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Topic Title: FUSE VALUE INDICATION-FCUs and 13 Amp PLUGS.
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Created On: 08 July 2014 10:49 AM
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 09 July 2014 12:21 PM
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mikejumper

Posts: 1803
Joined: 14 December 2006

Originally posted by: alancapon
My understanding is that the fuse in the plug / FCU is designed to protect the appliance flex, not the appliance.
Regards,
Alan.

Every boiler I've connected specifies a supply fused at 3 Amps.
I don't ever remember a specfication for the supply cable size.
So I don't think it's anything to do with protecting the cable;
more a primary level of protection for the circuits within the boiler.
 09 July 2014 12:51 PM
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rogerbryant

Posts: 868
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" I've for ages wondered why they don't fit mini-MCBs to plugs & FCUs"

Basic answer cost!

The ring final circuit may have saved a bit of copper and hence cost for the house builder, but burdened the consumer with a larger and more expensive plug than neccessary. When the consumer had to buy and fit his own plugs the industry didn't care. Now equipment has to be supplied with plugs cost comes into play.

Best regards

Roger
 09 July 2014 04:06 PM
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joepostle

Posts: 72
Joined: 14 September 2011

In reality the plug/FCU fuse protects <i>everything</i> on the load-side. The reason why I would support the theory why appliances should have their own fail-safe mechanisms in addition to the plug fuse is because of the nail & foil substitutes, or indeed a complete lack of protection. I vividly remember in the late 1990's seeing an external 300w/500w floodlight, wired in bell-wire and straight into the back of a socket (no word of a lie).
 09 July 2014 10:18 PM
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alancapon

Posts: 5811
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. . . Likewise domestic heating boilers often require a 3 amp fuse despite being equipped with 0.75mm flex that would be protected by a 13 amp fuse. . .

Most domestic boilers also have internal fuses for protection of certain motors / control units.

Regards,

Alan.
 10 July 2014 10:54 AM
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Zoomup

Posts: 607
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Originally posted by: daveparry1

The IEE recommends 3 amp fuses up to 700 watts, they don't recognise 5 amp fuses.


Yes but physics knows better than mere mortals.
 10 July 2014 11:08 AM
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Zoomup

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Originally posted by: alancapon

My understanding is that the fuse in the plug / FCU is designed to protect the appliance flex, not the appliance. As such, a 13A fuse will cover virtually all circumstances. Where the appliance needs extra protection, then the appliance manufacturer will provide this within the appliance.



Regards,



Alan.


Hello Alan,
that has always been my understanding too. The fuse is there to protect the appliance flex, not to protect the equipment connected from damage due to a fault. Appliance flexes can be subjected to all types of abuse, such as being run under mats or carpets so that over heating would be a problem, or eaten through by pet animals, or overloaded if used as extension leads, etc. with too much load connected. Short flexes are less prone to damage and vulnerability in my opinion, such as 400mm connected from a FCU to a fixed appliance. All interesting stuff on this thread though. I still prefer "close excess current protection" that is where the fuse is equal in value to or only slightly greater than the design current of the load, or in this case appliance, excepting large inductive loads like vacuum cleaners.

Bye,

Z.
 10 July 2014 11:30 AM
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Zoomup

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Originally posted by: OMS

Originally posted by: Zoomup



Originally posted by: OMS



What are you seeking to achieve ?



What specific instances where a 13A fuse has been installed to replace a 3A fuse are likley to be a problem ?



Hello OMS,



thanks for the reply. There must be many cases where a piece of electrical equipment should be fused at less than 13 Amps at a fused connection unit for safety, to prevent overloading of the flex connected and discrimination with an upstream protective device. . 1. A gas boiler central heating system wired in 0.5mm2 or 0.75mm2 flex in places, perhaps some time ago when 0.75mm2 was not the minimum recommended flex size. 2. A small frost heater (say 500 Watts) in an outhouse or greenhouse 3. An oil filled towel rail rated at 100 Watts or less in a bathroom. 4. A cooker hood with a small motor and a 25 or 40 Watt lamp. 5. A Electrocutor fly killer on the wall in a commercial kitchen. 6. A P.A. Amplifier for a public address system. 7. A door bell transformer. 8. A low wattage illuminated shop sign. 9. An electric clock supply. 10. A small door entry system with solenoid opening latch and intercom.



