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Old 30th Jun 2019, 4:15 pm   #21
Lucien Nunes
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Default Re: 13 amp fuse for 1.5mm mains flex?

To put this in perspective - your 1.5mm cable fused at 13A will probably run twenty of your HiFi systems at once without damage.

I did a demonstration a few years ago with a Philips CD650, a preamp that I can't recall, and a Quad 520 rackmount power amp (note, not a high-efficiency PWM type) driving Kef Concertos (note, not especially efficient speakers). I played rock music continuously at the highest volume I trusted the Kefs to withstand, that could be heard down the street and across the car park. The setup was supplied via.... wait for it.... a 315mA fast blow fuse connected in the mains feed. I.e. less than a third of an amp for more than living-room volume. The only caveat was the switch-on surge of the amplifier was too great for the fuse, it had to be bypassed for a fraction of a second at switch-on to let the reservoir caps charge.

You mention 2.5mm cable which will carry 20A safely and continuously. There is no need to use this for any normal domestic application fed from a 13A plug. A stage sound system with dozens of large speaker cabinets, powerful enough to fill a hall of 5000 people with ear-splitting rock complete with room-shaking bass, barely takes 20A running load.
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Old 30th Jun 2019, 5:06 pm   #22
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Default Re: 13 amp fuse for 1.5mm mains flex?

Ah, but everybody knows thick mains cable sounds better
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Old 30th Jun 2019, 6:40 pm   #23
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Default Re: 13 amp fuse for 1.5mm mains flex?

Indeed, under almost all circumstances I believe that extension leads should use 1.5mm flex and be fitted with 13 amp fuses.

Under certain circumstances, and under skilled supervision, other arrangements may be acceptable or even preferred, but for general use, 1.5mm cable and 13 amp fuses are preferable and allow for uses other than that first planned for.
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Old 30th Jun 2019, 9:18 pm   #24
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Default Re: 13 amp fuse for 1.5mm mains flex?

The current-carrying capacity of cables is mainly dependent on how much heat they can dissipate. A cable has a (small) resistance and that coupled with the current flowing through it will produce heat. A cable in free air can dissipate more heat than a cable rolled up on a cable drum. See post 18 where the cable on the drum had melted whereas the unrolled cable was OK. (A decent drum ought to have a thermal trip. That's quite a common problem). I use the rule of thumb that cables are fused at 6x cross section +1 amp. So for a 1.5mm2 cable I would use a 10A fuse/circuit breaker, for 2.5mm2, 16A and for 4mm2 25A. That's very conservative based on cables in conduits in walls. 13A for a 1.5 cable in free air should be no problem.
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Old 30th Jun 2019, 9:54 pm   #25
Lucien Nunes
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Default Re: 13 amp fuse for 1.5mm mains flex?

Your rule of thumb may serve you well; I prefer to stick to the published tables and derating factors based on extensive research, dealing with all sorts of operational parameters such as installation method, grouping, characteristic curves of protective devices, ambient temperature, etc.

However, what the OP needed to consider specifically was the rating of general purpose flexibles such as H05VV-F and H07RN-F, for which there are standardised ratings when used for portable appliances (as opposed to being part of an electrical installation).

0.5mm : 3A
0.75mm : 6A
1.0mm : 10A
1.25mm : 13A
1.5mm : 15A
2.5mm : 20A

Provided that overload protection is included in an appliance to which the flex is attached, only short-circuit protection is required from the plug fuse. All sizes from 0.75mm are adequately protected against short-circuit damage by a 13A fuse (and indeed by a 16A, hence 0.75mm cables are fully acceptable when used with unfused plugs in Europe protected by 16A OCPD in the distribution board).

Where overload protection is not provided at the load, such as in an extension cable, then either a suitable fuse must be fitted, or preferably, the cable should be able to withstand 13A continuously, for which a minimum of 1.25mm is specified. Most commercially made 13A extension leads use 1.25mm cable, although heavy-duty industrial types are typically 1.5mm. 13A plugs and trailing sockets to BS1363 are not designed to accept a larger cable than this.
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Old 30th Jun 2019, 11:04 pm   #26
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Default Re: 13 amp fuse for 1.5mm mains flex?

