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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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 14 April 2011 02:28 PM
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OMS

Posts: 22359
Joined: 23 March 2004

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.


Bugger - that's my retirement plan rumbled then, although I'm not sure a lada appeals to me that much

OMS

-------------------------
Let the wind blow you, across a big floor.
 14 April 2011 04:12 PM
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ebee

Posts: 6337
Joined: 02 December 2004

Originally posted by: briggsy6

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.



Aw right, an expert no less!

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 15 April 2011 02:12 PM
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marclambert

Posts: 329
Joined: 23 June 2010

This is the conversation I think OMS is referring to.
As you say ebee, an ex-spurt.
http://www.diynot.com/forums/v...a73b2a6b18ef625e403cf4
 15 April 2011 02:17 PM
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OMS

Posts: 22359
Joined: 23 March 2004

Ahhh - good old B.A.S - I do miss NonSparky from this forum on occasion

OMS

-------------------------
Let the wind blow you, across a big floor.
 15 April 2011 02:37 PM
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rocknroll

Posts: 9540
Joined: 03 October 2005

Originally posted by: OMS

Ahhh - good old B.A.S - I do miss NonSparky from this forum on occasion

OMS


I gotta admit he is missed as some of us enjoyed his posts of which believe it or not were mostly right, but he had to make way for the PIR and Part P appreciation society (the culprits who ousted him) who have continued to bore us to death over the years.

I always liked this bit, a classic!

Will people be led into carrying out potentially dangerous electrical installations?

There's already at least one out there - the author.


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!"
-------------------------
 15 April 2011 02:43 PM
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OMS

Posts: 22359
Joined: 23 March 2004

LoL - take a chill pill my friend - take an hour out to have a listen to Sugerland, The Incredible Machine - not my usual scene but worth a listen

OMS

-------------------------
Let the wind blow you, across a big floor.
 15 April 2011 03:01 PM
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John Peckham

Posts: 8783
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"If for example you were stood on a block of polystyrene when coming into direct contact with a live conductor, the human body will simply fill to capacity with earth leakage current. "

An interesting bit of theory. I would love to read the book but I am not to keen to pay the price.

Has anyone done any experiments to investigate the dialectric strength of the soles of shoes.? The bigger your feet the larger the capacitance and you know what they say about men with big feet?

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 15 April 2011 03:12 PM
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marclambert

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Is it ..."they have big socks"?
 15 April 2011 03:30 PM
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John Peckham

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No close but not the correct answer.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 15 April 2011 03:37 PM
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OMS

Posts: 22359
Joined: 23 March 2004

If socks is close, it must be big shoes then John

OMS

-------------------------
Let the wind blow you, across a big floor.
 15 April 2011 06:48 PM
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Legh

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I thought I was reading something about 'fusing factor' for MCBs and then ended up reading about some chaps out of date book on bonding. Did I miss the link somewhere ?

Legh


-------------------------

http://www.leghrichardson.co.uk

de-avatared
 15 April 2011 06:53 PM
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ebee

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Purely by coincidence perhaps, the chap who asked the question (The OP) seems to have the same name as the chap who wrote that book

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 19 April 2011 09:36 AM
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david911cockburn

Posts: 940
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OK,
Let's try this again only this time from the beginning and without the 'fusing factor' to begin with.
Storage heating:

If I want to supply 8 x 3.4Kw storage heaters, supplied by 2.5mm T&E, I can supply them via 8x 15amp fuse carriers each containing 15amp fuse wire. What I have created is a 'radial system' comprising 8 x spurs/stub end feeder emanating from a central point (the dist. board).

Alternatively I can supply 8 x 3.4Kw storage heaters using 4.0mm T&E, supplied via 4 x 30amp fuse carriers each containing 20amp fuse wire. But this example cannot be described as a 'radial system' as I no longer have a number of spurs/stub end feeders emanating from a central point, it can also not be used as an example of how 30amp fuses can be used to protect 4.0mm T&E supplying general purpose 13amp socket outlets as the general purpose socket outlets need to be protected from over-current as well as short circuit.

Dave.
 19 April 2011 09:53 AM
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david911cockburn

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Another 'trade secret',

A 30amp ring main supplying 13amp general purpose socket outlets via 2.5mm cable is protected from over-current using a 30amp fuse carrier containing 20amp fuse wire.

The 'fusing factor'.

If I am designing a circuit to supply a rotating machine, I select a cable with (in old money) a current rating which exceeds the full load current of the machine, I then simply multiply the current rating of my cable by the fusing factor (previously 1.5) in order to then choose the size of my fusing arrangments. By doing this the fuse element/wire will be of a size great enough to absorb the starting current; and because the starting current is for a short period only, my cable will not exceed it's full load running temperature during start up.
Alternatively move up to Star and delta.

