100a Switched Fuse?

[munch88]

Hi there,

I am carrying out a rewire and putting a new metal consumer unit under the stairs.

The customer wants the original position of the consumer unit to be moved but will still be under the stairs.

The distance away from the meter will be 4m and will run under the floorboards for approx. 2 meters
Therefore I am going to use 25mm 3 core SWA.

I believe I require a 100a switched fuse due to the length of the cable exceeding 3 meters.

Can anyone suggest a decent and reasonable prices 100a metal switched fuse which will be easy to terminate an SWA.

Thanks

 

[Octopus]

Why do you think you need a 100A switch fuse? What is the DNO fuse?

 

[telectrix]

why SWA if it's not buried in wall< 2" deep?

 

[munch88]

I was led to believe that tails exceeding 3 meters required a switched fuse as the DNO only protect tails up to 3 metres. I'm happy to be proved wrong as it's cheaper and less hassle

Also, the use of SWA was just to give the cable some extra protection under the floor. Also would look neater cleated up the wall and terminated via gland rather than knock outs.

 

[Spoon]

What about Murdoch's two questions "Why do you think you need a 100A switch fuse? What is the DNO fuse?"

 

[munch88]

What about Murdoch's two questions "Why do you think you need a 100A switch fuse? What is the DNO fuse?"

The DNOs fuse only protects tails up to 3 meters (so I believe) and if an additional fuse is required to protect tails exceeding 3 metres then surely it's better to fit a switched fuse so isolation in the future can be achieved without pulling the DNO fuse.

 

[Richard Burns]

Not such a bad plan as it goes, as indicated the fuse should be less than or equal to the DNO fuse, to try and hope yours may go first, if it ever comes to that.
A metal clad 100A switch fuse is probably your cheapest option something like this.

 

[Spoon]

Maybe my post should have been more clear, sorry.
No one was questioning your plan to use a switched fuse. A couple of us were wondering why you choose 100A and wanted to know what size is the DNO fuse.
I thought the fuse had to be smaller than the DNO fuse Richard? Do you not use discrimination in this case?

 

[munch88]

Maybe my post should have been more clear, sorry.
No one was questioning your plan to use a switched fuse. A couple of us were wondering why you choose 100A and wanted to know what size is the DNO fuse.
I thought the fuse had to be smaller than the DNO fuse Richard? Do you not use discrimination in this case?

The DNO is 100A so therefore was going to use 100A or even could use 80A (should of stated that in my OP)

I will check with the DNO whether I definately require an additional fuse due to the length.

 

[Midwest]

I would not recommend the one Richard suggested I'm afraid. It's a bit psychically small, especially terminating 25mm swa. Try something like Wylex 110M 100 Amp SPN Metalclad Switchfuse - Edwardes , you can get them most wholesalers. Install it before 1st Jan 16 though, cos its got a plastic cover :builder:. You can get one with an 80amp fuse or a 60amp.

OSG suggest meter tails should be minimum 25mm and that the DNO may specify maximum length and may require an isolator fitted (2.2.3 green OSG). And where their cut out provides fault protection for the tails, they may require certain method of installation. Where buried in walls further protection may be needed. Reg 434.2.1 & 434.3 (iv) and others :juggle2:

You would have to installed some sort of metal box to terminate the swa, next to the meter, so using a switch fuse is ideal. So I concur with you munch88, except you should use an 80amp fuse if the DNO one is 100amp.

You could run it pass the DNO. In my experience, they would be happy with what you are proposing.

 

[munch88]

I would not recommend the one Richard suggested I'm afraid. It's a bit psychically small, especially terminating 25mm swa. Try something like Wylex 110M 100 Amp SPN Metalclad Switchfuse - Edwardes , you can get them most wholesalers. Install it before 1st Jan 16 though, cos its got a plastic cover :builder:. You can get one with an 80amp fuse or a 60amp.

