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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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 21 February 2016 04:42 PM
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sparkingchip

Posts: 9953
Joined: 18 January 2003

Originally posted by: 0rlan

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



A British Gas engineer told me that his instructions for the procedure that your colleague is writing out are, to isolate by turning off the switch, then removing the fuse and inserting a cable tie through the fuse holder to attach a "Do not turn on sign". Then upon completion of the work they cut the cable tie to allow them to reinsert the fuse and turn the switch back on to re-energise the heating system.

I think that is also the method written up in a Corgi book I have somewhere on the electrical inspection and testing of heating systems.

I agree with you and think your colleague maybe writing a potentially flawed document.

Personally when the main supply switch is remote from the boiler I always put a secondary switch immediately next to the boiler,if you use a Schneider three pole isolator next to the boiler it is lockable and actually labeled as an isolator!





Andy

Edited: 21 February 2016 at 04:55 PM by sparkingchip
 21 February 2016 04:52 PM
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sparkingchip

Posts: 9953
Joined: 18 January 2003

I have never seen a combined two way light switch and isolator



ETC

Andy
 22 February 2016 08:06 AM
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Zoomup

Posts: 3269
Joined: 20 February 2014

Oh yes it does, (in true pantomime fashion ) see Definitions.

Isolation: ..... "by separating the installation or section from every source of electrical energy".

Neutral Conductor: "....... contributing to the transmission of electrical energy".

Now I know that the regs. allows exceptions, but I still prefer total disconnection from all sources of supply if I am working up to my armpits in a piece of electrical equipment.

The original post concerns itself with gas boiler maintenance and the necessary safety isolation requirements before mechanical or electrical maintenance. 537.3.2.4 may well apply.

Despite 537.1.2 allowing a neutral conductor to be left unswitched for isolation, it may be possible for an appliance to have a voltage difference between the exposed conductive parts and neutral inside an appliance within an installation. That is when disconnection of all poles is beneficial to anyone working within an appliance I believe.

Also, if a double pole isolator or double pole switched socket is used on a single phase load, a faulty load is unlikely to causer a N to E fault whilst standing idle. A faulty kettle is an example that could trip out a R.C.D. and cause domestic chaos with a freezer defrosting whilst the home owners were out for example, or on holiday.

We have double pole switches for cookers, showers, storage heaters etc. So why not use double pole switching for a gas boiler to play safe. It makes good engineering sense for isolation for mechanical maintenance.

Bye,

Z.

Edited: 22 February 2016 at 08:33 AM by Zoomup
 22 February 2016 11:42 AM
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geoffsd

Posts: 1702
Joined: 15 June 2010

Originally posted by: 0rlan

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

That is just defining the Yes and No.

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)

There are many more items similarly further defined which do not have fuses to remove.




Switched FCUs are double poled.
 24 February 2016 04:10 PM
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0rlan

Posts: 5
Joined: 07 December 2014

And just to throw a spanner in the works - what's the difference between 'lockable' and 'securable'?
 25 February 2016 05:20 AM
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Jaymack

Posts: 5356
Joined: 07 April 2004

Originally posted by: 0rlan
HOWEVER... I cannot convince the engineer who is writing a maintenance procedure that actually achieving 'isolation' requires the fuse to be removed.

My definition of Isolation would be to remove all sources of power to prevent danger. IMO a single pole, switched spur unit would not be suitable for isolation per se, since it would require other steps to prove this, it is however a means of switching off!

Regards
 25 February 2016 11:41 AM
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peterhoward

Posts: 72
Joined: 12 January 2011

Just checked BS 1363-4 again; a switched connection unit (AKA switched fused spur) is defined as "a fused connection unit as in 3.1 (fused connection unit) or 3.2 (cord outlet connection unit), with associated switch to disconnect the supply to both line and neutral terminals".

Single pole units (as far as I know) don't exist; if they do they don't comply with BS 1363-4 and shouldn't be used in the UK. All switched units which do comply will have been type tested to verify the design - clearances in particular - and will have been tested for isolation with a +/- impulse test at 6.2kV across both poles of the switch.
IET » Wiring and the regulations » Using Switched Fuse Spur as an Isolator

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