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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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 08 July 2014 10:49 AM
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Zoomup

Posts: 270
Joined: 20 February 2014

We have required fuse ratings indicated in or on fuse carriers in fuse boxes, consumer units and at distribution boards, either on the carriers or on inside the enclosures etc. But when it comes to fuse replacement by unskilled users, there is no required fuse value indicated at fused connection units or on or in B.S.1363 13 Amp. plugs. (sometimes just a small removable sticky label on a 13 Amp. plug). Does anybody else think that FCUs and 13 Amp. plugs should have a durable notice indicating the required value of replacement fuse required should it need to be changed? Perhaps FCUs could have a little dial (set by the installer|) which can be set before the FCU is finally fixed to its back box, which shows through a small window on the front of the FCU the correct rating of fuse to be fitted, e.g. 3, 5 or 13 Amp. It may add say 50p extra to the manufacturing cost, but it would cause less confusion amongst the "Great Unwashed". Suggestions as to how a 13 Amp. plug could have a durable and settable indication please. Any ideas?

Bye,

Z.
 08 July 2014 10:56 AM
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Simannjo

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Zoomup...
In a couple of your posts, to whom are you referring to with the term "Great Unwashed"?
 08 July 2014 11:00 AM
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OMS

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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 ?

Regards

OMS

-------------------------
Failure is always an option
 08 July 2014 11:45 AM
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AJJewsbury

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I think it's less of an issue with cartridge fuses than rewireable ones. Once a rewireable one has blown there's little or no evidence to the user what the correct rating of the fuse wire should be - hence a label is clearly needed. With Cartridge fuses however the old cartridge remains intact, with its value printed on it, so the old fuse provides the label automatically - admittedly one that can be removed, but we've provided the opportunity for people to get it right. If you want to force them, you'd need physically different cartridge sizes (like I've a feeling the old Wylex plugs did).

On the other hand, in 99% of cases, a 13A fuse will provide adequate safety from faults even where the flex is rated at very much less than 13A for continuous use, as it'll still withstand the fault current for the short duration until the fuse blows. As ever, it's normally left to the appliance to protect itself from overload. Remember that all CE marked appliances should be safe for use across Europe and every one one the continent uses unfused plugs on 16A or 20A circuits.

- Andy.
 08 July 2014 11:58 AM
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daveparry1

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One such instance I had yesterday. Went to check out a non-working cooker hood/extractor unit. I rarely get involved with repairing appliances these days but this was an old customer of mine. Anyway something had happened to one of the lamps, must have developed a short internally as it blew as I could see black burn marks inside the lamp. I then found a couple of tracks on the pcb completely blown away, checking the fuse in the fcu it was of course 13 amps! I've no doubt that if it had been the correct 3 amp fuse it would have blown instead of taking out the pcb tracks. (and would have saved me having to get the soldering iron out to bridge them out)
 08 July 2014 06:44 PM
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mikejumper

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Originally posted by: AJJewsbury
Remember that all CE marked appliances should be safe for use across Europe and every one one the continent uses unfused plugs on 16A or 20A circuits.
- Andy.

I wonder what they do for boiler supplies then?
Or any other appliances that need fusing down.
 08 July 2014 06:47 PM
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mikejumper

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Originally posted by: Simannjo
Zoomup...
In a couple of your posts, to whom are you referring to with the term "Great Unwashed"?

Electricians returning from Glasto perhaps.
 08 July 2014 08:31 PM
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AJJewsbury

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I wonder what they do for boiler supplies then?
Or any other appliances that need fusing down.

I'd guess in France they'd run a dedicated circuit on an appropriate rated DP MCB 3A say (have you seen the size of their CUs???), but suspect that the rest of the continent would just use a convenient room circuit (16 or 20A). Every boiler I've looked inside of had a 20mm fuse or two as the first component after the terminal block anyway, so I doubt it's an issue in practice provided the supply flex is protected from faults.
- Andy.
 08 July 2014 09:23 PM
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Zoomup

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Originally posted by: Simannjo

Zoomup...


In a couple of your posts, to whom are you referring to with the term "Great Unwashed"?


