank you. It has given me food for thought, so I will go away and digest it and research a little further.

The 1.45 given in 433.1.1 (iii) I understand is applicable @ 30 degrees hence the adjustment required for buried cable at @ 20 degrees. The 0.9 rating factor adjusts what would be 1.3 back to the required 1.45

I have spoken to Schneider and Draka, who are going to give me some detailed information on this subject.

Thanks again

Jobbo
 19 May 2013 07:31 PM
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Christof

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Joined: 20 April 2013

My understanding is it is based upon upper temp limits, so for thermoplastic 114 C.

The 0.9 is there due to the fact that using a lower ambient then exceeds the upper limit, so derating is required.

How is it you can have a BS 1361 with a 4 hour Conventional Fusing time and a BS 88 with a 1 hour?

Also if we adjust CCC due to lower ambient, this also then exceeds the upper limit of 114 C yet no derating is required as is the case for buried?
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