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?

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

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

Originally Posted by: 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.

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.

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.

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.