Hello Simannjo,

It is an old Victorian term I believe. It loosely means the mass or multitude of ordinary people. Electrically likened to an "unskilled person" in BS7671. An ordinary bloke or woman.

Bye,

Z.
 08 July 2014 09:31 PM
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rocknroll

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And there was me thinking you meant those who spend 75% of their working day on this IET facebook.

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!"
-------------------------
 08 July 2014 09:34 PM
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Zoomup

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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.
 08 July 2014 09:39 PM
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daveparry1

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The IEE recommends 3 amp fuses up to 700 watts, they don't recognise 5 amp fuses.
 08 July 2014 09:44 PM
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mapj1

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None of those examples are likely to fail in a way that gives a small overload that requires close fusing, and unless the wiring as astronomically long, a 13A fuse would still catch a dead short without actually risking a fire from 0.5mmsq.
this means that although we don't like it, its not likely to give trouble - which is good because elswhere in Europe it could well have an unfused plug into a 16A C type breaker.
The exception might be the transformer if it develops one or two shorted turns - but if a modern CE marked design should have a thermal trip built in -unless shorted out by an exuberant electronics guy in a hurry, as I did to my cordless drill charger when I convinced myself the thermal fuse was faulty and nothing else was wrong - then it all overheats terribly, but actually didn't blow a 3 amp fuse either. Luckily I hadn't bothered to put the covers on either so the plastic case was completely undamaged and I was alerted early by the smell. Not a particularly fine moment I admit..

-------------------------
regards Mike
 08 July 2014 11:29 PM
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broadgage

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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.


At what voltage ? presuming a nominal 230 volts for both appliance design and actual supply voltage, then 700 watts is very slightly over 3 amps, unlikely to matter much in the short term, but hardly conducive to long term reliability.

In the case of a resistive load designed for 700 watts at 230 volts, then at an actual supply of 255 volts, the current will be about 3.4 amps which is more significant.

If the load is a constant wattage load, such as a SMPS, then 700 watts at 200 volts is also significantly over 3 amps.

Allowing for both mains voltage variations, and for appliance manufacturing tolerances, I would consider about 500 watts nominal load rating to be the prudent limit for a 3 amp fuse.
 08 July 2014 11:39 PM
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alancapon

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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.
 09 July 2014 06:48 AM
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GeoffBlackwell

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BS 1363 2008 requires a 3A fuse for 0.5mm2 flex and a 13A fuse for 0.75 to 1.5mm2 flexes.

Fuses rated at 5A or 10A etc are no longer recognized in this standard.

Regards

Geoff Blackwell
 09 July 2014 07:33 AM
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rogerbryant

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We could end all these discussions regarding FCUs and plug fuses as well as the continuing saga of the ring final circuit by following the mainland Europe radial per room system with it's many benefits ;-)

A socket fitted together with the light switch, no unpleasent bending down.

Sockets switch by the light switch for standard/table lamps.

In case of a fault you only lose the lights in one or two rooms, not the whole floor.

Smaller, lighter and safer plugs.

Three sockets in the same space as one UK one.

Best regards

Roger
 09 July 2014 08:05 AM
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broadgage

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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.


I believe that was the theory years ago.
These days though it seems that the fuse in a fused connection unit IS being relied on to protect an appliance.
Consider small extract fans, often require a 1 amp fuse despite the fact that the flex into the fan would be protected by a 3 amp or even a 13 amp fuse.
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.
 09 July 2014 09:22 AM
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OMS

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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

-------------------------
Failure is always an option
 09 July 2014 11:11 AM
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joepostle

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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.


Whilst I agree with this principle (what Alan's saying) this thread does almost imply that the local plug/FCU is kind of redundant. Most of the 'electronic' type devices I can think of are internally fused (with the exception of mobile phone chargers etc) and most of the larger (non 'electronic' appliances) such as kettles & dishwashers I imagine would either burn themselves out (or cause a fire!) or cause the protection device upsteam to trip (kind of what others have been saying-ish).

I have always considered the UK electrical practice(s) to be some of the best in the world but also can see in practice why others are saying what they do.

Just as a curve-ball (to gauge others response), 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!

Sorry to ramble!
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