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Topic Title: Fuse switch or switch fuse?
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Created On: 14 May 2007 07:07 PM
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 14 May 2007 07:07 PM
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deapea

Posts: 370
Joined: 13 May 2007

Hi,
As far as I can remember a fuse switch isolator is the type where the switching action physically removes the fuse so in other words the fuse is the switch. A switch fuse has a fuse in series with the switch. Can anyone explain the applications of both isolators and and what criteria would influence the choice between the 2? Thanks.
 14 May 2007 07:38 PM
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deleted_johnwoods

Posts: 997
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You are right about the types.
I have only ever seen switch fuses and don't know what applications a fuse switch would be used for as such. I can't think what difference it would make, unless it relates to very high current, hopefully someone else will know that as I am interested myself now.

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 14 May 2007 10:08 PM
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mark2spark

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I've only seen the 'fuse switch isolator' that you describe in big stuff, in factory type enviroment. Looks a bit like a guillotine handle, which brings the fuse (in a holder) out of two spring loaded busbar type connections.
So, to my mind, the preference would be solely down to the physical size of the fuse, which in the instance above, is about 3" or 4" long, about the size of a plastic bullet.
I would also think that modern breakers make this type of fitting obselete now.

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 14 May 2007 11:45 PM
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Jaymack

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These definitions should explain all.

A fuseswitch has a mechanism by which upon operation a carriage moves in and out of the main contacts. The fuses are mounted on this spring assisted carriage - the fuses actually move. Usually bulky items, these are also used where both isolation and circuit protection are required. Commonly installed where high fault levels are present. Fuse switches are able to serve loads up to 800A

A switchfuse still has contacts but these are normally spade contacts on a hinge. The fuses are mounted separately in a fixed position, the fuses don't move. Used to serve electrical equipment where both isolation and circuit protection are required. Normally only available to serve load up to 125A, switchfuses do not have any form of internal compartmentation and are not able to be used where very high fault levels are present.

In my first year of apprenticeship in the steel industry, I was asked by a lampman to open the cantact blades of a switchfuse which had welded in, (it is easy to open these diecast iron covers, lampmen were unskilled and employed in the steel industry to change lamps and carry out minor work). Anyway this lampman had more confidence in my abilities than he should have had, I proceeded to place a screwdriver between the iron body of the switchfuse and the blades, not realised that the blades were live. Fortunately the only damage done was to my screwdriver.

Jaymack
 15 May 2007 09:26 AM
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deapea

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Thanks for the info'. I agree with your description Jay and as I recall the switching of the larger fuse switches sometimes involved the use of a telescopic handle to generate the leverage needed to get the snap action to "throw" the fuses out. So it seems that the choice between the 2 is down to current carrying capacity where fuse switches go higher.

Just a thought, because the fuse switch has such a positve snap action when operated does this mean it can be used as an "on load isolator."
 15 May 2007 09:57 AM
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Jaymack

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Originally posted by: deapea

Just a thought, because the fuse switch has such a positve snap action when operated does this mean it can be used as an "on load isolator."


The speed of operation of this fuseswitch, (on opening of the contact blades), is outwith the control of the user, due to the "pre-spring charging" feature - so I would think they were on load isolators, provided they were selected to withstand the particular fault level.

Jaymack
 15 May 2007 11:51 AM
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pnorton

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I agree with the replies given to this post.
But another benefit of the fused-switch is that there is freedom to have the supply cable entering the top and load cable at the bottom or vice-versa. This makes them ideal for use with bus-bar chambers where it may be necessary to install switchgear above or below the chamber.
A switch-fuse can normally only have the load cables taken out at the top - directly from the fuses which, of course, must be capable of being isolated from the mains. I have seen some bright sparks take the mains into the fuses and the load out of the switch!

