New circuit design question

[RyanAspire]

You are fitting a fused isolator, so you will not have 100A

Based the calcs on 100A. Really can't help but feel youre here to call me out rather than help?

 

[Spoon]

Based the calcs on 100A. Really can't help but feel youre here to call me out rather than help?

I think you have misread my posts mate. I have given you good info.
You say the main fuse is 100A and you are fitting a fused isolator, therefore the fused isolator can't be 100A as you would not have any discrimination mate. It has to be lower than 100A.
A few people have asked for more info on what power the kiosks are designed at, and rightly so. The more info we have the more we can help.

 

[pc1966]

You say the main fuse is 100A and you are fitting a fused isolator, therefore the fused isolator can't be 100A as you would not have any discrimination mate. It has to be lower than 100A.

There is no requirement for the fused isolator to protect the DNO side, that have their own fuse. It is so you can justify protection of the sub-main without relying on the DNO's permission.

True it won't provide selectivity at the DNO if you use the same value (assuming it meets the sub-main disconnection times) but if you are likely to blow the DNO fuse without a major sub-main fault (pick-axe through it, etc) you have under-specified the supply in the first place!

That is why I suggested a 60A fused-switch at each take-off point, as that should provide selectivity with the incoming fuses of both DNO and a customer.

 

[Spoon]

There is no requirement for the fused isolator to protect the DNO side, that have their own fuse. It is so you can justify protection of the sub-main without relying on the DNO's permission.

True it won't provide selectivity at the DNO if you use the same value (assuming it meets the sub-main disconnection times) but if you are likely to blow the DNO fuse without a major sub-main fault (pick-axe through it, etc) you have under-specified the supply in the first place!

That is why I suggested a 60A fused-switch at each take-off point, as that should provide selectivity with the incoming fuses of both DNO and a customer.

Ok, so in past posts, if someone was to extend their tails from the meter to the CU, and it goes over 3 meters they have been told that they need to install a fused isolator. Why doesn't this apply here then? The principle is the same.
Or have I misread something...

 

[David Prosser]

Ok, so in past posts, if someone was to extend their tails from the meter to the CU, and it goes over 3 meters they have been told that they need to install a fused isolator. Why doesn't this apply here then? The principle is the same.
Or have I misread something...

Yes you have missed the bit that says "There is no requirement for the fused isolator to protect the DNO side". Not there is no need for a fused isolator. The point being they can have 100A fuses the same as the DNO.

 

[pc1966]

As above, my comment was on the fuse size aspect.

 

[pc1966]

A quick look at 70mm has that run coming in at around 0.63 * 122 = 0.077 ohms R1+R2 if 3-core, and a 100A fuse to blow in 5s or less is around 0.3 ohms Zs max, so you need to check the supply Ze is no more than about 0.2 ohms otherwise you are going to need to do something else (e.g. 300mA delay RCD) to meet disconnection limits.

While 2-core is cheaper by a bit, the armour is around 2 ohm/km so the run would have R1+R2 of around 0.277 ohms leaving practically nothing for supply Ze & 100A fuse protection.

 

[Spoon]

Yes you have missed the bit that says "There is no requirement for the fused isolator to protect the DNO side". Not there is no need for a fused isolator. The point being they can have 100A fuses the same as the DNO.

Now I'm confused.
So you are saying that a fused isolator is required, due to the distance from the meter to the first CU.
What's the point in putting in a 100A fuse. We have always recommended a fuse lower than the main fuse size?

 

[David Prosser]

kiosk one can have 100A fuse switch giving you better flexability with your selectivity futher down the chain.

 

[Spoon]

kiosk one can have 100A fuse switch giving you better flexability with your selectivity futher down the chain.

But if there is a fault then either the main fuse or the isolator fuse could blow..