100a Switched Fuse?

[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!