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A Kurdziel  
#1 Posted : 19 June 2019 10:47:46(UTC)
Rank: Super forum user
A Kurdziel

Most of the equipment we have comes either with a 3 amp fuse or a 13 amp fuse.  Some of our PAT testers have asked whether it is acceptable to replace a 3 amp fuse with a 5 amp fuse if the rating moulded into the plug allows it?  My gut reaction is never raise the rating of a fuse, so always replace like with like.

Any guidance?

George_Young  
#2 Posted : 19 June 2019 10:56:25(UTC)
Rank: Super forum user
George_Young

Originally Posted by: A Kurdziel Go to Quoted Post

Most of the equipment we have comes either with a 3 amp fuse or a 13 amp fuse.  Some of our PAT testers have asked whether it is acceptable to replace a 3 amp fuse with a 5 amp fuse if the rating moulded into the plug allows it?  My gut reaction is never raise the rating of a fuse, so always replace like with like.

Any guidance?

My views would also be to replace like for like, the plug may be rated as 13amps, but there must be a reason why a 3amp fuse was installed by the manufacturer.

I would not be keen on uprating

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A Kurdziel on 19/06/2019(UTC)
chris42  
#3 Posted : 19 June 2019 11:22:41(UTC)
Rank: Super forum user
chris42

The piece of equipment the other end of the wire from the plug determines what size of fuse is required, not what the pug itself can take. So as you say if originally it was three amp then it was three amp for a reason. That reason is likely to be that it is specified either on the equipment ident or in accompanying info.

Chris

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A Kurdziel on 19/06/2019(UTC)
jmaclaughlin  
#4 Posted : 19 June 2019 11:24:44(UTC)
Rank: Forum user
jmaclaughlin

Depends on why your PAT testers are asking, if the equipment is blowing 3amp but not 5amp fuses, it’s less of a risk having  5amp as they don’t create a fire hazard.

Caveat being that you have Residual  Current Devices in place that eliminate any electrical hazard.

If they have simply run out of 3amp then of course not.

 

 

paul.skyrme  
#5 Posted : 19 June 2019 13:32:35(UTC)
Rank: Super forum user
paul.skyrme

The fuse is present to protect the flex and potentially the connectd load.

It fulfills two functions in doing this.

1. Protection against overload, i.e. limiting the current flowing in the cable due to an overload fault in the appliance or connected load, or a short circuit between live conductors, to the overload rating of the nominal capacity of the fuse (which is a funciton of current vs time).

2. Fault protection, by automatic disconnection of supply in the event of  a fault of negligible impedance within the appliance, or, within the cable.  A fault would be an "inadvertent" connection between line and protective conductors.  This will disconnect the supply via the fuse in accordance with the short circuit disconnection time of the fuse, (again a function of current and time).

The main question is why are the PA testing personnel asking this in the first place?

Are the 3A fuses blowing on power on of the equipment?  Are they blowing in normal use?  OR, are they just wanting to reduce the possibility of the fuse blowing?

If the latter, then tell them simply, no.

If the fuses are blowing on power up or in use, and these are IEC leads, are they the correct leads for the appliance. Not all IEC leads are the same.  It needs to be established if the leads are correct for the loads, and thus what the correct fusing is for the load.  Consult the OEM instructions, or contact the OEM.

Lastly, an RCD is NOT the panacea of all evils, they do not protect against overload or short circuit, only live to earth faults.

The for example  25, 40, 60, 80, 100A rating you see on an RCD, is simply the maximum current that must be allowed to flow through the device under normal conditions, ergo, it needs to be protected by ana upstream protective device of that size.  Unless it is an RCBO, a "normal" RCD to IEC 61008 (which is what most people are familiar with) will not detect overcurrent.  Hence the requirement for an overcurrent protective device of the saame current rating as the thermal rating of the RCD.

The 10, 30, 100, 300mA residual rating is the residual rating.  This is the live to earth fault current that the device will detect.

Edited by user 19 June 2019 13:33:57(UTC)  | Reason: Not specified

chris.packham  
#6 Posted : 19 June 2019 13:34:59(UTC)
Rank: Super forum user
chris.packham

I am happy to be proved wrong but it is my understanding that the fuse rating was to ensure that the cable did not take more current than it was capable of and thus presented a potential fire risk. Any electrical equipment requiring overload protection should have its own current overload protection.

paul.skyrme  
#7 Posted : 19 June 2019 13:45:36(UTC)
Rank: Super forum user
paul.skyrme

Originally Posted by: chris.packham Go to Quoted Post

I am happy to be proved wrong but it is my understanding that the fuse rating was to ensure that the cable did not take more current than it was capable of and thus presented a potential fire risk. Any electrical equipment requiring overload protection should have its own current overload protection.

You would hope so Chris, but it is often not so these days as manufacturers make things cheaper and cheaper so they reply on upstream protective devices, nobody wants to pay for safe equipment, everyone wants cheap equipment.