My main concern is a 13 amp fuse can at run at 20amps continually. I was a little concerned the 1.5mm cable rated at 15amps might have been a compromise. Hence the use of 2.5mm cable. Nothing to do with it sounding better.

But from the replys above, 1.5mm is more than plenty.
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Old 30th Jun 2019, 11:13 pm   #27
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Default Re: 13 amp fuse for 1.5mm mains flex?

My first house had a 100' garden. To minimise voltage drop, I bought a 50m drum of 2.5mm TRS cable to make up two approx 20m extension leads. The cable has string reinforcement laid in with the conductors. I found that the 13A plugs and extension sockets available at the time (1976) would just accommodate the cable. I have noticed that current extension sockets seem to have a smaller cable entry than my old Duraplug version. The mains cable of the circa 1953 washing machine that came with the house had a mains cable of about the same external diameter as my extension lead. I guess that experience has found that flexible cables used to be unnecessarily conservatively rated.
Attached Files
File Type: pdf 2.5mm extn cable.pdf (575.7 KB, 39 views)

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Old 1st Jul 2019, 12:30 am   #28
Lucien Nunes
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Default Re: 13 amp fuse for 1.5mm mains flex?

You do not have to apply any extra 'safety margins' to allow for the characteristics of ordinary fuses or circuit breakers; they have already been taken into account in the tabulated rating of a cable. For example, a cable rated at 13A will be adequately protected by a 13A fuse, or put another way, whatever a 13A fuse will withstand before rupturing, so will the cable.

The general principle to be observed is:
IB<=In<=Iz
where
IB = design load current
In = rating of protective device
Iz = tabulated rating of cable, multiplied by any applicable correction factors for temperature etc.

Actually it is a little more complex than this under some conditions and with non-standard types of protective device, but that rule holds good for normal cartridge fuses and domestic circuit breakers.


Quote:
I bought a 50m drum of 2.5mm TRS cable... I found that the 13A plugs and extension sockets available at the time (1976) would just accommodate the cable.
You might have had success installing that cable in some brands of accessories, but to the best of my knowledge BS1363 never required them to accommodate it. Judging from your picture, the sheath radial thickness is less than that of a modern H07-series cable that is usually specified for heavy duty outdoor applications, so it might have been more obliging.
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Old 1st Jul 2019, 1:05 pm   #29
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Default Re: 13 amp fuse for 1.5mm mains flex?

I've been wondering recently whether lowering the fuse in this situation is actively making things less safe due to increased risk of the fuse operating close to its rating and getting warm/hot but not blowing?
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Old 1st Jul 2019, 1:32 pm   #30
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Default Re: 13 amp fuse for 1.5mm mains flex?

There'd be a lot less power being dissipated in the lower fuse rating in this situation, so wouldn't be significant - unlike when running a 13 amp fuse close to its rating.
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Old 1st Jul 2019, 1:38 pm   #31
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Default Re: 13 amp fuse for 1.5mm mains flex?

Quote:
Originally Posted by winston_1 View Post
Fuses go in to plug bottoms, not their tops.
No, the plug top is the whole plug part that plugs into the plug bottom (if you like) which is the wall socket, so the fuses go into the plug tops
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Old 1st Jul 2019, 1:55 pm   #32
Lucien Nunes
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Default Re: 13 amp fuse for 1.5mm mains flex?

Quote:
No, the plug top is the whole plug part
There has been an epic thread on another forum regarding the significance of the term 'plug top' meaning 'plug'. It has been in existence for a century although now confined mainly to the electrical trade and considered archaic. My proposition, which has as much support as any other, is that it originated from the historical practice of specifying and supplying and a plug and socket as a pair. Sockets were often installed to serve a particular appliance, which itself was supplied without plug. Hence, the matched pair of components, and in an architectural context the socket alone, was listed as a 'plug', 'wall plug', 'switch plug' etc. When considered separately, the detachable part became the 'plug top'.