OSG suggest meter tails should be minimum 25mm and that the DNO may specify maximum length and may require an isolator fitted (2.2.3 green OSG). And where their cut out provides fault protection for the tails, they may require certain method of installation. Where buried in walls further protection may be needed. Reg 434.2.1 & 434.3 (iv) and others :juggle2:

You would have to installed some sort of metal box to terminate the swa, next to the meter, so using a switch fuse is ideal. So I concur with you munch88, except you should use an 80amp fuse if the DNO one is 100amp.

You could run it pass the DNO. In my experience, they would be happy with what you are proposing.

Helpful info, much appreciated

 

[davesparks]

Which DNO? The max length of tails varies between them. For example SSE say 3m and ukpn allow 4m

 

[Midwest]

Which DNO? The max length of tails varies between them. For example SSE say 3m and ukpn allow 4m

That distance, typically given, is, I think you'll find, for the installation of double insulated cables (6181y). The installation as suggested by the OP, i.e. extending the length using swa and suitable switch fuse, is accepted (as far as my research allows) by DNO's, and certainly is by my DNO, SSE.

 

[davesparks]

That distance, typically given, is, I think you'll find, for the installation of double insulated cables (6181y). The installation as suggested by the OP, i.e. extending the length using swa and suitable switch fuse, is accepted (as far as my research allows) by DNO's, and certainly is by my DNO, SSE.

No it is the maximum length of cable which may be installed without installation if additional OCPD, there is no restriction to the type of cable.
Also 6181Y is not double insulated cable, it is insulated and sheathed cable.

You have clearly missed my point, the OP is installing a switchfuse for a tails length of 4m when he has not yet confirmed what the maximum tails length permitted by his DNO is.

 

[munch88]

Which DNO? The max length of tails varies between them. For example SSE say 3m and ukpn allow 4m

Western Power, I think they may even be 2 metres

 

[Midwest]

No it is the maximum length of cable which may be installed without installation if additional OCPD, there is no restriction to the type of cable.
Also 6181Y is not double insulated cable, it is insulated and sheathed cable.

You have clearly missed my point, the OP is installing a switchfuse for a tails length of 4m when he has not yet confirmed what the maximum tails length permitted by his DNO is.

As ever Dave, I bow to your superior knowledge. But in reply to your first point, OSG suggests in addition to maximum length; minimum csa & method of installation (if relying on DNO fuse), and agreed I've seen tw & e for tails.

Second point, you got me there. But google the two, and you get 6181Y.

Thirdly, I did suggest to the OP to contact the OP, as you and many others have.

 

[7029 dave]

This is an rewire, so I think the DNO will may not even be aware, 4 meters is no big deal, I have seen over 20mts tails in duct. Not wright but just saying.

 

[munch88]

I just contacted Western Power and they said it's down to my energy supplier as it's after the meter!

 

[Richard Burns]

Maybe my post should have been more clear, sorry.
No one was questioning your plan to use a switched fuse. A couple of us were wondering why you choose 100A and wanted to know what size is the DNO fuse.
I thought the fuse had to be smaller than the DNO fuse Richard? Do you not use discrimination in this case?

An 80A fuse and a 100A fuse will not discriminate on fault current and generally installations do not go out on overload because downstream protection goes first. To avoid the overload situation you can use a lower fuse but this then limits the installation capacity.
Belt and braces would be a fuse that does fully discriminate and then you would hardly be able to boil a kettle!

I would not recommend the one Richard suggested I'm afraid. It's a bit psychically small, especially terminating 25mm swa. Try something like Wylex 110M 100 Amp SPN Metalclad Switchfuse - Edwardes , you can get them most wholesalers. Install it before 1st Jan 16 though, cos its got a plastic cover :builder:. You can get one with an 80amp fuse or a 60amp.

Yours is a better link, but just more expensive, depends if installation problems or upfront cost are your key priority, but, if you can, yes, I would go for your version.

I just contacted Western Power and they said it's down to my energy supplier as it's after the meter!

