500 V 125 Amps fuse body getting overheated(80 c) in only L1 Phase issue.

Have you checked that the connections have been properly torqued (etc)? If you have access to a micro-Ohmeter I'd be inclined to quickly measure across the terminals to check that they are consistent which should also verify that the fuses are intact.

Why was the temperature measurement being made... Is there some other issue being checked for? Have you ruled out optical effects (e.g. reflections)?

Is there a neutral or is it 3W only?

I'd suggest asking the forum mods to move this to the Wiring & Regulations section since while strictly off topic it's better frequented by people who know more about this kind of issue than me.

The most likely causes in my view are either an overload or very heavy loading that was not present when you took the current readings. Or poor  connections between the fuse link and the fuse carrier, or between the fuse carrier and the base, or between the fuse base and the external conductors.

Another less likely possibility is high frequency or harmonic currents that are not accurately shown by whatever instrument was used to measure the current. Not likely to affect only one phase, but possible.

When you replaced the fuses, did you still see L1 fuse hotter than the other two? If so I'd be looking to see what is different between the phases, either about the detail of temperature measurement method , or the cooling of the fuse by it's immediate environment - does the fuse board and its lid allow more airflow at one end than the other for example? Or is one set of terminals heavier because it is at one end.

Depending on the cable type 80C may be not dangerousm but f the fuse is rated 125A, it is running at less than half the rated current, something like a quarter of the rated self heating, it should be running cool.

How did you verify temperature? I have used thermal labels in the past on parts which overheat when no-one is looking or are not safe to touch.

If the load is resistive you could swap two phases to the load and see if the problem swaps with them?

Let us know how you get on.

Mike

Thanks Mr. Jam, This is 3phase load without neutral. You had mentioned about Optical effect.. could you pl tell me bit more about it so that i can check at my end. 

Regarding your forum suggestion i shall change it. 

Thanks Mr. Broadgage for your reply. Can harmonics effect only one phase out of three phase....Will try to check the all fuse links. 

Thanks Mr. Mapj1 for your reply. Even after replaced all fuses with new one, still L1 fuse getting high temperature. We are using Infrared thermometer gun for measuring the fuse temperatures. 

These SFU cabinet doesn't have any forced cooling they just have its environment cooling. Pl refer the picture of SFU cabinet. it will give you more details about its cooling arrangement. 

Cable type is 1kV 2XCRY, Size is 3x35+16 rm & length is 84 mtr, Revisions A.

Be careful with infra-red guns firing at things that are not perfectly matt black - what is measured will be a mixture of the heat radiated from the object itself and reflected heat from other objects, perhaps  outside the box, forming  a 'mirror image' in the surface of the object (not a visible mirror image of course, but an infra-red one - and some items have more or less infra red reflection than they do in the visible wavelengths.).  It is harder to do but more reliable to verify with a contact measurement in cases where there  is  doubt. Also is it like your photo, where L1 is on top ? If so it may be nothing more serious than it is being pre-heated by the ones below.

Mike

521.5.1?

Z.