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Topic Title: Over-current protection & the 'fusing factor'.
Topic Summary: Why are we using a 'fusing factor' when calculating the size of RCBOs' and circuit breakers?
Created On: 13 April 2011 03:12 PM
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 13 April 2011 03:12 PM
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david911cockburn

Posts: 940
Joined: 13 April 2011

Hi All,

I have been asked to bring up this subject on the IET forum because it was thought too complicated for a DIY forum.

Therefore; why are we using a 'fusing factor' in our over-current protection calculations when our over-current protective devices are MCB's and RCBO's?
I will highlight the fact that 4.0mm T&E has a maximum current carrying capacity of 36amps and it has become common practice to 'protect' 4.0mm T&E against over-current, by using a 32amp BS EN MCB or RCBO that does not begin to operate until the current reaches a minimum of 46amps.
 13 April 2011 03:44 PM
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OMS

Posts: 22359
Joined: 23 March 2004

A "fusing factor" is simply a slang term (which has it's origin in the days of fuses) for the multiple of In at which disconnection will occur in conventional time.

Every device has a fusing factor, but for common devices this will be 1.45 x In

As for :

by using a 32amp BS EN MCB or RCBO that does not begin to operate until the current reaches a minimum of 46amps


It's not true to say that the device doesn't begin to operate until circuit current reaches 46A.

The device should not trip at 1.13 x In and must trip at 1.45 x In within one hour - anything in between is a variable.

Hence the origin of the relationship in BS 7671 between minimum cable curent carying capacity in defined conditions (Iz) and the current ensuring effective operation of a protective device in conventional time (I2) in the relationship:

I2<1.45Iz

Regards

OMS

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 13 April 2011 03:44 PM
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AJJewsbury

Posts: 15838
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why are we using a 'fusing factor' in our over-current protection calculations when our over-current protective devices are MCB's and RCBO's

The thermal elements of MCBs and RCBOs have similar characteristics to that of fuses - i.e. if they have to survive indefinitely when carrying their nominal currents, the restrictions of the technology mean that they have to carry significantly more current before they'll open in a reasonable length of time. 'Fusing factor' might be more an historical rather than currently accurate description, but the principle is the same.

I will highlight the fact that 4.0mm T&E has a maximum current carrying capacity of 36amps and it has become common practice to 'protect' 4.0mm T&E against over-current, by using a 32amp BS EN MCB or RCBO

I suspect that the tabulated current carrying capacities of cables have an in-built safety margin which generally takes account of the characteristics of circuit breakers and cartridge fuses. If you look back at older versions of the regulations I believe you will find different tables, with lower current carrying capacities for the same cable sizes, where rewireable fuses were used. Rewireable fuses have a larger difference between carrying current and operating current than other devices (nowadays we have to apply a 0.725 factor manually).

a 32amp BS EN MCB or RCBO that does not begin to operate until the current reaches a minimum of 46amps

I'm not sure that's 100% accurate. The breaker should operate within conventional time (usually an hour) at 1.45x its rating - but that's not to say it won't operate at lower currents over a longer period (down to 1.13x if memory serves). That just reflects the performance of cables - they don't just burst into flames the instant the current goes beyond their rating - they gradually get hotter over a period - the smaller the overload the longer it takes. Usually a cable will carry a small overload for a very long period without coming to significant harm (although the regs do caution us against allowing small overloads of long duration).

- Andy.
 13 April 2011 03:45 PM
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AJJewsbury

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As ever, OMS is quicker than me!
- Andy.
 13 April 2011 03:47 PM
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OMS

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Joined: 23 March 2004

LoL - quicker maybe, but as ever , less eloquent Andy

OMS

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Let the wind blow you, across a big floor.
 13 April 2011 06:12 PM
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david911cockburn

Posts: 940
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Hi All,

Those are without doubt very interesting replies.
But if you look at your history I don't think that you would claim that "every device has a fusing factor", a 'fusing factor' is simply added to our calculations in order to absorb the current spike when a rotating machine starts where the supply cable is protected from over-current by a fuse.

Now the advice we are given in order to avoid over heating our supply cables generally, is to avoid small amounts of over-current for a long duration. Therefore compare a 40amp load being drawn through a 4.0mm (36amp) T&E protected by a 20amp MCB/RCBO (3mins. disconnection time); and protected by a 32amp MCB/RCBO (no disconnection at all).

Now are you beginning to see my point?
 13 April 2011 06:26 PM
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OMS

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But if you look at your history I don't think that you would claim that "every device has a fusing factor", a 'fusing factor' is simply added to our calculations in order to absorb the current spike when a rotating machine starts where the supply cable is protected from over-current by a fuse.


I think I would claim every CPD has a fusing factor as it's almost impossible to engineer a device that can continuously carry current at In but trips when fractionally above In as Andy pointed out.

A fusing factor is useful in dealing with inrush curents but it certainly isn't the origin of it.

Now the advice we are given in order to avoid over heating our supply cables generally, is to avoid small amounts of over-current for a long duration. Therefore compare a 40amp load being drawn through a 4.0mm (36amp) T&E protected by a 20amp MCB/RCBO (3mins. disconnection time); and protected by a 32amp MCB/RCBO (no disconnection at all).


3 mins is hardly detrimental to a cable and I would discount your claim that a 32A won't operate at 40A - it will operate, but in unknown time as you are way above the 1.13 x In "no trip" threshold

Now are you beginning to see my point?