Now are we getting somewhere?

Dave.
 19 April 2011 10:07 AM
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michaelbrett

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Originally posted by: david911cockburn

OK,

Let's try this again only this time from the beginning and without the 'fusing factor' to begin with.

Storage heating:



If I want to supply 8 x 3.4Kw storage heaters, supplied by 2.5mm T&E, I can supply them via 8x 15amp fuse carriers each containing 15amp fuse wire. What I have created is a 'radial system' comprising 8 x spurs/stub end feeder emanating from a central point (the dist. board).



Alternatively I can supply 8 x 3.4Kw storage heaters using 4.0mm T&E, supplied via 4 x 30amp fuse carriers each containing 20amp fuse wire. But this example cannot be described as a 'radial system' as I no longer have a number of spurs/stub end feeders emanating from a central point, it can also not be used as an example of how 30amp fuses can be used to protect 4.0mm T&E supplying general purpose 13amp socket outlets as the general purpose socket outlets need to be protected from over-current as well as short circuit.



Dave.


If you are assuming 230V nominal supply, load current is 14.78A per heater. Can't use a 13A socket. Has to be connected via 20A DP switches with 20A type B MCBs.

Overload protection is no longer an issue. Just need fault protection.

So, your example does not make sense

Regards

Mike
 19 April 2011 10:11 AM
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david911cockburn

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Hi Mike,

There are two 3.4Kw storage heaters in the room where I am at the moment, both use 13amp spurs and have done for many years.
 19 April 2011 10:42 AM
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michaelbrett

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Whilst your 13A Sw. FCUs are working 'okay' for many years, you cannot deny that they are effectively working beyond their rated max load current.

I have seen many Sw. FCUs fail where they are loaded to the limit (or just over). This is poor installation practice.

Surely with the discussion you have started, you cannot advocate the practice of 'overloading' a 13A fuse - even if it will work at near to 15A for many hours.

Regards

Mike
 19 April 2011 11:04 AM
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david911cockburn

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Joined: 13 April 2011

Hi Mike,

It's just a trade secret mate.
We all know that a 13amp fuse won't rupture at 15amps even when it carries on at full load for seven hours at a time, it easy and cheap so people do it.

The important point to recognise is that when supplying storage heating we are only protecting against short circuit, but when protecting general purpose 13amp socket outlets we are protecting against short circuit and over-current. This explains the different cabling arrangements.
For two 3.4Kw storage heaters we need to supply a 30amp fuse carrier because the design current is greater than 20amps, but the fuse wire cannot be greater than 20amps or we risk damaging the 4.0mm cable during short circuit.
For a 2.5mm T&E ring main supplying general purpose socket outlets the design current/fuse carrier is 30amps, but in order to avoid overheating the cable during long duration over-current the fuse wire can be no greater than 20amps.

Please now compare the characteristics of 20amp fuse wire and a 30amp BS circuit breaker.

Dave.
 19 April 2011 11:39 AM
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michaelbrett

Posts: 1042
Joined: 28 December 2005

Originally posted by: david911cockburn



Please now compare the characteristics of 20amp fuse wire and a 30amp BS circuit breaker.



Dave.


Ok. Just done a comparison between 20A SE Fuse & (30A) 32A Type B MCB using Amtech Protect.

At 0.4 secs, they trip-out at almost the same time. At high fault currents they seem to operate at almost the same time. The SE fuse pops in & out of overlaying the MCB curve.

Fastest MCB trip time is 0.1s. For all intensive purposes, this can be assumed to be instanteous.

Now whilst I can see your point, unless there is a good reason, the use of MCBs are preferred from a number of standpoints.

However, your SE fuse will take longer to fail under overload conditions. MCBs tend to trip out faster under fault conditions.

I need to go out now, but, will return to this discussion later.

Regards

Mike
 19 April 2011 12:07 PM
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david911cockburn

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No worries Mike,

The point is whether or not we have to use a 'fusing factor' to make the figures add up.
Using either 20amp fuse wire in a 30amp carrier, or a 30/32amp BS circuit breaker the I2 value does not exceed the current carrying capacity of a 2.5mm ring main/final circuit.
Unfortunately using a BS EN 32amp circuit breaker/RCBO the I2 value does exceed the current carrying capacity of a 2.5mm ring final circuit; and the only way to make it seem as if it doesn't is to use a what is essentially a fusing factor (now 1.45) in the calculation.

The correct size of BS EN over-current protective device for a 2.5mm ring final circuit, is 25amp.
The correct size of BS EN over-current protective device for a 4.0mm final circuit supplying general purpose socket outlets, is 20amp.
If you know your history and work things out logically using a modicum of common sense.

Dave.
IET » Wiring and the regulations » Over-current protection & the 'fusing factor'.

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