Apart from reducing the 'splat' in the rare event of a cable being severed, by reducing the It let-through, I see no advantage in fusing an extension cable down below its rating. Conversely, in the general case (i.e. not the OP's particular situation) cables with undersized fuses that blow unexpectedly in the absence of a fault are prime contenders for fuse-bypass-bodgery in the heat of the moment. I would therefore always advocate the use of the correct fuse for the cable rating.

An exception might be to detect misuse in specific circumstances, where it is known that heating appliances are being plugged into tech supplies etc by the night guards. Of this I have direct experience; a 5A fuse was found to reliably discriminate between a 20" monitor and a Rhino heater.
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Old 1st Jul 2019, 2:49 pm   #33
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Default Re: 13 amp fuse for 1.5mm mains flex?

Thank you Lucien Nunes for the advice, and thank you to everyone else for your comments.

I tend very much to over think things when it is not necessary.

It did puzzle me why audio cable manufacturers were fitting 13amp fuses in their mains cables, for low powered hifi components. The reason being (I found out later) is to reduce the impedance of the cable, leaving the internal fuses in the equipment to protect the device. Whether the low impedance from the cable has any impact on how your hifi sounds, I have no idea.
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Old 1st Jul 2019, 3:15 pm   #34
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Default Re: 13 amp fuse for 1.5mm mains flex?

Part of the issue is switch-on surges: a 'beefy' amplifier can draw a surprisingly large surge-current if the power-switch is closed at the right point in the AC cycle. Anti-surge fuses in the amp itself are specified to handle this but there are no anti-surge fuses available for fitting in the plugs themselves.

A 13A fuse in the plug avoids such 'nuisance blowing', even though it may seem vastly overrated for steady-state working.

It's not just amplifiers etc. that can have issues like this: at a previous gig the company contracted to PAT all the IT equipment discovered to their horror that all the computer-monitors were fitted with 13A fuses in the plugs, despite the monitors - big CRT ones - being rated to take 300 Watts. So they swapped all the 13A fuses for 3A ones 'to be safe'. After a few days we started having monitors failing to power-up in the morning, the issue being that though they only took an Amp or so in steady-operation, when powered-on the degauss system activated and this caused quite an impressive current-spike for a mains-cycle or two. Though this didn't blow the fuse the first couple of times it sufficiently weakened it that it was doomed to failure.

I sent someone out with the company credit-card to scour the neighbourhood and buy up every 13A fuse they could find. We needed a couple of hundred of them!
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Old 1st Jul 2019, 3:49 pm   #35
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Default Re: 13 amp fuse for 1.5mm mains flex?

I can't be the only person here to have a large jar of (in many cases new) 13A fuses that I have taken out of mains plugs to replace with 3A (or 5A or...) ones. The new ones, of course, came from new plugs which almost always come with a 13A fuse.

As for things that need 13A fuses, my Philips P850 minicomputer has a pair of linear PSUs. The normal running current is quite low, but the switch-on surge (which causes the transformes/chassis to give a very loud clonk as things move under the magnetic field) will blow a 3A plug fuse. As there are internal anti-surge fuses in the PSUs, I have no worries about puting a 13A fuse in the plug.

I doubt the different impedance of a 3A and 13A fuse makes any difference to the sound (at least to non-audiophools). I can't see it making much difference to the total impedance of distribution cable from the substation, electricity meter, ring main cable, power cable, etc.
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Old 1st Jul 2019, 3:52 pm   #36
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Default Re: 13 amp fuse for 1.5mm mains flex?

