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Topic Title: Using Switched Fuse Spur as an Isolator
Topic Summary: Fuse removed to be considered an isolator?
Created On: 17 February 2016 11:00 PM
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 17 February 2016 11:00 PM
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0rlan

Posts: 5
Joined: 07 December 2014

Hi guys...

This probably sounds like a pedantic request but it's something I can't find an answer to in black and white.

My company is using a Switched Fused Spur (BS 1363-4) to feed a domestic boiler, and based on Table 53.2 of BS 7671 this device is shown as 'use for isolation'

HOWEVER... I cannot convince the engineer who is writing a maintenance procedure that actually achieving 'isolation' requires the fuse to be removed.

He's using the words 'that's not what the book says' and is insisting that it can just be switched off because the Unswitched Spur in the table specifies ' Remove Fuse to Isolate' and the Switched Spur doesn't

Now common sense obviously counts for nothing against a degree, so I need to find an authoritative written source to show that it's only an isolator if you remove the fuse (to my mind it's just a switch if you leave the fuse in...)

Can anyone help with a definitive written source on this?

Thanks all

Allan

Edited: 17 February 2016 at 11:20 PM by 0rlan
 17 February 2016 11:16 PM
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74jools

Posts: 477
Joined: 02 March 2008

Table 53.4 in the Big Yellow Book provides the answer, the fuse only needs removing if its non switched type.

Regards

Julian
 17 February 2016 11:18 PM
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mapj1

Posts: 9689
Joined: 22 July 2004

if you want something you can 'lock off' then a removable fuse carrier, or one with the provision to be fixed in the fuse out position is needed. The question you should really ask is, do you need lockable isolation - and I can tell you that you probably do (health and safety at work) and certainly you do if it is not under the effective supervision of the guy servicing the boiler - if it was in the next room for example, say if he was working on the pump in a cupboard, and some one else might casually flick it back on, the consequences could be severe.

-------------------------
regards Mike
 17 February 2016 11:24 PM
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Zoomup

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Doesn't BS 7671 describe an ISOLATOR as a mechanical switching device......that disconnects from every source of energy, i.e. either double or tripple pole switching? A double pole switched fused connection unit does just that. And if it is under the close supervision of a person working on the boiler is safe enough.

I have met some who like an appliance to be connected via a 13 Amp. plug and socket. That way you can see that the plug is disconnected if pulled out of the socket. With a switched fuse connection unit you have to have faith in the installer, that it is connected correctly.

Z.
 17 February 2016 11:31 PM
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0rlan

Posts: 5
Joined: 07 December 2014

Thanks Julian and Mike

I have always believed that an isolator has to have a minimum air gap (among other things) to be considered an isolator.

As the unfused version of this switch is BS 60996-1 (i.e. NOT suitable as 'use for isolation' in that table) I cannot see how this device can be considered an isolator if you *don't* remove the fuse to obtain that air gap - regardless of the requirements for securing in the off position or not?

Then again that's why I'm asking the question as it's not something I can see in black and white anywhere...
 17 February 2016 11:58 PM
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Zoomup

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Now I have only my Green Book with me at present, the Yellow one is in the vehicle, but does not Table 53.4 list a switched fused connection unit to BS 1363-4 as being suitable as an isolator?

Z.
 18 February 2016 12:03 AM
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mapj1

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tell him to read page 6 of This

especially the bit referring to reg 12 of the electricty at work act.
Or for a truly riveting read, go to the paragraph of the act direct.

In paragraph (1), "isolation" means the disconnection and separation of the electrical equipment from every source of electrical energy in such a way that this disconnection and separation is secure.


my bold...

the legal precedent is to interpret the law so that this does mean you should provide a lock off if you can't otherwise guarantee it can never be accidentally switched to live while work is ongoing.

-------------------------
regards Mike
 18 February 2016 01:27 AM
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geoffsd

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Is anyone saying Table 53.4 is wrong?
 18 February 2016 07:54 AM
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Tiggywiggler

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Table 53.4 tells if you the method can be used for isolation of electrical energy. I do not think it is telling you if you can use the device for safe isolation for the purpose of maintenance. Clause 537.2.1.2 states that there must be a method to prevent it being unintentionally or inadvertently energised. Clause 537.2.1.5 tells you if it should need to be 'locked off'
 18 February 2016 09:53 AM
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peterhoward

Posts: 72
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Switched fused connection units (switched FCUs AKA switched fuse spurs) are type tested for isolation, defined in BS1363-4. The test is done by connecting L+N input together and L+N output together then impulse testing across input and output with three 6.2kV 1.2/50 microsecond pulses of both positive and negative polarity.

On both switched and unswitched units, the fuse is in the line side only, so removing just the fuse does not isolate the neutral.

The point is a switched FCU is electrically suitable for isolation; an unswitched FCU is not. I think it's up to the person doing any work to ensure the job is isolated and safe. If the FCU is next to the job (ie boiler supply situated next to the boiler), the FCU switch should be under control. If it's yards away (sorry, metres) with a door between then it may not be under control and other precautions may be necessary.