Hope this helps
Paul

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 09 February 2010 04:03 PM
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TCSC

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Joined: 25 April 2007

Some apologies for resurecting this old string. I recently needed to find the difference between switched fuse and fused switch and this string was very helpfull.
The last answer by pnorton makes alot of sense to me. However then, why do the BS EN 60617 symbols for the pair. both show the fuse on the left side of the switch?

see http://www.electrics-home.co.uk/symbol2.htm#a1

I suppose I am correct in assuming the power flows from left to right? Maybe it dosen't have to, but I think most of us would think so. Maybe that is why Paul came across the things being wired backwards!

Just to be quite sure, would either of these devices be said to be found in a typical domestic consumer unit?
Cheers
 09 February 2010 05:34 PM
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Jaymack

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If you shine a torch behind the paper, you may be able to transpose the symbol to the other direction, matters not a jot. These were 3 phase, industrial type switches.

Regards
 09 February 2010 09:02 PM
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ScottLangston

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the answer is on the tin, an isolator is an off load device and a switch is an on load device. Light switch
 09 February 2010 10:39 PM
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TCSC

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Originally posted by: ScottLangston

the answer is on the tin, an isolator is an off load device and a switch is an on load device. Light switch


Sorry I should have made my post clearer. I'm not asking for the difference between isolator and (functional) switch. I'm asking about a switched fuse and fuse switch. The first one has the fuse separate from the switch and the second apparently has the fuse integral to the switch mechanism. I've never had anything to do with a switch fuse and wanted to check exactly what the difference is, but more so, why we have the two types.
I think jaymac's earlier answers give me what I need to know but I was just checking.

What I would also like to find out, is if they are used much nowadays, or fairly obsolete. It is a question in an exam (which shall remain anonymous), and I want to guage whether it is a fair question for present day apprentices. Again, it appears as though they may be yesterday's technology from the previous posts and if so, I would want to think it is a bit of an irrelevent question.

Edited: 09 February 2010 at 10:46 PM by TCSC
 09 February 2010 10:44 PM
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paulskyrme

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As far as I now they are still available and in use.
As has already been stated they have a snap action on the operation which is useful on higher switching currents / under fault conditions.
Less common now even in Ind. installations though.
Paul
 09 February 2010 10:46 PM
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TCSC

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Cheers
 09 February 2010 10:56 PM
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TCSC

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Originally posted by: paulskyrme

As far as I now they are still available and in use.

As has already been stated they have a snap action on the operation which is useful on higher switching currents / under fault conditions.

Less common now even in Ind. installations though.

Paul


Oh dear, I hope people don't start to think I am OTT on this question, but would there be any link between the snap action and the position of the fuse? I am still perplexed as to exactly why the fuse is placed in the moving part of the switch.

Thanks for the info on how common they are.
 09 February 2010 11:01 PM
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paulskyrme

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You're not oTT esp. if you are still in formal training / education which the posts suggest.

The moving fuse was never somethign I could get my head around even though I have dealt with them since I was an apprentice (not saying how long ago)!

I suppose the fuses whilst on the moving carrier would be further away from the live terminals, and the arc gap in the mechanism may be larger?

Not a definitive answer as I never have studied the technicalities or the design detail of these items.

HTH
Paul.
 09 February 2010 11:08 PM
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TCSC

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Cheers again Paul
 10 February 2010 10:56 AM
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OMS

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The whole point of the fuse moving away from the supply side of the switch contacts is that in the "off" (and hence open) position the fuse body is not "live". If the fuse remained in place connected to the supply side ypu would need to either unbolt the fuse assembly live or find further upstream points of isolation.

Switch fuses and fused switches (or combination units) are still in wide use (I've just specified a 4000A panel with BS 88 fused switches on the outgoing ways (principally due to thier breaking capacity and a requirement for specific discrimination functions of the electrically "aggressive" load).

At a much smaller level, the advantages of fuse characteristics over circuit breakers in terms of fully discriminating systems are widely known - and may well be a cheaper solution

Why don't you post the exam question - that's the only way anyone can really comment on the validity of that question for learning purposes

Regards

OMS

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 10 February 2010 02:42 PM
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TCSC

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Thanks OMS.
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