I'm afraid I don't

Regards

OMS

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Let the wind blow you, across a big floor.
 13 April 2011 06:30 PM
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Jobbo

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You are suggesting that Ib exceeds In, therefore a departure with BS7671 alone. PD's cannot be relied upon for overload protection, hence why assessment of the maximum demand of the circuit is essential

Regards

Jobbo
 13 April 2011 06:45 PM
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sparkingchip

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I was going to post this in the one about the shower and the MCB by Grandfortune, might be appropiate here. Whilst under my desk I found a CEF wholesalers handbook, this note is put in after several entries:

"Adjacent MCB's or MCB/RCD's should not be continuously loaded at or approaching their nominal rated currents when mounted in an enclosure.
Under these circumstances, and in common with other manufacturers we recommend a diversity factor of 66% is applied to MCB nominal rated current, especially if the MCB runs at full rating for more than one hour."

Andy
 13 April 2011 07:22 PM
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OMS

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Originally posted by: sparkingchip

I was going to post this in the one about the shower and the MCB by Grandfortune, might be appropiate here. Whilst under my desk I found a CEF wholesalers handbook, this note is put in after several entries:

"Adjacent MCB's or MCB/RCD's should not be continuously loaded at or approaching their nominal rated currents when mounted in an enclosure.

Under these circumstances, and in common with other manufacturers we recommend a diversity factor of 66% is applied to MCB nominal rated current, especially if the MCB runs at full rating for more than one hour."

Andy


There are probably two issues going on there Andy;

1 - using the factors gives some diversity across the distribution boardand avoids overloading of teh DB copperwork

2 - it reflects that "ganged" MCB' s have a mutual heating effect and as such In will drop as the stated In is at a declared temperature (either 30 or 40C) - if the breaker runs hot, it's already into the very first part of its thermal tripping component

Given a derating of 66% is supect it's encompassing both of teh above - it's more usual for derating factors to be given just for point 2

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 13 April 2011 07:31 PM
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sparkingchip

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So it would be good practice to spread spare ways across the board rather than grouping the MCB's at one end thus leaving all the spare ways at the other end based on this guidance.

Andy
 13 April 2011 07:34 PM
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GeoffBlackwell

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First the terminology - fusing factor is not, and has never been, defined in BS 7671 or its predecessors back at least to the 12th Edition - however it was used in notes in the 14th Edition attached to the definitions of 'Coarse Excess Current Protection' and 'Close Excess Current Protection'.

It is defined in BS 3036 and BS 1361 and it was used in BS 88, but it has been replaced in the current version by definitions of conventional tripping current and conventional non tripping current. This terminology is also used in BS 60898 and BS 61009. BS 3871 defines its tests requirements a little differently to other standards, but the net result is similar.

Its effects have been specifically included in the regulations and tables of current ratings since the 14th Edition. It is indirectly implied in the 13th Edition by the fact that all tables assumed that all types of CPD would have the same characteristics as a rewireable fuse!

It has nothing to do with current spikes or the like - it is, as OMS and others have indicated, simply related to the way CPDs work and engineers have known all about it since the year dot .

If current spike protection is required there are a range of methods of dealing with it such as mechanical time delays in circuit breakers or alloy slugs on the silver elements of BS 88 fuses (type 'M').

Regards

Geoff Blackwell
 14 April 2011 06:45 AM
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briggsy6

Posts: 500
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Credential check:

http://www.amazon.co.uk/Illust...&qid=1302759770&sr=1-1

Use the facility to read a few pages and see what you think.
 14 April 2011 08:21 AM
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alancapon

Posts: 6881
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Oh yes. . . I remember this from discussions here a few years ago!

Regards,

Alan.
 14 April 2011 08:24 AM
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OMS

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Joined: 23 March 2004



OMS

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 14 April 2011 09:59 AM
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OMS

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Originally posted by: sparkingchip

So it would be good practice to spread spare ways across the board rather than grouping the MCB's at one end thus leaving all the spare ways at the other end based on this guidance.

Andy


In principal yes, although it's not common practice to do that - good design should really ensure Ib is significantly less than In for numerous reasons (protective conductor currents, avoidance of nuisance tripping, avoidance of inrush tripping etc etc) - the outcome of which is that the breakers shouldn't be running hot anyway (try to design circuits etc such that conductor temps aren't greater than say 62C in normal use is one method0

regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 14 April 2011 10:50 AM
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pmenetwork

Posts: 353
Joined: 20 September 2009

"in the days of fuses"

nice one! [we are up to BS88-3:2010 now]

the tables, [41] are being prepared as we speak for these new fuses,

Amendment,[ sorry correction 2] awaits!

RRP £120, concessions for retired forum members.

half day, [sorry 3 hour] seminar only £199 + vat.

completion of this important seminar leads to a prestige certificate.
"C and G, low voltage fuse calculations."

A "must" for anyone using these devices in their profession.







from the OMS archive of rewireable fuse publications .
 14 April 2011 10:53 AM
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OMS

Posts: 22359
Joined: 23 March 2004

Do you actually have a point you are failing to make ?

OMS

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Let the wind blow you, across a big floor.
 14 April 2011 11:57 AM
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perspicacious

Posts: 7854
Joined: 18 April 2006

I think he's spat them out on the floor

Regards

BOD
 14 April 2011 01:16 PM
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rocknroll

Posts: 9541
Joined: 03 October 2005

RRP £120, concessions for retired forum members.


Why should they have any concessions, most of them are running around in their Lada estates earning a bit of easy pocket money doing some PIR's to supplement their pension.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
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IET » Wiring and the regulations » Over-current protection & the 'fusing factor'.

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