Even if people are willing to pay for safe, the manufacturers still insist on minimum safety and effor for maximum profit, as it is almost impossible to pin the issue on them even if their apliances are dangerous.

Dave5705  
#8 Posted : 19 June 2019 13:54:59(UTC)
Rank: Super forum user
Dave5705

Originally Posted by: jmaclaughlin Go to Quoted Post

Depends on why your PAT testers are asking, if the equipment is blowing 3amp but not 5amp fuses, it’s less of a risk having  5amp as they don’t create a fire hazard.

Caveat being that you have Residual  Current Devices in place that eliminate any electrical hazard.

If they have simply run out of 3amp then of course not.

 

 

IMHO you must investigate why the device is blowing the three amp fuse if indeed the 3 amp fuse is the recommended value, it may have a fault. Simply changing it for a 5 because it is bothersome is foolhardy, and certainly not best practice. If it is blowing the fuse it was designed to operate with, take it out of service till you have rectified the fault.
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SJP on 25/06/2019(UTC)
Connor35037  
#9 Posted : 19 June 2019 15:56:03(UTC)
Rank: Forum user
Connor35037

Originally Posted by: A Kurdziel Go to Quoted Post

If you the know the power consumption (in Watts) divide that by the voltage (240v)

eg 1000w appliance divided by 240 = 4.16 therefore use a 5 amp fuse.

Most of the equipment we have comes either with a 3 amp fuse or a 13 amp fuse.  Some of our PAT testers have asked whether it is acceptable to replace a 3 amp fuse with a 5 amp fuse if the rating moulded into the plug allows it?  My gut reaction is never raise the rating of a fuse, so always replace like with like.

Any guidance?

paul.skyrme  
#10 Posted : 19 June 2019 18:54:48(UTC)
Rank: Super forum user
paul.skyrme

Originally Posted by: Connor35037 Go to Quoted Post

If you the know the power consumption (in Watts) divide that by the voltage (240v)

eg 1000w appliance divided by 240 = 4.16 therefore use a 5 amp fuse.

Unfortunately that only works for purely resistive loads, these are very few and far between these days, stroage heaters, kettles, immersion heaters, showers, generally heating loads, everything else has a "power factor" and, the Ohms law calculation is just an estimate of steady state current.

Also, the nominal voltage which everything is calculated around is now 230V in the UK and has been for many years now.

Roundtuit  
#11 Posted : 19 June 2019 19:44:58(UTC)
Rank: Super forum user
Roundtuit

In my many decades i have noted the decline of in/on device fusing with a switch to rely upon the fuse mounted in the plug where it is only heavy power demand (kettle, toaster) that need the full 13 Amp fuse. Keep with the fitted fuse if it is 3 Amp replace with 3 Amp
Roundtuit  
#12 Posted : 19 June 2019 19:44:58(UTC)
Rank: Super forum user
Roundtuit

In my many decades i have noted the decline of in/on device fusing with a switch to rely upon the fuse mounted in the plug where it is only heavy power demand (kettle, toaster) that need the full 13 Amp fuse. Keep with the fitted fuse if it is 3 Amp replace with 3 Amp
Acorns  
#13 Posted : 20 June 2019 08:14:55(UTC)
Rank: Super forum user
Acorns

I'd be wondering why the PAT testers are asking.  have they found devices with blown fuses, are they doing the replacing of fuses.  If so, even a small degee of electrical competence would help them make their decision. If my device was fitted with a 3a fuse and it kept blowing.  I'd go back to basics and look at the device, the cable and then the fuse to make sure they are all correct.  Many moons ago a relative had a fuse keep blowing on his car - the solution as to roll up a piece of silver foil and voila, all is well and the fuse doesn't blow anymore. Never found out the fault. Simply swopping a 3a to a 5a and ultimately a 10a / 13a is hiding the issue.  I would not uprate the fusing until I had a reason to do so.

 

A Kurdziel  
#14 Posted : 20 June 2019 08:24:43(UTC)
Rank: Super forum user
A Kurdziel

Thanks for these responses; they seem to confirm what I was thinking. So, if a plug comes with a 3 amp fuse you replace it with 3 amp fuse. As far as I know there has not been a spate of equipment suddenly blowing and I would not allow people to uprate fuses if that was the case. I was a bit ingenuous when I said it was PAT testers asking the question; it was only one, morning Nigel, who likes to plough his own furrow and I was preparing myself for a discussion with him about this issue.

I now know what to say!

Neil P  
#15 Posted : 20 June 2019 12:24:55(UTC)
Rank: Forum user
Neil P

If required, you can calculate the fuse rating for each device by looking on the information plate which usually gives a power rating. You already know the mains supply is 240v and this will be on the information plate too.

Dividing the power rating by the supply voltage will give you the current drawn by the device. You can then fit the closest fuse greater than the result.

For example, our kettle in the office is rated at 3000W. Supply voltage is 240V.

3000/240=12.5A so you would fit the closest fuse greater than 12.5A which is 13A.

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billstrak on 21/06/2019(UTC)
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