So, to answer the original question, the fuse does not need to be removed (although I would).

Another point. All the FCUs I've seen have fuse drawers which can't be removed unless forced, ie they're not designed for removal. Another test in the standard is access to live parts with a test pin when the fuse is out. Those I've seen where the fuse drawer can be forced out would fail this test without it in situ.

Edited: 18 February 2016 at 09:59 AM by peterhoward
 18 February 2016 01:07 PM
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mapj1

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Actually - if the OP is writing instructions for a general maintenance routine, to work in any building of unknown condition where you have no idea where the spur will be or even if it is double pole switched, then you need to cover all bases. At least advising removal of the fuse, is the safest case.
We all know that in a hurry on a Friday after lunch, the switch will be flicked and that will be all, or even opened up with the juice still on, but its a judgement call of the guy when he gets to site - the 'official process' however, needs to maker it clear it may be required to remove the fuse or provide a similar lock-off in all or some cases.

-------------------------
regards Mike
 18 February 2016 01:42 PM
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0rlan

Posts: 5
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Thanks all for your comments...

I am not particularly interested in whether or not the isolator should be locked off or secured at this point - all I'm really asking is if a Switched Fuse Spur can be considered an 'isolator' by just switching it off? If the fuse is to be left left in why can't we just use DP switches on a dedicated radial to achieve 'isolation'? Is it the case that by having a 'local' fuse (as also in a plug-top into a socket) that this is considered as preventing high local currents which will protect the contacts from welding in some way? BTW i'm asking this because I want him to add *and remove fuse* to the *switch off* part of the procedure.
 18 February 2016 03:19 PM
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peterhoward

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The answer is yes. A switched fuse spur - fused connection unit in BS1363-4 has sufficient contact gap (and is type tested to confirm it) to be classed as an isolator. Most DP switches of the wiring accessory type are classified to BS EN 60669-1:1999 and this does not cover isolators. Switches classified to BS EN 60669-2-4:2005 are isolators and are type tested by impulse testing as per FCUs. A switch can pass the requirements of BS EN 60669-1 without being impulse tested.

The fuse in an FCU is protection for the cable coming from it and nothing to do with isolation, excepting that removing it will isolate the line side (but not the neutral).
 18 February 2016 07:36 PM
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AJJewsbury

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Doesn't BS 7671 describe an ISOLATOR as a mechanical switching device......that disconnects from every source of energy, i.e. either double or tripple pole switching?

Not quite - the N may be left connected where it's reliably connected with earth - e.g. on TN system supplied by the UK distribution network. See reg 537.1.2. Other than for the main switch of household or similar which must be DP regardless (reg 537.1.4).

So removing a single fuse wouldn't count as isolation for a TT system (or possibly TN systems abroad).

- Andy.
 18 February 2016 07:47 PM
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aligarjon

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Its a shame common sense cannot be used A switched spur is an isolator, not that difficult really. If it is under the control of the person working on the circuit then it is not necessary to remove the fuse. If you are working on remote pumps/ programmers etc then anyone with half a brain would also remove the fuse.

I suspect a 3amp fuse is stipulated in the boiler instructions so a dedicated mcb would not suffice unless you can get a 3amp one, and then it would only be single pole..


Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 20 February 2016 09:19 PM
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0rlan

Posts: 5
Joined: 07 December 2014

Hi everyone following this and thanks for the comments

I have now had a definitive answer and thought I'd share...

A Switched Fused Spur is given BS 1634-4 as shown in Table 53.2 of the Regs

BS 1634-4 is defined as: "A device is suitable for on-load isolation, i.e. disconnection whilst carrying load current.

However the telling point is that the 'Yes' in the isolation column of that table is further defined directly below the table as: Yes = Function provided. No = Function not provided

The 'isolation' function is provided because a BS 1634-4 device has the ability to have the fuse removed (as a comparison a socket is also a BS1634 device because it has the ability to have a plug removed)

If you look at an exactly same item without the fuse (e.g. an MK Logic Plus 20A Double Pole Switch) this has exactly the same switch contact arrangement as the Fused version but only gets a BS-60889 (non isolation function) rating

The BS 1364-4 is awarded on the basis that this device has a FUNCTION of becoming an isolator - and without removing the fuse it is therefore only a switch!

Hope that helps...

Allan

Edited: 20 February 2016 at 09:27 PM by 0rlan
 20 February 2016 09:59 PM
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AJJewsbury

Posts: 16095
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I have now had a definitive answer

who's definition?
- Andy.
 21 February 2016 02:54 PM
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TeesdaleSpark

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Originally posted by: mapj1
the legal precedent is to interpret the law so that this does mean you should provide a lock off if you can't otherwise guarantee it can never be accidentally switched to live while work is ongoing.


Do you have anymore details of that judgement please? I would be interested in looking at that in more detail.
 21 February 2016 02:56 PM
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TeesdaleSpark

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The MK SFCU can be locked off with a padlock.
 21 February 2016 04:36 PM
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Zs

Posts: 3830
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You've just made my day Teesdale. I've never realised that before.

Zs
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