Personally, I tend to fuse down to the requirement of the equipment at the other end of the plug. A lot of my plugs have 1A or 2A fuses in them, as the equipment itself often has no fuse. 'Fusing down' takes nothing away in terms of protection, but can add some (across-line RFI capacitors failing short-circuit, for example). But an extension lead, I'd fuse to the rating of its cable, as I don't know what could be plugged in, and nuisance fuse-blows are a pain. And, switch-on surges (motors, big incandescent lamps, degaussing coils) need to be considered when selecting fuses (you can't get anti-surge in 1" mains-plug fuses).

Quote:
Originally Posted by Techman View Post
Quote:
Originally Posted by winston_1 View Post
Fuses go in to plug bottoms, not their tops.
No, the plug top is the whole plug part that plugs into the plug bottom (if you like) which is the wall socket, so the fuses go into the plug tops
Quote:
Originally Posted by Lucien Nunes View Post
There has been an epic thread on another forum regarding the significance of the term 'plug top' meaning 'plug'. It has been in existence for a century although now confined mainly to the electrical trade and considered archaic. My proposition, which has as much support as any other, is that it originated from the historical practice of specifying and supplying and a plug and socket as a pair. Sockets were often installed to serve a particular appliance, which itself was supplied without plug. Hence, the matched pair of components, and in an architectural context the socket alone, was listed as a 'plug', 'wall plug', 'switch plug' etc. When considered separately, the detachable part became the 'plug top'.
And of course, the thing on the wall with a switch and 3 small slots, is a 'plug socket.' Any estate agent will confirm that.

And the thing that's 0.25" diameter and has a sleeve and an end electrode and goes into a hole, is a 'jack plug.'
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Old 1st Jul 2019, 4:43 pm   #37
Lucien Nunes
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Default Re: 13 amp fuse for 1.5mm mains flex?

Quote:
The reason being (I found out later) is to reduce the impedance of the cable
The idea circulates in the HiFi world that this is significant, but it isn't. A typical mains transformer in a large domestic power amplfiier has a primary resistance of a few ohms, possibly 10-20 ohms in a smaller one. Yet the resistance of a 5A fuse is a tiny 12 milliohms cold and a 13A fuse perhaps three milliohms. Saving nine milliohms in a circuit with a total resistance of many ohms is trivial.

If one insists on disregarding the load resistance and considering only the supply impedance, in a domestic situation that is likely to be in the order of half an ohm, so the fuse still only accounts for a few percent. Moving the equipment plug from one socket to another just one metre further from the fuse box would have a greater effect on the sound. Clearly, no commercial equipment could be made that intolerant of external conditions.

Or could it... further discussion invited in the audiophool thread!
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Old 1st Jul 2019, 7:41 pm   #38
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Default Re: 13 amp fuse for 1.5mm mains flex?

I am not an audiophool, just curious to learn, and experiment with different ideas. This way I can learn from experience, as apposed to just excepting what I am told. Of course I apply common sense to anything electrical, and seek advice if I am unsure how to proceed.

I think audiophool is overused sometimes, and I am certainly not one.
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Old 2nd Jul 2019, 12:54 pm   #39
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Default Re: 13 amp fuse for 1.5mm mains flex?

Quote:
Originally Posted by kalee20 View Post
Personally, I tend to fuse down to the requirement of the equipment at the other end of the plug.
The correct way is to fuse to the requirements of the cable conencting the equipment to the mains supply. In nearly all cases, the fuse is there to protect this lead - the equipment itself will usually have its own fuse that the manufacturer has calculated appropriately.

What disturbs me the most is the number of items bearing a PAT Test sticker that come my way, and have the wrong fuses in the mains plug!
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Old 2nd Jul 2019, 1:30 pm   #40
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Default Re: 13 amp fuse for 1.5mm mains flex?

I have seen 4 way extension leads on sale marked 10 amps and fitted with a 10 amp fuse.
With 6 amp C13 cables I normally fit a 5 amp fuse.
If the equipment has a big heave transformer I have a small number C13 of cables from the US that have a thicker cable and fit 13 amps to them.
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