IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Fuse Switch protection for cable
Topic Summary: Fuse Switch protection for cable
Created On: 13 September 2017 02:23 AM
Status: Read Only
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 13 September 2017 02:23 AM
User is offline View Users Profile Print this message



mzanlongo

Posts: 4
Joined: 16 April 2016

Hello, I'm a little confused in regards to the whole fuse-switch or combined fuse switch set-up. I have a 35mm2 cable rated at 120A that is currently been protected with a 125A fuse-switch (SSFDN1253P NHP with 3x 170M1568 BUSSMANN fuses).

Now, my first query was the fuses dont look like they will protect my cable. I looked for the trip curve of the fuses, but for some reason the trip curve starts at 200A for the 125A fuse. Why is this? Should it not start at 125A? The fuse says its 125A 690V and im using it on a 415V system, so should i reduce the entire curve by 415/690?

Also, looking at the curve, the fuse can protect the cable form a short circuit, but it cant protect it form an overload. I would thus need some other sort of protection for this, like a simocode or a MCB, which would then make the fuses obsolete?

Thanks for the help in advance!
 13 September 2017 10:09 AM
User is offline View Users Profile Print this message



OMS

Posts: 22388
Joined: 23 March 2004

The fuse is rated at 125A and can operate on a system voltage not exceeding 690V

Current is current is current - ie Amperes flow regardless of system voltage - the fuse will operate at about 1.13 x 125A in an indeterminate time

Not idea what curves you are looking at - but you may be seeing the effect that a fuse will carry in excess of 125A for a long period of time if you are looking at a trailing edge fuse curve

Basically it should carry about 1.12 x In forever and should operate at about 1.45 x In within an hour (or possibly 4 hours) - I think this is where you think you are seeing circa 200A on the curve

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 13 September 2017 11:46 PM
User is offline View Users Profile Print this message



mzanlongo

Posts: 4
Joined: 16 April 2016

Hello OMS,

The curve for the "170M1568" 125A 690VAC fuse can be found by Googling "BIF#17056310", the first link is the fuse curve pdf.

For some reason, all fuse sizes shown have curves that start 1.4x to 1.6x the fuse rating, way too high. Not sure why that is?

Thanks for the help
 14 September 2017 01:21 AM
User is offline View Users Profile Print this message



mapj1

Posts: 9557
Joined: 22 July 2004

NO, it is not way too high. you misunderstand how fuse ratings work.
A fuse does not blow at its rated current.

The rated current is the maximum current it can carry 24/7 at all extremes of its environmental specification, and is guaranteed NOT to blow..

To blow however, as you see, needs quite a lot more than this, especially if the fuse is cold.
The same is true of cable rating.
As a noddy example, cable rated for 30A will warm up by 40 degrees at that current, after an hour or so of running.
So in a room at 25 degrees ambient the cable will reach 65.
IF you put 50 A through it instead, it will carry on heating, and after a similar time reach an equilibrium temperature that is rather higher.
Now if you drop an off-cut of cable into your mates tea at just under 100 degrees, it does not explode or burst into flames liek things do in the films, it does not even melt, it just goes a bit floppy.
And in the same way it won't suddenly fail at 10% overload. Or even 50%, although it will get painfully hot to touch after a while
So it is perfectly safe to protect a 30A rated cable with a 30A fuse and the concept scales for other sizes.

-------------------------
regards Mike
 14 September 2017 03:13 AM
User is offline View Users Profile Print this message



mzanlongo

Posts: 4
Joined: 16 April 2016

Hello Mapj1,

The problem I see is that my 125A 35mm2 cable, derated to 100A, is been protected by a 125A fuse that does nothing untill you hit 200A. Doesn't that mean that my cable could be overloaded at 180A forever ,damaging insulation, and my fuse will never do a thing?
 15 September 2017 07:51 PM
User is offline View Users Profile Print this message



Nedryerson

Posts: 105
Joined: 12 December 2009

Hi,

Fuses are useless for overload protection.

ned
 15 September 2017 11:22 PM
User is offline View Users Profile Print this message



mapj1

Posts: 9557
Joined: 22 July 2004

That is a bit sweeping, and I'd say not really true in many cases. Actually by relaying on the same mechanism, heating as would damage the cables they protect, they can be tailored to match the cable characteristics, and be 'slow blow', and allow short duration currents that would if they were sustained be an overload.
Of course if the fuse is inappropriately selected then it may be true, and things like 1mm2 fed from a 30 A fuse would handle a 20A overload badly, but then the same is true if it was an MCB. It is worth noting that normal MCBs do the curved time delay thing with a thermal part anyway and the more complex MCCBs have quite complex electronics to emulate the delay vs over-current characteristic to be a bit like a fuse.

-------------------------
regards Mike
 16 September 2017 06:49 AM
User is offline View Users Profile Print this message



alanblaby

Posts: 754
Joined: 09 March 2012


The problem I see is that my 125A 35mm2 cable, derated to 100A, is been protected by a 125A fuse that does nothing untill you hit 200A. Doesn't that mean that my cable could be overloaded at 180A forever ,damaging insulation, and my fuse will never do a thing?


That's pretty much sums it up.
It wont run forever at 180A, but you havent got the correct over current sized fuses for your cable, so it will make the cable pretty warm if run at those power levels for a long time.

What is the load?

I'm working on a similar thing at the moment, 35mm cable protected by a 160A fuse. The rating of the cable is 154 Amps. A previous inspection says the cable is under-rated for the fuse.
It is under=rated. However, after 4 hours at full load, the cable never reaches 50 degrees, (I've measured it), and using a clamp meter has shown the typical load is not much over 100 amps, so well within the capacity of the cable.
It's been installed for 10+ years, and shows no signs of damage, but, because it has been highlighted as a fault, the cable has to be changed for a larger one - changing the fuses was not considered due to possible future expansion.
 19 September 2017 08:32 AM
User is offline View Users Profile Print this message



Nedryerson

Posts: 105
Joined: 12 December 2009

I repent in sackcloth and ashes.

For example BS1362 fuses

Short circuit.

The disconnection times for a 3A fuse and 13A fuse are identical at normal domestic fault levels.

Overload

A 3A fuse will rupture at anywhere between 5A and 18A and assuming a 1.00mm sq flex then overload protection of the cable is assured within a best case of 0.2 seconds and worst case of 3 seconds.

Whether or not the load is protected would depend on the load itself. ( a flat screen telly drawing 5 - 18A would likely not survive)

So yes, fuses can certainly provide overload protection for a cable. However, if the ruptured 3A fuse is exchanged for a 13A fuse scrounged from an plug on a knackered fan heater in the shed which you could not bear to throw out then overload protection is lost.

As has been pointed out in this forum before a 13A fuse will carry 20A indefinitely (twice the rating of the 1.00mm flex).

Hello, is that the Fire Brigade ?

Ned
Statistics

New here?


See Also:



FuseTalk Standard Edition v3.2 - © 1999-2017 FuseTalk Inc. All rights reserved.

..