That is just typical of a DNO, not our responsibility.
Caution, male deer crossing here, please pass the buck.

 

[davesparks]

you should use an 80amp fuse if the DNO one is 100amp.

Why's that then?

 

[Midwest]

Why's that then?

Why not?

 

[Midwest]

An 80A fuse and a 100A fuse will not discriminate on fault current and generally installations do not go out on overload because downstream protection goes first. To avoid the overload situation you can use a lower fuse but this then limits the installation capacity.
Belt and braces would be a fuse that does fully discriminate and then you would hardly be able to boil a kettle!

I was taught to provide discrimination with fuses in series. Indeed I believe I asked the same question, possibly on this forum. There is the consideration of reducing the customer supply by 20amps, when placing an 80amp fuse after DNO 100amp fuse. I was advised a 100A BS 1361 should blow in 0.4s at 1200A (cannot cite my source, haven't got BRB). A 80A BS 88 should do the same at around 750A (from BYB). Suggesting to me that the switch fuse would disconnect before the DNO fuse? As I said I was was taught to provide discrimination with fuse in series, but I can see that whatever fuse does go, the end result is the same. I'm willing to be told otherwise.:uhoh2:

 

[Octopus]

The DNO fuse could be 60A

Just saying

 

[Midwest]

The DNO fuse couled be 60

Just saying

Yep could be. I was suggesting if DNO was 100a, then switch fuse could be 80. But this appears to be of no real use?

 

[Octopus]

Yep could be. I was suggesting if DNO was 100a, then switch fuse could be 80. But this appears to be of no real use?

It just goes to show how we are speculating. And if it is 60A discrimination could be tricky

 

[Midwest]

OP does say DNO fuse is 100a on post 9.

 

[Octopus]

OP does say DNO fuse is 100a on post 9.

Yup. Missed that but had he actually seen it or is he going on what the holder says?

 

[Spoon]

I'm sure the OP will clarify everything in his next post.....

 

[Richard Burns]

I was taught to provide discrimination with fuses in series. Indeed I believe I asked the same question, possibly on this forum. There is the consideration of reducing the customer supply by 20amps, when placing an 80amp fuse after DNO 100amp fuse. I was advised a 100A BS 1361 should blow in 0.4s at 1200A (cannot cite my source, haven't got BRB). A 80A BS 88 should do the same at around 750A (from BYB). Suggesting to me that the switch fuse would disconnect before the DNO fuse? As I said I was was taught to provide discrimination with fuse in series, but I can see that whatever fuse does go, the end result is the same. I'm willing to be told otherwise.:uhoh2:

Discrimination is required for fuses in series to prevent nuisance tripping, however this is a specialist case here where you are just providing another protection device that is not the DNO property.
To discriminate fuses you usually as a rule of thumb need to go down two sizes or have a 1.6 ratio between them, this would mean at least dropping to a 60A fuse from a 100A supply, which is starting to seriously limit the installation, obviously for a 60A supply you would have to drop to 40A or less.
To discriminate the prearcing energy of the upstream device must be less than the total let through energy of the downstream device.

 

[Spoon]

But isn't putting an 80A fuse better than putting a 100A fuse in? A little discrimination is better than nothing isn't it?

 

[munch88]

I'm sure the OP will clarify everything in his next post.....

I can confirm that the current fuse is 100A.

 

[Spoon]

I'll have to tell my missus about this. I was right for once. Thanks for the post munch88.

 

[Midwest]

Discrimination is required for fuses in series to prevent nuisance tripping, however this is a specialist case here where you are just providing another protection device that is not the DNO property.
To discriminate fuses you usually as a rule of thumb need to go down two sizes or have a 1.6 ratio between them, this would mean at least dropping to a 60A fuse from a 100A supply, which is starting to seriously limit the installation, obviously for a 60A supply you would have to drop to 40A or less.
To discriminate the prearcing energy of the upstream device must be less than the total let through energy of the downstream device.

I'm not clever enough to argue against that!