I hope that helps.



Bye,



Z.




It doesn't help - no



Basically none of the items you mention requires a 3A fuse (or even a 13A fuse)



The fuse is simply protection to the flex - nothing else



Whilst I wouldn't reasonably expect to see flex less than 0.75mm protted by anything other than a 3A fuse, if you accept that it's only short circuit we are concerned about then a 13A fuse will protect a pretty small flex.



Good practice suggests we replace the fuses with the right rating, but in practice it's not a real problem



So back to my original post - what are you seeking to achieve and what evidence is there that replacing a 3A fuse with a 13A fuse is a problem



From that point, you should be able to answer the question as to why we don't have a positive and foolproof indication in current product standards (aside from colour of course, and manufacturers instructions) to ensure that incorrect fusing can't be achieved



Regards



OMS


Hello OMS,
What am I seeking to achieve?" I hope to achieve additional safety. Closer protection of flexes due to short circuits AND POSSIBLE SUSTAINED OVERLOADS as well. Long duration overloads that could cause a flex to overheat and cause a fire.

Table 4F3A (Flexible cables, non armoured, copper conductors) shows that 0.75mm2 flex is rated to carry 6 Amps. I would fuse that at 5 Amp, even though that value of fuse is not recognised by BS 7671 to be available for use by the Great Unwashed, because presumably the general public is assumed not to be able to use more than two values of plug fuse, a 3 Amp or a 13 Amp. I would consider that a 0.75mm2 flex would be badly protected if fused by a 13 Amp fuse. Also I can use the flex to carry a load of up to 5 Amps comfortably in most cases. Would I fuse a 2.5 T@E cable at 60 Amps? Why should I fuse a 0.75mm2 flex at 13 Amps., when the flex is only rated at 6 Amps? Irrespective of the "actual" CCC of the flex for short duration faults.

We spend ages calculating Ib for fixed cables circuits, and calculating Iz cable values to provide safety, to prevent damage to cables by over heating, but when flexes are used we just say, oh well a 13 Amp fuse will do, even though that fuse can allow an enormous amount of heating to occur before it even thinks about rupturing and opening the circuit.

I believe in "close" protection and I still use 3, 5, 10 and 13 Amp fuses, with appropriate labelling to show the value of fuse to be inserted as a replacement in FCUs.

Bye,

Z.

Edited: 10 July 2014 at 11:46 AM by Zoomup
 10 July 2014 11:37 AM
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Zoomup

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Hello,
a mini trip in a FCU seems like a good idea. But possibly there are locations where a mechanical switch is not allowed to be sited. Also a cartridge fuse to BS 1362 is simple cheap and reliable. It also has a reasonably high rupturing capacity for such a small fuse.

Bye,

Z.
 10 July 2014 11:47 AM
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OMS

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I still prefer "close excess current protection" that is where the fuse is equal in value to or only slightly greater than the design current of the load, or in this case appliance, excepting large inductive loads like vacuum cleaners.


I believe in "close" protection and I still use 3, 5, 10 and 13 Amp fuses, with appropriate labelling to show the value of fuse to be inserted as a replacement in FCUs.


OK - I guess the concepts of "close" and "coarse" protection tend to date you somewhat - it's been a while since we've had that debate on here.

Take a look at the current a 13A fuse can pass for what time - I think you might be suprised.

Then take a look at time constants for cables and determine how long it takes to heat up a 0.75 flex to say 60C conductor temperature and contrast that with the fuse (or actually the fusing factor of the fuse and the actual current passed over time)

Then take a look at relevant appliance standards to see the manufacturing approach to thermal overheating/overload conditions etc

I'm not suggesting your approach is wrong - just not based on the full set of facts related to appliances, flexes and the means of connection to fixed wiring systems

Regards

OMS

-------------------------
Failure is always an option
 10 July 2014 01:22 PM
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AJJewsbury

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We spend ages calculating Ib for fixed cables circuits, and calculating Iz cable values to provide safety, to prevent damage to cables by over heating, but when flexes are used we just say, oh well a 13 Amp fuse will do, even though that fuse can allow an enormous amount of heating to occur before it even thinks about rupturing and opening the circuit.

Indeed - but the situation of fixed wiring supplying say a number of socket outlets and a flex supplying a single appliance is rather different. Essentially the fixed wiring is at risk of overload because the user can plug in what they like - so we need to protect against that. The flex on the other hand only has the one known appliance drawing current through it - so it can't be readily overloaded, since there's no facility for connecting extra loads. The single appliance could be subject to faults of course - ones that appear like short circuits (i.e. draw very large current) would be quickly cleared by even a 13A fuse. Ones that appear more like an overload (e.g. because a motor has jammed or a winding had shorted turns) have to be dealt with internally by the appliance - often by internal fusing or by thermal cut-outs or similar. Either way the flex is usually fine.

Things like home-made extension leads (were the load is unknown) may need the plug fuse need to provide overload protection for the flex (commercial ones for domestic use usually have a downstream fuse or thermal trip or have a dedicated connector so that only a single appliance can be connected - typical of ones for garden tools).

I guess the concepts of "close" and "coarse" protection tend to date you somewhat

And Zoomup seems to be using the term in a slightly novel way, which might be confusing. I believe the original terms related not to the relationship between the load and the fuse, but between the fuse's nominal rating and the current that would cause it to open (I think in 4 hours), course was under 2.0 (e.g. rewireable fuses) and close under 1.5 (e.g. many cartridge fuses) - which then influenced the size of conductor where overload protection was required. We have the same idea still - but have 1.45 instead of 1.5, and a 0.725 correction factor for sizing cables for overload protection when using rewireable fuses.

- Andy.
 11 July 2014 10:41 AM
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perspicacious

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"I've for ages wondered why they don't fit mini-MCBs to plugs & FCUs. Just in the same way MCBs are now standard in DBs which has the obvious benefits of faster response than fuses (which is kind of implied in this thread re plug fuses) and virtually eliminates 'user error' of incorrect fusewire and nails etc in DBs; with the advent of moulded plugs the manufacturer could pre-fit a correctly rated mini-MCB. Would be interested in your thoughts to my whacky thought!"

On being called out to a tripping circuit-breaker, how many electricians reset the device and then when it trips, get the test gear out?

If electricians do this when they have the kit, how many times do you think the householder will "stress out" the circuit if this can be readily achieved with a circuit-breaker?

Regards

BOD
 11 July 2014 11:51 AM
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daveparry1

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I think it would be unreasonable to expect even an electrician to start doing tests without trying a re-set first BOD, unless there was something obvious. Different with a re-wireable fuse of course where molten fuse wire can be seen!
 11 July 2014 12:16 PM
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OMS

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Originally posted by: daveparry1

I think it would be unreasonable to expect even an electrician to start doing tests without trying a re-set first BOD, unless there was something obvious. Different with a re-wireable fuse of course where molten fuse wire can be seen!


Matter of scale I guess Dave - personaly I wouldn't want to reclose say a 4000A ACB without knowing why it tripped - call me chicken, but they go off with a hell of a bang if there is still a short circuit or similar in place

At the other end of the scale however, when I had a real job, I made plenty of money on the bonus scheme by getting the apprentice to flick the MCB back on a number of times whilst I wandered round firstly listening/looking and then splitting circuits.

Usually you could find a fault at the second or third "split" - of course you claimed bonus for every single termination on the whole circuit being "tested" - and "flick operation" no doubt plays merry hell both with the MCB and the fixed wiring

Regards

OMS

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Failure is always an option
 11 July 2014 12:27 PM
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daveparry1

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True OMS, this is where my "sheltered life" as doing mainly domestic work shows up!
 15 July 2014 03:10 PM
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AJJewsbury

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At the other end of the scale however, when I had a real job, I made plenty of money on the bonus scheme by getting the apprentice to flick the MCB back on a number of times whilst I wandered round firstly listening

Our local DNO still uses that method... (although with big fuses of course rather than MCBs)
- Andy.
 15 July 2014 05:56 PM
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OMS

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For sure Andy - I've seen it done with 800A distribution fuses in the hope of getting a bit of burn back on a cable so either it is clear or full down to earth/between

It's a bit worrying watching a 0.2" PILC burning away for what seems like an eternity before the fuses clear

Regards

OMS

-------------------------
Failure is always an option
IET » Wiring and the regulations » FUSE VALUE INDICATION-FCUs and 13 Amp PLUGS.

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